Advanced Field Epi:Manual 1 - Disease Control and Eradication Programs

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Disease control and eradication programs

What is meant by control or eradication of disease

Control: is the reduction of the morbidity and mortality from a disease by

- Treating diseased animals to reduce the prevalence

- Preventing disease to reduce incidence and prevalence

Eradication: is the regional extinction of an infectious agent

Many animal diseases are endemic in a population and are either not sufficiently serious to warrant control, or are amenable to well recognised treatment, control or preventive measures implemented at the farm or individual level.

Most countries will have a list of animal diseases that must be reported to relevant authorities as soon as diagnosis is suspected or confirmed. In Australia these are called notifiable animal diseases. Diseases are generally included on a notifiable diseases list because of their potential to cause adverse impacts on animal health and production, international trade, biodiversity and human or ecosystem health.

There is little reason to invest effort in maintaining a list of notifiable diseases and the necessary infrastructure to detect and report diseases unless there is also a commitment towards controlling or eradicating these diseases.

Control or eradication programs may be applied to endemic diseases or as a planned response activity that is implemented only if specific exotic diseases occur in a particular country or part of a country that is normally free of that disease.

For example:

  • Milk fever and grass tetany are affected by seasonal and management factors and are generally managed at farm and individual animal levels.
  • Clostridial diseases of sheep and cattle are widespread and generally controlled by on-farm vaccination programs.
  • Internal and external parasites in sheep and cattle are generally managed at the farm level, but can be very costly on an industry basis (10’s – 100’s of millions of dollars per year) and on-farm control may be supported by regional programs providing technical advice and support.
  • Some diseases, such as ovine brucellosis or caprine arthritis-encephalitis virus, affect only some herds or flocks, and can be managed at a regional or industry level through voluntary quality assurance (QA) type programs.
  • Control of Johne’s disease in many countries is moving towards voluntary, industry-based programs.
  • Zoonotic disease such as anthrax, rabies, bovine spongiform encephalopathy and highly pathogenic avian influenza are subject to strict regulatory programs in many countries.
  • Brucellosis and TB in cattle have been eradicated from Australia and are subject to national eradication programs in some countries.
  • Exotic disease outbreaks in some countries are usually subject to emergency eradication programs (for example foot-and-mouth disease) in countries where the disease doesn’t usually occur.
  • Global freedom from rinderpest was declared in 2011, following a lengthy eradication program.

Why have a regional control or eradication program?

Regional disease control or eradication programs have been an important facet of livestock production since at least the 18th century. Early programs were directed at eradication of outbreaks of severe diseases such as rinderpest and foot-and-mouth disease from Europe and the UK in the 18th and 19th centuries. Also in the 19th century, Australia eradicated sheep scab from its national flock, while in the mid-late 20th century contagious bovine pleuro-pneumonia brucellosis and tuberculosis were also eradicated from the Australian cattle population. More recently, in 2011 international freedom from rinderpest was proclaimed after a protracted eradication campaign. This is only the second time global eradication of a disease has been achieved (following the eradication of smallpox in the mid 20th century) and the first time for an animal disease.

Regional or national programs may be implemented for any of the following reasons:

  • To control or eradicate diseases with severe productivity and economic consequences including trade in animals and animal products (e.g. foot-and-mouth disease);
  • To protect human health from zoonotic infections (e.g. bovine spongiform encephalopathy, bovine tuberculosis, anthrax);
  • To maintain product quality (e.g. chemical residues);
  • To protect unaffected producers or regions from disease that may be endemic in other regions (e.g. footrot, ovine brucellosis, Johne’s disease, cattle tick);
  • To reduce indirect effects of disease on unaffected producers who are not in a position to take action themselves to effectively prevent or control the impacts of the problem on their enterprise (e.g. chemical residues, Johne’s disease); and
  • To reduce the impact of disease on affected herds and flocks (e.g. mastitis, internal parasites in sheep).

An important aspect of diseases requiring regional or group action to control or eradicate is that they are often diseases where producers can take individual action if they wish, but where the risk of re-infection or break-down of control because of external factors is sufficiently high to discourage individual action.

For example, many sheep producers in the Australian State of New South Wales were reluctant to attempt eradication of footrot in sheep until a regional program started and provided some reassurance that they were not likely to get reinfected.

Table 5. lists the characteristics of conditions that determine whether a disease is more suited to individual farm or regional control.

Table 5.: Characteristics of conditions suited to farm-level or regional control (adapted from Hanson & Hanson, 1983)


Farm-level control Regional control
Spread can be stopped by a physical barrier such as a fence Physical barriers of limited effectiveness in preventing spread
Rate of transmission is slow enough to allow intervention before the entire herd is infected Transmission is too fast for intervention before the entire (or majority of) herd is infected
Carriers are readily detectable on farm Apparently healthy carriers can only be detected by laboratory tests
No public health, food safety or product quality implications Condition is a public health, food safety or product quality risk
Low or no mortality rate High morbidity and high mortality rates
Highly effective vaccine or treatment is available Vaccine or treatment is only poorly to moderately effective

Types of programs

Regional animal health programs can vary substantially in their design, the tools used and the way they are implemented, depending on the rationale and objectives of the individual program. Programs can be broadly classified according to their objectives as either eradication or control programs.

Eradication programs

Eradication programs are generally directed at the elimination of a disease agent from a region. This is usually achieved by the implementation of measures directed at reducing prevalence on infected farms and interrupting spread from infected to uninfected farms.

Eradication programs often require a strong regulatory framework, with significant government input to the management and implementation of the program. Funding for eradication programs may be largely from governments, or shared by governments and affected industries, depending on the nature of the disease and the capacity and willingness of governments (and industry) to contribute.

Eradication programs are generally time-limited and aim to eliminate the disease within a relatively short or manageable period. Once disease is eliminated there is assumed to be no ongoing cost associated with eradication but there may be substantive ongoing costs associated with surveillance programs to prevent, detect and respond effectively to any future incursion of the disease into the area where it has been eradicated.

In cases of endemic diseases, eradication may be preceded by a period of control to reduce the prevalence of disease to a level where eradication becomes feasible and economic.

Control programs

As defined elsewhere, control implies any program directed at reducing the level of morbidity, mortality or production losses due to a disease. Control can be achieved by:

  • treating diseased animals; and/or
  • preventing infection occurring; and/or
  • reducing the impact of disease in infected animals

Control programs are expected to have ongoing costs associated with disease detection and control, while-ever the disease or the reasons for its control persist.

Regulatory programs

Some control programs may be supported by government regulation to allow enforcement of compliance. Regulations may relate to movement controls, animal treatments, destruction of animals and compensation. Regulatory programs are more common for diseases that have a “public good” component, such as zoonotic diseases. Over time, if a control program is successful it can be extended and adapted into an eradication program. Examples of diseases where regulatory control programs are used include anthrax, rabies and bovine spongiform encephalopathy.

Voluntary (Industry-based) programs

Governments in many countries are moving to reduce regulation of the livestock industries, and this move is often accompanied by a move towards voluntary or industry-based control programs. These programs rely on farmers complying voluntarily with recommended practices to reduce disease risk to themselves and other producers, rather than using regulations to enforce compliance. Voluntary programs depend heavily on an effective communication and education program to change the behaviour and attitudes of farmer and their advisors and to get farmers to adopt the recommended practices.

Voluntary programs may have some regulatory support (for example legislative support for the use of vendor declarations or movement controls), but are being used increasingly as an alternative to regulatory programs, particularly where most of the benefits of the program flow to producers, rather than consumers or the general public. Examples of voluntary programs include the early stages of enzootic bovine leucosis eradication in dairy cattle in Australia and Johne’s disease control programs in many countries.

Assurance-based programs

Assurance-based programs rely on on-farm implementation of a quality assurance approach to management and production on some farms to provide a source of quality-assured stock for other producers. Quality assurance programs require participating farmers to implement a range of recommended practices to achieve a quality outcome and are supported by an audit process to ensure compliance and demonstrate program integrity. Stock from qualifying farms may be assured as low-risk for a particular disease or for chemical residues, depending on the program(s) in which they participate and the level they have achieved.

Although assurance-based programs may not significantly reduce the regional prevalence or impact of the disease or condition of concern, they can reduce further spread by providing sources of low-risk stock for producers who wish to avoid introducing unwanted diseases to their farm. They can also be used as part of a broader regulatory or voluntary control program. Examples of assurance-based programs include the various Johne’s disease Market Assurance Programs in Australia and similar programs in other countries, as well as industry-based product quality programs.

Strategies that may be used for disease control

Maintenance of infectious disease in an animal population depends on presence of infectious individuals and herds, presence of susceptible individuals and herds, and contact between infectious and susceptible individuals and herds. Disease will persist in the population while ever these conditions remain.

The main strategies for control and eradication of animal diseases (Thrushfield 2005) include:

  • Quarantine: Isolation of diseased animals (or animals suspected of being diseased) so the risk of spread to other susceptible animals is reduced. Often accompanied by other biosecurity measures relating to movement controls, hygiene and disinfection.
  • Slaughter of diseased animals: May be accompanied by slaughter of high-risk contact animals in emergency disease control (eg Foot-and-Mouth disease outbreaks in some countries) and disposal of carcasses and other infectious material.
  • Vaccination: May be to reduce spread of disease during an outbreak or as part of longer term eradication programs to reduce circulating infection.
  • Treatment: Administration of drugs (antibiotics or anthelmintics) may be used as part of a control program or to reduce risk of diseases from occurring.
  • Control of animal movements: Often part of quarantine measures to prevent disease spread. May also be used more routinely eg controlled grazing for management of internal parasites or movement of animals out of high risk areas at certain times of the year to avoid vector borne diseases or bringing animals indoors at night in Africa to minimise risk of exposure to African horse sickness virus carried by night flying midges.
  • Vector and reservoir control: Infectious diseases may be transmitted by insect vectors or different reservoir hosts (Nipah virus). Control of the vectors or reservoir hosts will help in disease control.
  • Biosecurity measures: Measures include hygiene, disinfection, and other management measures that may reduce disease spread. May be applied at animal, mob, farm or regional levels.
  • Genetic selection: May be useful in control of some diseases by elimination of inherited diseases or selection of animals with increased resistance.

These strategies are generally applied through four separate activity pathways:

  1. Detection of the infectious agent responsible for the disease;
  2. Reduction in the number of infected hosts;
  3. Increase in the resistance to infection of susceptible hosts; and
  4. Reduction in contact between infectious and susceptible hosts.

Detecting the disease agent

Surveillance

The term surveillance describes an active process in which disease occurrence data is collected, analysed, evaluated, and reported to animal health agencies tasked with disease control. The term monitoring is usually used for a more passive process although in common usage both terms are often used interchangeably. Because of the substantial cost involved, programs often encompass several diseases at the one time.

An effective surveillance program will be able to answer a number of important questions relevant to disease control:

  • Is the frequency of the disease remaining constant, increasing or decreasing?
  • What is the relative frequency of one disease compared with another?
  • Are there differences in the geographical pattern of the condition?
  • Does the disease have any impact on productivity and/or profitability?
  • Is the disease absent from a particular herd, region, or nation?
  • Is a control or eradication program cost-effective?

The potential sources of data for surveillance programs include clinical evaluations, laboratory reports, slaughter inspection data, screening tests, owner reports, and on-farm screening programs.

Surveillance or monitoring programs may be developed at a number of different levels, depending on the level of need for the information. Some examples are listed below:

1.Individual farms - these usually include monitoring of economically significant production parameters, such as mortality rates, somatic cell counts in milk as an indicator of mastitis, growth rate, milk production, mortality rates, etc. Monitoring of temporal patterns of these variables is important for early detection of potential disease problems or failure of on-farm control programs.

2.Regional levels within a country (district, province, state etc) – including testing to detect infected animals or herds and to support disease freedom at a regional level.

3.National – National surveillance programs can be very costly. To help defray costs these programs may predominantly be based on passive surveillance (investigation initiated by the owner) or involve testing of only a sample of the national herd.

Surveillance to identify infected animals or infected herds/flocks is an essential component of any control or eradication program. For such programs, surveillance could be targeted at individual animals on-farm (for example, test-and-slaughter programs for brucellosis or bovine tuberculosis eradication), or could use aggregate samples, such as bulk-milk or pooled faeces, or could use off-farm sampling such as through milk factories or abattoirs (for example, milk-ring testing to identify brucellosis-infected herds).

Farmer notification of suspected cases also forms an important component of surveillance for case-detection. For surveillance to be effective, an economically justifiable test with known sensitivity and specificity should be used. Once an infected animal or farm has been identified, further action is likely using one or more of the other tools discussed below.

Tracing

Tracing of livestock movements is an important tool particularly for the detection of infected herds or flocks. For disease control purposes, tracing usually involves the identification of potentially infected farms through the tracing of movements of infected or exposed animals. Further testing is usually undertaken on the identified farms to establish their true infection status. If a farm’s infection status cannot be determined immediately, quarantine measures may be imposed until the situation is resolved.

Tracing can involve any of the following activities:

  • Identification of the property of origin of animals identified as infected or suspect through testing at abattoirs or saleyards (abattoir/saleyard traceback);
  • Identification of the property of origin of animals suspected as a potential source of infection on an infected farm (trace-back);
  • Identification of farms that have received possibly exposed animals from an infected farm (trace-forward)
  • Identification of farms with animals potentially exposed during movement of infected animals, such as at saleyards or during transport;
  • Identification of neighbouring farms or other farms potentially exposed to an infected farm by local movement of animals or infectious material; and
  • Identification of vehicles used to transport potentially infected animals or vehicles, people or other fomites that have had possible contact with infected animals or environments.

Tracing activities are made much easier and more reliable by the consistent use of unique animal identification and national animal identification systems that are capable of tracking animal movements over time. An example of this sort of system is the National Livestock Identification System (NLIS) in Australia.

In the absence of a comprehensive database of animal movements, tracing relies on interviews with the owners of infected or exposed animals to identify potential animal or other movements that might have spread infection. Investigations may also include discussion and examination of records from livestock agents, stock selling centres, milk processors and abattoirs.

Effective tracing can also consume large volumes of resources for both the identification of movements to/from infected farms, and also the subsequent identification and investigation of the source or destination properties. However, examination of tracing records can often help understand the epidemiology and distribution of a disease during an outbreak.

Reducing the number of infected hosts

Slaughter

Slaughter of individual infected animals, in-contact animals or entire herds may be an option, depending on the nature of the disease and the program involved. Slaughter of infected animals and herds has an immediate effect of reducing the number of infected animals in the population and greatly reduces opportunities for further spread of the disease.

However, this comes at a significant cost in terms of surveillance to detect the infected animals and the costs of compensation and disposal if the animals are not salvaged through normal slaughtering.

Depending on the type and scale of the program, slaughtering of stock can be undertaken in a number of ways:

  1. Immediate destruction of infected and in-contact animals generally in emergency situations such as response to an exotic disease outbreak (for example, foot-and-mouth eradication programs, bovine spongiform encephalopathy).
  2. Test-and-slaughter programs have often been used in the past for eradication of specific diseases (bovine brucellosis and tuberculosis in Australia). In this case animals are tested and only those that are deemed to be disease positive are then slaughtered.
  3. Herd depopulation may be used in extreme situations or for problem herds where eradication using other methods has failed (for example, foot-and-mouth disease, bovine spongiform encephalopathy in the UK; problem herds for bovine tuberculosis and brucellosis late in the Australian brucellosis and tuberculosis eradication programs)
  4. Slaughter or early culling of individual animals may also be used in non-emergency situations as part of a voluntary or regulatory control program for some diseases (for example footrot or ovine brucellosis in sheep, chronic mastitis in dairy cows).

Animal treatments

Where available, treatments (either therapeutic or preventive) can be used to treat infected or exposed animals and reduce prevalence. For example, antibiotic preparations can be used to treat mastitis cases and teat disinfection preparations can be used to prevent new infections occurring.

Increasing resistance of susceptible hosts

Vaccination

Vaccination is an important tool for the control and eradication of many diseases, and can be used in two main ways:

  • Routine animal management where commercially available vaccines may be used as part of routine on-farm disease control for diseases such as clostridial diseases, leptospirosis, vibriosis, Marek’s disease, etc.
  • Prevalence reduction where specific vaccines may be used either on individual farms or at a regional level to reduce the prevalence of disease as part of a regional control or eradication program, by increasing the level of herd immunity. This can be used solely for control purposes, or as a prelude to eradication, with eradication attempts only proceeding subject to reducing prevalence of infection to an acceptable target level. For example, a key aspect of the brucellosis eradication program in Australia was the use of Strain 19 vaccine to reduce prevalence in high-prevalence regions before eradication commenced.

Progress of a disease in a population is affected by herd-immunity effects. Herd immunity effects appear when a meaningful proportion of the population is immune to a disease either from innate immunity (although this may not always have an immunological basis), natural infection or vaccination.

Herd immunity will slow the rate of transmission of a disease within a population, with the magnitude of the effect depending on the level of herd immunity. If herd immunity is high, infection may fail to establish or can be eliminated from the population. It is not necessary for all individuals in a group to be immune to eliminate infection. The level of herd immunity (proportion of immune animals in the population) must simply be sustained at a level which exceeds a critical threshold value at which the contact rate between infectious and susceptible individuals is insufficient to sustain the epidemic. This means that if a minimum critical proportion of animals can be kept immune to infection, a disease can be eliminated from the population. For many infectious diseases, effective vaccination rates of 70-80% provide sufficient herd immunity to prevent an epidemic being sustained.

Genetic manipulation

Many diseases have some level of genetic resistance or susceptibility. For these diseases it may be possible to breed for resistance to infection (for example internal parasites in sheep). However, any such breeding program is likely to be long-term, and must consider competing priorities for selection on production traits.

Reducing contact between infectious and susceptible hosts

Quarantine

Quarantine is the physical isolation of infected or potentially infected animals to prevent further spread of infection. Quarantine can be applied to farms that are known or suspected to be infected to prevent spread of infection to other farms. It can also be applied within farms to prevent spread between infected and uninfected groups of animals, or to isolate introduced animals until the farmer can be confident that they are disease free. Occasionally groups of farms may also be quarantined, particularly if they are potentially exposed to a highly infectious disease.

Movement controls

In a similar way to quarantine of infected farms, regional or inter-property movement controls can be used to reduce the risk of spread of infection from areas of high prevalence to areas of lower prevalence. These movement controls can be supported by official disease “zones” and regulatory requirements for movements between zones, or by a less regulated approach and voluntary implementation of recommended movement controls to minimise disease spread by farmers.

If a regulatory program is implemented it is appropriate not only to have regulatory support for movement controls (including quarantine), but also the willingness and resources to enforce the regulations. Under such programs it may be necessary to have regulatory staff available to maintain movement check points, check movement documentation, carry out saleyard inspections and enforce other regulations, as appropriate. However, regulation does not necessarily mean that the program will be complied with. In fact, a voluntary program with effective education and ownership of the program by farmers may be more effective than an unpopular regulatory approach.

In a less-regulated or voluntary program, it is still important to know the level of farmer compliance with recommended control measures. Therefore, even in completely voluntary programs it is essential to monitor or audit compliance rates against targets on a regular basis.

If farmer compliance is poor, the program is unlikely to succeed and progress and future options should be urgently reviewed.

Vector control

For vector-borne diseases, control measures may be more easily directed at the vector than at the actual disease agent. For example, effective control of tick fever in cattle in many parts of Australia is achieved mainly by controlling its cattle-tick vector. Similarly, effective long-term control of liver fluke in sheep and cattle can be achieved by either eliminating the snail vector or restricting access of stock to the snail’s habitat area. Vector control also should include consideration of mechanical vectors such as syringes/needles, which can be important vectors for some diseases such as enzootic bovine leucosis or caprine arthritis-encephalitis virus.

Management measures

Grazing or animal management

For some diseases, grazing management strategies can be used to reduce exposure of susceptible animals to contamination. For example, many internal parasite control programs are based on grazing susceptible young animals on pastures that have previously been grazed by low-risk older animals. Similar strategies have been tried for control of Johne’s disease, although low-risk animals may be difficult to identify, and may be a different group to animals that are low-risk for parasites.

Many diseases are also affected by factors under the control of the farm manager, such as housing, nutrition, stocking rates, feeding practices, etc. For these diseases, effective control can often be achieved by changing management practices or housing to reduce the transmission or impact of the disease. For example, inadequate ventilation is an important contributor to respiratory disease in pigs, so that severe respiratory disease problems can often be overcome by improving shed ventilation. Similarly, bovine Johne’s disease transmission relies on ingestion of contaminated faecal material by susceptible calves, so that the incidence of Johne’s disease in dairy cattle can be reduced by changing management to minimise the exposure of young calves to adult faecal contamination.

Biosecurity

Biosecurity measures complement other control measures and generally involve two quite separate components, bioexclusion, aimed at keeping diseases out and biocontainment, aimed at preventing onward transmission from infected herds or flocks.

Bioexclusion is the implementation of measures to prevent the introduction of unwanted pathogens into a livestock (or other) population.

Biocontainment is the implementation of measures to prevent the onward transmission of unwanted pathogens from a (potentially) infected livestock (or other) population.

Bioexclusion measures are focussed on disease-free farms, and are made up of a range of measures designed to keep disease out. These can include isolation of introduced stock, only sourcing introductions from farms with a specified level of testing or assurance, disinfection of equipment and clothes/boots coming onto the farm, management of boundary fences and contact with neighbouring stock, vaccination, testing of introductions and any other measures designed to keep disease out or for early detection and response to disease introduction.

Conversely, biocontainment measures are aimed at control of disease on infected farms to reduce prevalence and other measures to reduce the likelihood of onward transmission. Although quarantine is one important biocontainment measure, biocontainment is broader than just quarantine and includes a range of other measures, including many of the same activities as for bioexclusion. Specific additional measures include vaccination, culling or treatment of affected animals, selling animals for slaughter only, testing animals prior to sale, disinfection of people and equipment leaving the farm, maintenance of boundary fences, etc.

Disinfection

For highly infectious diseases such as foot-and-mouth disease, disinfection of premises and potential fomites (including veterinary equipment) is an essential component of any control or eradication program. Disinfection can also be an important part of on-farm biosecurity programs to keep farms free of disease.

Supporting activities

Communication, education and training

Support of producers and the general public for the program and compliance of producers with program requirements are essential requirements for a program’s success. Without an effective communication and education program, high levels of producer support and particularly of producer compliance are unlikely to be achieved. Program messages must be simple and consistent, and in many cases a substantial effort will be required to change the attitudes of farmers and their advisors to disease control and also their actions in managing disease risk. Education and training are also critical elements, to inform and educate producers and advisers about technical aspects of the disease and the program.

This is increasingly important with the shift from regulatory to voluntary programs, so that farmers are being asked to voluntarily change their practices to reduce disease risk, possibly at a significant short-term cost to themselves.

Risk assessment

Traditional disease control programs have relied on regulatory management of quarantine and movement controls to limit the spread of disease, with the underlying assumption that the measures imposed would be effective. Movement controls were generally based on a perceived “no-risk” approach to prevent spread of infection.

With the move towards more voluntary programs, and the recognition that there is no such thing as a “no-risk” policy, risk assessment has become an important aspect of any control or eradication program. A risk assessment approach makes a thorough understanding of the epidemiology of the disease much more important, so that the true risk associated with various options can be properly evaluated and communicated.

It is also important to note that in risk analysis terminology, “risk” includes elements of both likelihood of occurrence of an event and the expected consequences, should it occur. This is in contrast to the epidemiological definition of risk, which relates to likelihood of occurrence only.

The increasing move to a risk-based approach and voluntary control programs has been developing has coincided with an environment of decreasing government expenditure on disease control, placing increased reliance on the livestock industries to fund and manage programs with fewer government inputs.

Economic analysis

Just as a cost-benefit analysis is essential in determining whether or not a program is worthwhile in the first place, it is also essential that any program is subject to ongoing economic analyses. Such analyses should be directed at determining if the achievement of the program objectives is still economic, as well as determining which are the most economic and cost-effective of a range of potential control options.

Animal Identification

Identification of individual animals to their property of origin (and even their property of birth) is an essential component of an effective surveillance program for the detection of infected herds and flocks.

For example, abattoir inspection of adult sheep is an important part of surveillance for ovine Johne’s disease in Australia. Australia’s flock identification system allows rapid tracing of the origin of sheep that are inspected and found to be either positive or negative, so that an inspection history can be built up for each flock and region over time, providing better levels of assurance for low-risk flocks and areas and allowing estimation and monitoring of flock-prevalence on an area basis.

Many countries now have mandatory cattle identification and passport systems in place to support traceability of animals and product in the wake of the bovine spongiform encephalopathy outbreak.

Identification of animals to the property of origin is important both at the abattoir, and for sales between properties, to support rapid tracing of animal movements in cases of emergency disease outbreaks, such as for foot-and-mouth disease or bovine spongiform encephalopathy or for chemical residue incidents.

Permanent individual identification of animals on farms is also an important and useful tool in any program that depends on animal testing or examination. Unique animal identification allows animals requiring further action (such as culling or treatment) to be easily identified for such action as may be required.


Pre-requisites for a successful program

Before embarking on a potentially difficult, costly and often controversial disease control or eradication program, it is essential to evaluate the proposed program in terms of its technical feasibility and likelihood of success.

The critical elements required for a successful disease control or eradication program are summarised below (adapted from Yekutiel, 1981 and Thrusfield, 2005). Although it may be possible to successfully control or eradicate a disease without meeting all of the criteria listed, the likelihood of failure increases as more criteria remain unfulfilled.

  1. Adequate knowledge about the cause of the disease and its epidemiology

Knowledge of the cause (at least in epidemiological terms) and the epidemiology of a disease is essential for the development of effective strategies for the prevention of transmission and spread of the disease and for the application of screening tests to detect cases.

  1. Adequate veterinary infrastructure and resources, including administrative and operational personnel

Adequate infrastructure and veterinary staff are essential for the effective implementation of a program. Inadequate staffing of the program is likely to result in failures in the application of the selected control measures and significant delays in meeting program objectives. Important components of the infrastructure required for a successful program include:

    • field veterinary staff;
    • lay staff to assist with field activities;
    • administrative staff to manage the program; and maintain databases and reporting capability;
    • regulatory staff to implement and enforce legislative support measures;
    • diagnostic facilities and staff; and
    • research facilities and staff.
  1. Accurate, reliable and economic diagnostic tests

Reliable and cost-effective tests that have been adequately characterised for sensitivity and specificity are essential for the identification of infected animals and herds or flocks, for appropriate follow-up action. Reliable tests are also required for herd/flock classification and identification of low-risk replacement stock. A good understanding of test sensitivity and specificity and factors that may affect these characteristics is also required for the development of appropriate testing and surveillance strategies.

  1. Epidemiological features which facilitate case detection and effective surveillance

Diseases that are mainly sub-clinical or for which diagnostic tests have a poor sensitivity are likely to be difficult and expensive to detect, making the reliable identification of cases and implementation of control measures difficult. Diseases which can be detected through screening of routinely available samples or by simple testing at the herd/flock level (for example abattoir screening, bulk milk samples) are more suited to an effective program than diseases which require on-farm testing of large numbers of individual animals for the identification of infected individuals and/or herds/flocks.

  1. Control measures that are simple to apply, relatively inexpensive and highly effective at preventing transmission of infection

Any control or eradication program depends on the implementation of one or more control measures to interrupt transmission and reduce prevalence. While it is possible to control and even eradicate diseases with imperfect tools (for example brucellosis, TB), the more effective the measures are, the more likely a program is to succeed. The less that is known about disease transmission and on-farm control measures, or the harder it is to control on-farm, the more difficult it will be to control the disease on a regional or national level. Measures must also be effective at preventing spread between farms, as well as at reducing or preventing transmission on infected farms.

Formal risk assessment should be completed of the risk (probability of disease event and consequence) of disease occurrence and spread under various scenarios (no control and under each control or eradication option).

  1. A reliable source of sufficient numbers and quality of disease-free replacement stock for those destroyed or culled during the campaign

Any program requiring slaughter or compulsory culling of infected stock is heavily dependent on a source of disease-free replacements. If disease prevalence is high, this becomes more difficult. Also, if available tests have a poor sensitivity it may be difficult to reliably identify low-risk animals or populations as a source of replacements.

  1. Support for the program amongst producers and the general public, and cooperation by producers with the requirements of the program

If there is not a high level of commitment to the program among producers it is likely to be affected by criticism, unrest and even active resistance, hampering implementation and potentially undermining the effectiveness of the program. This is even more important for voluntary programs, where farmer education, support and compliance are critical for program success.

  1. Appropriate justification for eradication or control, supported by independent cost-benefit analysis

Without a clear and well-argued rationale for eradication or control, any program is likely to lack the support of producers, industry leaders and governments. The most common reasons for eradication or control have been discussed previously, but include public health effects or the cost of the disease to the industry or community. If eradication is proposed, there also must be a valid reason for recommending eradication rather than control.

For a program to be supported, a social cost-benefit analysis will generally be required, demonstrating that the program is economically justifiable and that the expected returns (in terms of savings in cost of disease or productivity losses) exceed the cost of the program over the longer term.

  1. Supporting legislation to enable the program to proceed, including provision for compensation

Appropriate legislation is required to implement movement controls, compulsory slaughter, compensation and other measures included in regulatory type programs. However, even in voluntary programs, some level of legislative backing may be required to provide a legal basis for area declarations and movement restrictions and for enforcement of program requirements.

  1. The ecological consequences of the program must be assessed and addressed

There is increasing public concern over environmental and ecological issues, such that they must now be an important consideration in any animal health program. If the proposed program is likely to have adverse environmental or ecological effects it is unlikely to be supported by governments or the general public. However, programs that have a positive impact on the environment (for example by reducing the feral animal population) are likely to be well-supported.

  1. Adequate funding committed to the program

Without adequate funding, any animal health program is doomed to failure. In the current economic climate, governments are reluctant to commit large amounts of public money unless there is a positive return on their investment and an obvious public benefit from the program. Where the livestock industries are the major beneficiaries of disease control, they are also expected to be the major funders in some countries. A requirement for industry contribution also raises the issue of how to collect money from producers at a State or regional level, usually through some form of levy at sale or slaughter.

Application of control measures based on infection status

One way to think about control and eradication measures is to consider measures that may be applied based on the infection status of an individual farm or village (Toma et al 1999).

Control measures applied to an infected premise

In Australia the most common unit at which these measures would be applied is the farm. A farm is generally one enterprise at a single location though it may cover a large area and have large numbers of animals of multiple species. In other areas of the world it may be more logical to think of a village or some other unit. A unit is likely to be a relatively small area where animals can co-mingle during feeding or management.

For contagious diseases the critical first step in control is to implement quarantine measures usually accompanied by restriction of movement of animals into and out of the premise. In many cases all movement of animals and animal products and other related material (animal feeds, equipment, etc) may be stopped and even movement of people onto and off infected premises may be carefully controlled.

A second critical step for contagious diseases is generally to slaughter all susceptible animals on the infected premise and dispose of them in a way that eliminates infectious risk (burial or burning) along with any other infectious material such as bedding or other material. For diseases that are not highly contagious, it may be appropriate to test animals and only slaughter those that are known to be infected. Alternatively animals may be able to be sent for processing but not transferred to any other property.

These measures are accompanied by disinfection of the premise to minimise the probability of infectious agent surviving in the environment and application of biosecurity measures to reduce the risk of disease inadvertently being carried off the premise. This may include disinfection and changes of clothes and vehicles at entry and exit points to/from the property, control of movement of people and other things into and off the property.

Trace-forward and trace-back should be used to identify any high risk contacts of movements onto the property in the period before the disease was detected and off the property. Other properties identified through these procedures should be visited and examined to look for any evidence of infection. Knowing the incubation period of the disease, the likely date when the premise was infected and dates of movements onto or off the farm can all be used to identify windows of interest for tracing. If the date when the premise was likely to have been infected can be identified then all movements off the farm from that time up until quarantine is imposed should be checked.

Once infection has been eliminated from the premise, there is then usually a period when the premise may be left without any livestock. This is to help ensure that once animals are permitted back onto the farm (restocking) that they do not get infected from infectious agent that may have survived for some time in the environment. The period of time will depend on the longevity of the infectious agent in the environment.

If eradication is an aim of the response policy, then it will be necessary to demonstrate that eradication has been achieved before the area or country can declare itself free of the disease. Proof of freedom from disease is an important function of surveillance and will be discussed in more detail in a separate manual.

Control measures that may be applied to a disease free premise

A disease free farm can be considered like a remote and isolated castle with a moat around it. The principle is that measures have to be adopted that prevent disease from getting in.

Animals and any product capable of carrying infection should only be allowed to enter the premise if they can be confidently declared to be disease free. This may be difficult to achieve. It may be safest to not allow any animals to enter. If animals must enter then they should be subjected to sufficient testing and examination to be confident that they are not infectious.

Strict biosecurity measures should be imposed including restriction on visitors and disinfection and other measures (change of clothes, vehicle etc) at controlled entry and exit points.

If the disease is able to be spread through the air or via water (streams or overland flow), or wild animals, insects, birds, etc then each of these risks will need to be assessed and measures applied to mitigate risks. It may be useful to move animals away from the boundary of the farm and develop a destocked barrier region along the boundary. Additional control measures for rodents or other wild animals may be appropriate.

In many cases and particularly where the prevalence of infection around a premise may be high, even if all applicable control measures are implemented, the farm may become infected.

Example: Rabies in Bali

When the current rabies outbreak in Bali began in 2008, the island had no policies for rabies post-exposure prophylaxis (PEP), no dog bite surveillance, no rabies diagnostic facilities and no vaccination program for dogs. In subsequent years the Indonesian government provided PEP for humans and vaccines for dogs, diagnostic testing was established at the Disease Investigation Centre in Denpasar and surveillance was implemented of dogs that died or were killed either as part of culling programs or that were showing neurological signs. Culling of unconfined dogs was instituted in some areas but was found to be counter-productive because people reacted strongly by hiding or moving dogs to avoid culls and replacing dogs that had been culled.

While most dogs are owned, many are unconfined and this combined with the need for frequent booster vaccination using the locally produced vaccine meant that there were serious problems in achieving effective vaccine coverage. Long-lasting vaccines were supplied from about 2009 and by mid 2011 about 250,000 dogs had been vaccinated (coverage >70%). A second island wide vaccination program was completed by late 2011.

Mass vaccination has been shown to be effective at reducing rabies incidence in dogs, reducing human exposures and reducing cases of rabies in people. Mass vaccination must be continued along with effective surveillance and control of inter-island movement of dogs if rabies control is to be maintained and spread of the disease to other islands is to be prevented.

A recent publication (Townsend et al 2013) applied the use of mathematical modelling to the question of whether mass vaccination might be able to successfully eradicate rabies form the island of Bali.

The authors concluded that a single mass vaccination program that achieved 60% or lower coverage of the dog population had no chance of successful eradication while two campaigns of 80% coverage or three campaigns of 60% coverage was predicted to achieve eradication in 90% of model runs.

A related publication (Agung Gde Putra et al 2013) indicated that there has now been two mass vaccination programs that have achieved ~70% coverage rates of dogs but that rabies continues to be present and dog bites of people (exposures) continue to be high (more than 4,000 per month). Townsend et al (2013) suggested that although the overall coverage in the first mass vaccination program was as high as 70% the long time taken to complete the program meant that the average island wide coverage was probably closer to 40% due to ongoing animal turnover and waning immunity.

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Figure 5.: Rabies incidence on Bali prior to island wide mass vaccination. From Townsend et al (2013)

Townsend et al (2013) suggest that eradication is achievable with a third and possibly further mass vaccination campaigns, but that ongoing control and surveillance will be critical to prevent re-introduction of rabies in the future even if eradication is achieved. Eradication would save about 55 human deaths per year while requiring ongoing surveillance and control.

Continued control without eradication was predicted to save about 44 human deaths per year (some deaths would continue) and would require ongoing expenditure on mass vaccination programs and human PEP.


Designing an appropriate animal health program

The challenge for designing an animal health program is to bring together the most cost-effective mix of tools to achieve the desired goal.

Key issues in planning and designing an appropriate regional animal health program include:

  • What is the current situation (how common is the disease, what inputs and tools are available, etc)?
  • What is the desired situation?
  • Is a regional program the right approach?
  • Is a regional program feasible and likely to be successful?
  • Is the proposed program likely to be a voluntary or regulatory type of program?
  • What control tools are available for use in the program that are likely to be effective for the disease of concern?
  • What level of resourcing is available for implementing the program?
  • Is the proposed program feasible and likely to be successful?
  • Who are the main beneficiaries of the program?
  • How will the program be funded?
  • How will the program be managed?

In most cases, any program will be made up of a one or more of the various strategies discussed above. Once the appropriate programs and strategies have been identified, and the ways in which they will be applied have been determined, detailed business and operational plans for the program should be developed.

The Program plan describes the overall management and operations of the program and should:

  • Define the overarching goals or aims of the program;
  • Identify specific objectives against which progress can be measured and reported;
  • Provide a detailed description of how the program will be managed
  • Define roles and responsibilities for participating organisations and key personnel;
  • 3333Include a detailed budget and funding sources for the program;
  • Identify supporting legislation and regulatory powers required or available to support the program
  • Identify the resources required for implementation and where these resources will come from;
  • Define timelines, targets and monitoring processes to evaluate progress of the program; and
  • Provide decision points and criteria for key decisions as to whether to continue, modify or abandon the program.

In some cases a program plan may be split into a business plan that covers broad goals, management, responsibilities and funding; and a separate operational plan (often reviewed annually) which provides the specific details of targets, resourcing and day-to-day operational activities of the program.

Monitoring program performance

The success of animal health programs is highly variable, depending mainly on the factors outlined previously. However, if program performance is not monitored and regularly reviewed, stakeholders will not know whether it is succeeding or not. Therefore, ongoing monitoring of program performance and review of achievements against targets and objectives is essential for any animal health program.

It is also important that performance is monitored against both financial and animal health objectives. A program can be operating very efficiently on a financial basis, and remain well within budget, but fail to achieve any of its animal health objectives, and vice versa, either of which represents significant failure of the program.

As part of the planning process, milestones should be set, at which progress can be reviewed against targets. Failure to meet targets at a review point should trigger a response to identify why targets are not being met, and to implement measures to correct any deficiencies. In some cases the program business or operational plans and budgets may need review and refinement, or in severe cases a major overhaul of the program may be required.


Economics of animal disease control

Animal disease has a range of potentially adverse effects that can be presented in economic terms. Economics uses monetary units (dollars) to inform rational decision making about allocation of scarce resources between competing options. Economics is generally focused on the use of resources (inputs) that in turn produce goods (outputs). Outputs then produce some form of human benefit through a market (products used or purchased by consumers).

Economic analyses can be complex and difficult to understand, reflecting the complexity of animal production systems and the difficulty in describing and valuing the potential impacts at both farm and national levels.

Data requirements for economic evaluation

In order to perform economic analyses to compare options for disease control or eradication, it is often necessary to collect an extensive set of data and information including (Rushton et al 2012):

  • The livestock production system or systems (if there are multiple separate systems) in sufficient detail to be able to describe production with and without disease. This will usually require some form of model to simulate the production system (number of females bred, proportion that get pregnant, number of calves born, annual loss rates, growth, turn off etc).
  • The occurrence of disease and the effects of disease on the livestock production system (mortality, morbidity, production effects) and on factors outside the production system.
  • Possible control measures including their effects on disease occurrence, livestock production and on market prices.
  • Details of the costs associated with implementing different control options.


  Products

(outputs)Human benefit


 Resources

(inputs)

             
                 
                 
                 
  land     animals        
  labour     meat / milk / other      
  capital   manure        
  animals     work (traction)    
  feed     other      
  other            

Figure 7.: Livestock production system pathway showing effects of disease (from Otte and Chilonda, 2001)

Animal disease has the potential to produce adverse effects at each step along the livestock production pathway. Disease effects can be group as direct or indirect losses.

  • Direct losses include:
    • Mortality of breeding or production animals
    • Reduced production efficiency eg reduction in feed conversion, fertility rate, growth rate etc. May be presented as a higher rate of inputs to sustain required outputs.
    • Reduction in product quantity (fewer offspring, less milk, less eggs, less meat or fleece etc), or reduction in product quality (poor hides because of tick damage, discarded milk because of mastitis etc)
    • Costs incurred in diagnosing and treating sick animals (veterinary fees and drugs costs).
  • Indirect losses include:
    • Additional costs for disease control measures or eradication
    • Human health costs associated with health impacts from zoonotic diseases (BSE, HPAI, Salmonellosis) or from unintended consequences of control measures (chemical residues in products)
    • Negative animal welfare impacts of disease or control measures
    • Trade restrictions due to disease and control measures
    • Loss of consumer confidence in a market sector leading to reduced demand or altered consumer behaviour
    • Range of possible negative effects such as a move towards production systems that may be resistant to disease but that are relatively inefficient or have other potentially negative effects (use of resistant genetics with reduced production efficiency).

The direct cost of disease has been defined as the sum of the production losses and expenditures that are incurred because of the disease. The components contributing to direct cost include L + R + T + P (Bennett 2003):

  • L= value of loss resulting from reduced output in the presence of disease when compared to no disease;
  • R= increase in expenditure on non-veterinary resource resulting from the presence of disease (increased labour, feed, vehicle running costs, equipment etc);
  • T= expenditure on diagnosis and treatment of disease in affected animals;
  • P= expenditure on prophylactic measures to prevent infection and disease from occurring in healthy animals.

Methods for economic evaluation

Economic analyses of disease impacts may be performed at the micro-economic level (farm or household) or at the macro-economic level (industry sector or country). There is a bewildering array of terms and methods used in economic analyses of production systems and the effects of disease on these systems.

At the micro-economic level the most common approaches involve partial budgets and gross margins analysis. At the sector or national level it is more common to see benefit-cost analysis (BCA) of some form. These terms require some brief explanation.

The term budget simply means estimation of expected income and expenses.

A whole farm or enterprise budget estimates the income (outputs) and costs (inputs) for the enterprise or farm. Input costs include both fixed and variable costs.

Fixed costs for a farm or enterprise vary only in the long run and are still incurred even if output is zero. Fixed costs usually include permanent labour including paid staff and the owner’s family, depreciation (infrastructure, vehicles, machinery, equipment), maintenance and repairs, fuel & oil costs (where they cannot easily be assigned to one enterprise), rent, interest.

Variable costs are those costs that are related directly to the amount of output produced and would decline to zero if output was set at zero. Variable costs are able to be allocated to specific enterprise activities (cattle production vs cropping for example). Variables costs include feed, veterinary inputs, seed, fertilizer, marketing costs and casual labour employed for specific jobs such as castration of calves. Vehicle running costs are generally not included in variable costs unless they can be clearly allocated to a specific enterprise. If the number of breeding cows doubles, then the variable costs associated with carrying the additional stock, such as feed costs and costs of medication (drench, vaccination) will also double.

Gross margins analysis is defined as the gross income from an enterprise less the variable costs incurred in achieving it and is generally calculated on a per-year basis. It does not include any fixed costs. The gross margin for an enterprise is the gross income minus the variable costs over a one-year period. Gross margins are generally produced in units such as $ per animal or animal equivalent or per unit of land area (hectare). A gross margin is not a profit measure because it does not include fixed costs which have to be met regardless of enterprise size. Gross margins do allow comparison of similar enterprises and allow assessment of the impacts of changes in management practices.

A partial budget means summarising just those changes in expenses and in income when some minor change is made to management or some other input in the production system (using a new feed supplement or vaccinating/drenching animals). Partial budgets generally consider four components:

Table 7.: Table showing components of a partial budget for assessing economic impacts of a minor change in management practice(s).


Costs Benefits
new costs costs saved
income lost new income

Partial budgets are relatively simplistic and may not represent all the factors that might be relevant in a decision about investing in some change in management practices.

Table 7.: Table showing approach to partial budget estimation


Change Amount Unit price Gains Costs
New feed supplement S kg $s /kg S*$s
Additional feed required F kg $f /kg F*$f
Additional labour D days $d /day D*$d
Additional weight gain of cattle W kg $w /kg W*$w
Additional manure sold M kg $m /kg M*$m
Partial budget [W*$w + M*$m] - [S*$s + F*$f + D*$d]

 

The example above shows a simple partial budget that attempts to assess the impact of a new feed supplement on weight gain and manure output in a cattle production enterprise. If the benefits are larger than the costs then the management change may be considered worthwhile on economic grounds.


[[Image:]] [[Image:]]

Figure 7.: Production function showing budget impacts before and after a change in management practices. The left side shows measures or total benefits and costs and the right side shows marginal benefits and costs (Rushton 2009).


Often changes intended to control or eradicate livestock disease and benefit animal production measures may take years to fully implement. In addition costs may be higher in the beginning and then reduce over time and benefits may be lower in the beginning and slowly rise over time. It is very difficult to assess the impacts of these sorts of changes using partial budgets or gross margins analysis alone, mainly because of the time change in the value of money. One dollar earnt (or spent) now is not the same as one dollar earnt or spent in five years time, mainly because of the effects of things like inflation and discounting.

Often a method like gross margins analysis is extended using additional criteria that allow for the time-changing value of money. Converting future values (benefits or costs) generally involves application of a discount rate.

{\mathit  {Present}}{\mathit  {Value}}={\mathit  {PV}}={\frac  {{X}_{{t}}}{{\left(1+r\right)}^{{t}}}}

wherePV=present value

Xt= amount of money in year t

R is the discount rate expressed as a proportion (5%=0.05)

T= number of years from the present date

The discount rate is also described as the opportunity cost of money. There are many different approaches to setting the discount rate. A reasonable approach is to use the real rate of interest which can be estimated as the nominal interest rate (cost of borrowing money) minus the inflation rate. If the market interest rate was 7% and inflation was 1.5% then the real rate of interest would be 5.5%. An alternative approach is to use an estimate of the rate of return you could get if you invested the money in an alternative investment with a similar risk profile (ie investing in a bank or in a financial market).

If all current future benefits and costs are adjusted so that they are all measured in present value (PV), then it is possible to perform comparisons of different strategies that may have different patterns of benefits and costs over time. These comparisons are generally done using one of three criteria: net present value, internal rate of return or benefit-cost ratio.

The net present value (NPV) is the difference between the sum of the present value of all benefits and the sum of the present value of all costs. If the NPV is positive (present value of benefits is greater than present value of costs) then the investment is worth considering.

The internal rate of return (IRR) is defined as the discount rate that must be applied to make the NPV equal to zero. If the IRR is greater than the conventional discount rate than the project is worth considering because the findings are suggesting that the investment will provide a better return than if you had invested in an alternative investment.

A benefit-cost ratio (BCR) or benefit-cost analysis (BCA) or cost-benefit analysis (CBA) is calculated by dividing the present value of the benefits by the present value of the costs. If the ratio is greater than 1 then the benefits exceed the costs and the investment is worth considering. Benefit-cost analysis is often of most value when performed at the sector or national levels.

Rushton et al (2012) also describe the use of cost-effectiveness analysis as an application of economic evaluation that can be applied in the early stages of a disease outbreak response and used to guide decisions relating to implementation of policy to achieve the most effective result per unit of investment. The principle is based on identifying possible interventions and associated costs and effects (outcomes), in order to achieve a pre-existing policy goal. The evaluation may be presented in terms of cost per positive animal detected or cost per animal saved and the application of this approach may guide decisions about which strategy to implement to achieve a given target in the most cost-effective manner.

It is also important to recognise that decisions at any level will only partially rely on rational economic measures. Farmers may choose one option over another because of risk perception or for other personal reasons rather than solely based on estimates of economic benefit. These issues may explain why individual farmers may choose options that are not necessarily associated with the highest NPV or best BCR based on economic analyses.

Macroeconomics vs microeconomics

Microeconomics refers to estimates of costs and benefits at the farm level.

Macroeconomics refers to economic analyses conducted at an aggregated level such as across an industry sector (livestock or agriculture) or across the national economy as a whole. Macroeconomics is a very complex area particularly when the approach involves trying to determine the interactions between animal disease (including effects of control or eradication programs) on different sectors in the economy such as domestic consumption, foreign trade, tourism, biodiversity and others.

Where animal disease or control programs targeting a disease have effects that are beyond the farm gate or even beyond the livestock production system, there is a strong case for government involvement in disease control programs to better manage investment in risk management for the benefit of the entire population of the country.

Countries may develop a shared responsibility for development and implementation of animal health and welfare policies including disease control programs. At one end of the scale where livestock producers are the primary beneficiaries of any improvement in outcomes, then the producers may be expected to pay for most or all of the costs associated with the program(s). At the other end of the scale where the benefits of any outcomes may be considered to mainly involve people or areas other than the livestock producer (public health, animal welfare, environmental benefit), then there is a stronger case for having government bear some or most of the costs (Bennett 2012).

References – economics of animal disease control

Gde Putra, A.A., Hampson, K., Girardi, J., Hiby, E., Knobel, D., Wayan Mardianna, I., Townsend, S. & Scott-Orr, H. (2013). Response to a Rabies Epidemic, Bali, Indonesia, 2008-2011. Emerging Infectious Diseases, 19(4):648-651.


Bennett, R. (2003). The ‘Direct Costs’ of livestock disease: The development of a system of models for the analysis of 30 endemic livestock diseases in Great Britain. Journal of Agricultural Economics, 54(1):55-71.


Bennett, R. (2012). Economic rationale for interventions to control livestock disease. EuroChoices, 11(2):5-11.


Chilonda, P. and Van Huylenbroeck, G. (2001). A conceptual framework for the economic analysis of factors influencing decision making of small-scale farmers in animal health management. Rev. sci. tech. Off. Int. Epiz. 20(3):687-700.


Dohoo, I., Martin, W. & Stryhn, H. 2010. Veterinary Epidemiologic Research, Charlottetown, Prince Edward Island, Canada, VER Inc.


Hanson, R. P. & Hanson, M. G. 1983. Animal Disease Control, Ames, Iowa, Iowa State University Press.


Hueston, W. D. 2003. Science, politics and animal health policy: Epidemiology in action. Preventive Veterinary Medicine., 60, 3-13.


Martin, S. W., Meek, A. H. & Willeberg, P. 1987. Veterinary Epidemiology, Ames, Iowa, Iowa State University Press.


Martin, S. W., Shoukri, M. & Thorburn, M. A. 1992. Evaluating the health status of herds based on tests applied to individuals. Preventive-Veterinary-Medicine, 14:33-43.


Otte, M.J. and Chilonda, P. (2001). Animal health economics: an introduction. Livestock Information Sector Analysis and Policy Branch, Animal Production and Health Division (AGA), FAO, Rome, Italy.


Rushton, J. The Economics of Animal Health and Production (2009). Wallingford, Oxfordshire, CAB International.


Rushton, J., Raboisson, D., Velthuis, A. & Bergevoet, R. (2012). Evaluating animal health investments. EuroChoices, 11(2):50-57.


Thrusfield, M. 2005. Veterinary Epidemiology 3rd edition, Oxford, UK, Blackwell Science.


Toma, B., Dufour, B., Sanaa, M., Benet, J.J., Ellis, P., Moutou, F. & Louza, A. (1999). Applied Veterinary Epidemiology and the Control of Disease in Populations. Paris, France, AEEMA.


Townsend, S.E., Sumantra I.P., Pudjiatmoko, B.G.N., Brum E., Cleaveland S., Crafter S., Dewi A.P., Dharma D.M., Dushoff J., Girardi J., Gunata I.K., Hiby E.F., Kalalo C., Knobel D.L., Mardiana I.W., Putra A.A., Schoonman L., Scott-Orr H., Shand M., Sukanadi I.W., Suseno P.P., Haydon D.T., Hampson K. (2013). Designing Programs for Eliminating Canine Rabies from Islands: Bali, Indonesia as a Case Study. PLoS Negl Trop Dis 7(8): e2372. doi:10.1371/journal.pntd.0002372.


Yekutiel, P. 1981. Lessons from the big eradication campaigns. World Health Forum, 2:465-490.