Early reaction is to carry out without delay the disease control activities needed to contain the outbreak and then to eliminate the disease and infection in the shortest possible time frame and in the most cost-effective way, or at least to return to the status quo that existed previously and to provide objective, scientific evidence that one of these objectives has been achieved.
It is far too late to leave the planning of an emergency disease eradication or control programme to the time when a disease outbreak has actually occurred. There will then be intense political pressure and pressure from livestock farmer groups for immediate action. In such a climate mistakes will be made, resources misused, deficiencies rapidly highlighted, and there will be unavoidable delays resulting in further disease spread and higher costs-unless there has been adequate forward planning and preparation.
This chapter first highlights the importance of effective quarantine services for the prevention of exotic animal diseases. It then describes the principles and strategies of epidemic livestock disease control and eradication that need to be taken into account in the preparation of early reaction contingency plans.
The old maxim that “prevention is better than cure” is particularly relevant to exotic animal diseases. Quarantine should be regarded as one of the most important core functions of government veterinary services. Transboundary and other exotic animal diseases can be introduced to countries in many ways. These include entry of infected animals or germplasm (semen or ova), entry of contaminated animal products or biological products (e.g.vaccines), contaminated food waste from aircraft or ships, infected people (in the case of disease transmittable to animals), migrating animals and birds, or even by natural spread of insect vectors or by wind currents. While governments may be powerless to prevent some of the latter methods of disease introduction, the others can be considerably mitigated by efficient quarantine services.
Quarantine programmes should include the following:
A number of basic approaches may be used to control and eliminate epidemic livestock diseases. They are usually used in combination. The weighting that is given to the different approaches will be determined by the nature of the disease in question, the epidemiological circumstances and their acceptability and cost. The approaches to be used are summarized below.
Denial of access of the disease agent to susceptible host animals
This may be achieved by:
Swill feeding - a common source of transmission of animal diseases
While the other approaches to be described could be considered as subsets of denying access of the disease agent to susceptible hosts, they are conveniently considered separately.
Avoiding contact between infected and susceptible animals
This is one of the most important approaches and may be achieved by:
Removing infected and potentially infected animals
This is often referred to as an eradication policy. Susceptible species on infected farms or in designated infected areas are immediately slaughtered on site and their carcasses disposed of safely, usually by burial or burning. It is often combined with cleaning and disinfection procedures for the infected premises. Because of the rapid spread of epidemic diseases, all susceptible animals are slaughtered, whether obviously infected or not. For some infectious disease control programmes, such as for brucellosis and tuberculosis, it is possible only to slaughter animals that have been tested positive, but this is not appropriate for rapidly contagious epidemic diseases.
A component of an eradication policy may also be selective reduction of susceptible wild and/or feral animal populations in infected areas, but before embarking on such a programme a careful evaluation should be made.
Reducing the number of susceptible animals
This is an important approach used in many countries. In emergency disease control it is usually achieved by vaccination of susceptible animals. Vaccination may be done selectively (for example as “ring vaccination” around infected areas) or as “blanket” vaccination programmes in susceptible animal populations. Depending on the nature of the disease and of available vaccines, it may be possible to eliminate infection completely. More usually vaccination is used to reduce the level of infection in animal populations to an acceptably low level where other disease elimination policies are more feasible. In fact, in some cases routine vaccination may mask underlying infection in animal populations.
Reducing access of vectors to susceptible animals
This may be appropriate for insect-borne diseases and, in some cases, may be achieved by reducing vector numbers in an area by treatment and/or elimination of potential breeding sites. Large-scale insecticide spraying is generally too costly, ineffective in the long term, and/or environmentally unacceptable. Other approaches might be to treat susceptible animals with longacting insecticides during critical periods or remove animals from high-activity insect vector areas either continuously or during times of the day or year when insect vectors are most active.
To date, there has been only one emergency disease situation for which biological control has proved effective. This has been for the New World screwworm fly (Cochliomyia hominovorax) in the Americas and North Africa using the sterile insect release method (SIRM). SIRM techniques are also currently under evaluation for the Old World screwworm fly (Chrysomia bezziana).
Containment of an outbreak of an epidemic disease is the first priority. Stabilizing the situation is the prelude to eradication.
In order to contain the outbreak, one must be able to determine where the disease is - which farms or areas are infected and which are free. This means that all the active disease surveillance procedures discussed in Chapter 4 should be put immediately into effect. There needs to be an intensive search for new foci of infection for the disease, with priority given to:
As can be appreciated, the task of following up tracebacks and traceforwards and other epidemiological leads becomes very complicated if, for example, suspect animals have been through livestock markets. This points to the need for countries to have in place livestock identification mechanisms or at least effective “paper trails” (e.g. movement permits) for animals that have been sold or moved. As new foci of infection are identified, starting from where the disease was first detected, appropriate disease control actions must be put into place immediately and strictly enforced to prevent further spread of the disease from these foci. In most cases this will involve quarantining the infected farm or area and placing bans or restrictions on the movement of susceptible species animals and dangerous animal products or other materials in surrounding zones. The disease control/eradication strategies selected for the particular disease (e.g. eradication or ring vaccination) are then carried out.
The proclamation of geographic areas in which specific disease control strategies are to be carried out is known as “zoning”. Zoning almost always takes place in the form of concentric “circles” around known or suspected foci of infection, with the most intensive disease control activities in the inner zones. The actual size and shape of the zones may be determined by administrative boundaries or geographic barriers or be driven by epidemiological or resource imperatives. The nature of the disease control zones and the activities carried out in each zone are dependent on the particular disease control/eradication strategy selected. These are described in the next sections.
For disease control zones to have the desired effect, they must be made well known to the local farmers
Finally, disease-free zones or regions of the country may be declared. In these, the emphasis of surveillance shifts from detecting infection to proving freedom from infection. More emphasis should thus be given to such techniques as serosurveillance. In the early stages of a disease eradication campaign, while the extent of the disease is still being assessed, it could be expected that the disease control zones are comparatively large and the disease-free zones comparatively small. As the disease control campaign progresses, it is to be hoped that the situation would reverse with the ultimate aim of the whole country being declared disease free.
Zoning is now recognized as an important principle in the definition of the animal health status of countries by OIE.
Stamping out by slaughter of affected herds or flocks
This is usually the most efficient method for the rapid elimination of an introduced exotic or other emergency disease. It is also often the most cost-effective. Not only is the disease eradication campaign shorter and achieved for a lower overall cost, but there is a much shorter waiting period before the country can be recognized as being free of the disease and the export of livestock and animal products resumed.
Several social, economic and other factors need to be carefully evaluated before eradication is selected as the desired strategy for any specific disease contingency plan. These factors include:
In an eradication campaign, activities carried out in designated disease control zones are described below.
Infected premises. It is here that the disease has actually been detected and includes all areas where there are susceptible animals that could have become infected through contact with the diseased animals. The premises may be a single farm, household or herd/flock, but could also be an entire village, settlement, common grazing land or even livestock saleyards. Activities to be undertaken are itemized below.
Dangerous contact premises. These are premises where overt disease has not yet appeared, but for which epidemiological investigations indicate that there is a high likelihood that infection has been introduced. This circumstance might occur with an immediate neighbour to infected premises that have introduced animals from infected premises during the critical period for transfer of infection. A worst case scenario of a highly contagious disease being detected in a livestock market may lead to many dangerous contact premises.
These premises are put under the same tight quarantine as infected premises and are subject to intense surveillance (at least daily). Provided there is no evidence of infection, they may be released from quarantine after a period equivalent to at least two quarantine periods for the disease.
In certain circumstances a decision may be taken to slaughter animals from dangerous contact premises.
Infected zone. This is the area immediately surrounding infected premises. While its size and shape are influenced by topographical features, physical barriers, administrative borders and epidemiological considerations, OIE recommends that it should be at least a 10-km radius around a disease centre in areas with intense livestock raising and 50 km in areas where extensive livestock raising is practised. Activities to be undertaken are itemized below.
The infected zone should be left in place for as long as can be reasonably expected, on the basis of epidemiological evaluations, that infection may still be present. However, there is a risk in maintaining restrictions for too long as resentment may build up in the community, with a resulting reluctance to maintain the livestock movement bans and other restrictions.
Surveillance (or control zone). This zone is much larger and surrounds one or more infected zones. It may cover a whole province or administrative region (or clan or tribal area). Activities undertaken are described below.
Vaccination supplemented by other disease control measures
Well-planned, comprehensive vaccination programmes, supplemented by other disease control measures, can go a long way towards eliminating many epidemic livestock diseases. This may be the strategy of choice in areas where large-scale eradication is unacceptable for one reason or another.
There are a number of important issues to be evaluated before selecting a vaccination strategy. These issues are described below.
Vaccine type. Different types of vaccine may be available and their comparative advantages and disadvantages should be evaluated. Live attenuated vaccines generally provide a more durable immunity and require fewer doses. However, assurances need to be obtained that the vaccine has been thoroughly tested on the types of animals for which it is to be used and it has been found to be safe and free of potential problems with teratogenicity if administered to pregnant animals, reversion to virulence or reassortment/recombination with field strains. Some live vaccines (e.g. oral Newcastle disease vaccine) can be administered in ways that involve little or no handling of animals. Inactivated (killed) vaccines should be safe in all circumstances. However, they often require two doses in a primary immunization course, together with periodic booster doses. Several new-generation genetically engineered vaccines show great promise, but few have yet come to commercial reality.
For epidemic livestock diseases such as FMD for which the causal agent exhibits antigenic variation, it is important to select the correct antigenic type and subtype vaccine in order to achieve good levels of immunity. Field isolates of the agent should therefore be regularly collected from different parts of the country and submitted to a world or regional reference laboratory for antigenic characterization. The most appropriate vaccine strain(s) can then be chosen.
Vaccine quality. There have been several well-documented disasters where vaccines have actually caused the diseases that they have meant to prevent, often in previously free areas. This has happened because killed vaccines have been improperly inactivated and because both live and killed vaccines have been contaminated with virulent virus, perhaps through cross-contamination with challenge virus cultures in the same laboratory. Just as serious has been the use of ineffective vaccines, which have either lost their potency or perhaps were never potent even when they left the manufacturer. Not only does this cause waste of money and scarce resources, but also leads to a false sense of security.
Vaccines should always be sourced from highly reputable manufacturers who follow internationally accepted quality assurance procedures and codes of good manufacturing practice. The manufacturers should be subject to approval and quality control verification by independent national or international biological control authorities.
Vaccination cover. The aim in vaccinating a population of animals is not only to protect the animals that are actually immunized, but also to cut down the rate of transmission of the pathogen in the target population to a level where infection is no longer sustained in that population. The latter is often referred to as herd immunity and a 70 percent vaccine coverage quoted as the figure to achieve this, but in many cases the justification is somewhat vague. In fact, in some cases, including FMD, it has been shown that a higher vaccination cover is required to achieve really good herd immunity.
Vaccine protection. The ideal vaccine not only protects animals from the clinical disease if they are subjected to challenge by the disease agent in the field, but also prevents infection and virus growth. Not all vaccines match this ideal and a proportion of animals can develop a silent infection, especially in the respiratory tract after nasal aerosol challenge. Fortunately, virus multiplication is generally at a lower level than in unimmunized animals and the excreted virus is usually insufficient to establish transmission. However, in partially and suboptimally immunized populations the virus can continue to circulate within the non-vaccinated sector of the population. Thus, the impact of the disease can be reduced to a point where mortality is unremarkable against the normal background level of disease from diverse causes, particularly under extensive range management. Once vaccination ceases and the level of herd immunity falls, the disease becomes more visible. For this reason it is necessary to maintain enhanced active disease surveillance to detect any possible breakdowns until well after vaccination campaigns are stopped and freedom is confirmed.
Vaccination programmes are pivotal in the control of many emergency animal diseases, but should be carefully planned and targeted to meet a well-defined objective
Vaccine storage and application. Vaccines must be stored at the correct refrigeration temperature at all times and used before expiry dates. This means that cold chains must be maintained for vaccines up to the time of their injection. Inactivated vaccines may require more storage space, as the dose volume is generally larger than for live attenuated vaccines. Heat-stable, live vaccines, if available, reduce cold storage problems.
Too often, injection of vaccines in the field becomes a hit-or-miss affair because animals are inadequately restrained. Vaccination teams must be trained in proper techniques and equipped to restrain animals properly.
It may be possible to give more than one vaccine at the same time, either at different sites or in the same injection, thus saving resources and possibly improving the acceptability of the vaccination programme to farmers. However, manufacturers should be consulted to determine whether this practice is safe and efficacious.
Vaccination and disease surveillance Vaccination campaigns may complicate disease surveillance activities in two ways. First, if vaccination campaigns are not carried out in a comprehensive way and there is a mixture of immunized and unimmunized animals in the population, clinical surveillance may be more difficult. This is because the disease, if present, may be very unevenly distributed. Second, there are few serological tests available that can discriminate between antibodies that have been derived from vaccination or from natural infection. Therefore interpretation of the results of serological surveys may be difficult. This problem can be alleviated somewhat by having a permanent identification system for vaccinated animals, so that it is at least known whether or not an animal has been vaccinated if it gives a positive or doubtful result to a serological test.
Vaccination programmes may be used as a tool for the elimination of epidemic livestock diseases in different ways, as described below.
Ring vaccination. Ring vaccination is the rapid creation of an immune belt around an infected area and may be carried out to contain a rapidly spreading epidemic disease outbreak or in situations where the effectiveness of other methods to prevent the spread of the disease in and around infected zones, e.g. quarantine and livestock movement controls, cannot be guaranteed, or where these areas may be relatively inaccessible.
A decision to implement ring vaccination needs to be made quickly or else the size and number of infected areas may make this unmanageable. The width of the immune belt should be determined by epidemiological factors and resource availability considerations but, as a general guide, should be of the order of 20 to 50 km. Speed is of the essence and vaccination in the target ring should ideally be completed within a week or so. It is preferable to select a narrower ring for which human resources, vaccines and other resources are available for comprehensive vaccination within this time frame rather than to select a larger ring where gaps may be left in the immune belt for longer periods. The vaccination ring would then be extended later as necessary. Having selected the target area for the ring, vaccination should commence at the outer circumference and move centripetally towards the infected herds or flocks. Separate vaccination teams should be used for herds/flocks in which there is a high suspicion of infection.
Ring vaccination should be supplemented by other disease control measures including disease surveillance, livestock movement controls and, where possible, quarantine of infected premises. The movement of susceptible species animals into or out of the combined infected/ring vaccination zones should not be permitted. Livestock markets and other congregations should also be suspended in this area.
Intensive disease surveillance should be carried out within and around the infected/ring vaccination, with the greatest concentration of effort being in the area immediately surrounding the vaccine ring.
A decision could be taken to extend the vaccination ring inwards or, if necessary, to have a second outer vaccination ring.
Blanket vaccination. This involves the comprehensive vaccination of all susceptible species animals over a larger area. It may be the preferred option when the disease outbreak has become well established and there are multiple foci of infection, or when other disease control methods are impractical for one reason or another. The vaccination area should cover known and suspected infected areas together with those areas considered to be at high risk for spread of the disease.
The latter may include known livestock movement routes. It may be necessary to carry out several rounds of vaccination over a few years in the target area, until the clinical disease apparently disappears, or the incidence is at least reduced to a level where other disease control measures can be followed.
The vaccination campaign should be supplemented by heightened disease surveillance activities both inside and outside the vaccination area(s), together with publicity programmes. The movement of animals from vaccinated areas to disease-free areas should be regulated in such a way as to minimize the possibility of spread of infection.
Whichever vaccination programme is selected, the following guidelines should be followed:
Although the previous two strategies have been presented as alternatives, they are not mutually exclusive. It is quite sound to combine elements of both to suit different epidemiological or resource availability circumstances or to suit different phases of an eradication campaign.
For example, it may be decided to slaughter infected herds or flocks and then to use ring vaccination in a control zone around them, or targeted vaccination in other strategically important areas. One disadvantage is that it will complicate the interpretation of disease surveillance, particularly that of serological surveys. However, a combination of eradication and vaccination may well be selected in a number of countries or areas where there may be some doubt about the ability to maintain strict quarantine or animal movement controls or where there are inadequate resources for comprehensive disease surveillance. Vaccination may also be used to dampen down the rate of spread of an epidemic disease to the point where “stamping out” can be applied.
Strategies for dealing with special circumstances
Nomadism and transhumance. The presence of an epidemic disease in highly mobile cattle herds and sheep and goat flocks in the semi-arid lands complicates the eradication process greatly.
Nomadic and transhumant pastoralists are among the most knowledgeable of livestock farmers and they are amenable to cooperation with veterinary authorities if their confidence has been gained and they are given the opportunity to participate actively in decision-making. Many are amenable to quarantine procedures as a part of their traditional disease management practices, providing they are carried out sympathetically with full consultation. This is important because changes in climate and weather, which have profound implications for the seasonal availability of feed and water, may affect their willingness to conform to quarantine regulations. Virtually all pastoralists are now familiar with the value of vaccines in controlling major epidemic diseases.
Knowledge and mapping of traditional livestock movements are the keys to anticipating the spread of emergency animal diseases and the risk of disease introduction
Confidence building achieved largely through communication and improvements in the veterinary-farmer interface must start well in advance of any disease emergency. This is a most important and fundamental activity of animal health services. A specialist unit of veterinarians and livestock production specialists is desirable to develop and implement strategies for animal health service delivery to such communities. Livestock graziers' organizations (or similar cooperative organizations representing the interests of pastoralists) and participatory animal health programmes involving community animal health workers have an important role in building confidence and cooperation as well as in undertaking many of the actions of disease control.
Mapping of migration routes and an understanding of the factors that drive migrations are the keys to anticipating future livestock movements and managing the risk of disease introduction.
Should a disease emergency involve migratory communities, it is essential to involve community elders in decision-making and implementation of control activities from the outset.
Insecure or otherwise inaccessible areas. Relative inaccessibility of areas as a result of natural causes (climate or topography) or insecurity resulting from civil unrest present a major challenge to the successful control and elimination of epidemic diseases.
These areas often share a number of characteristics:
These characteristics have precluded the successful implementation of conventional vaccination programmes which have a "top-down" approach with predetermined targets for vaccine coverage and sero-surveillance results, a tight time schedule for predefined activities and contact with communities is primarily only through local officials. Such a model fails to accommodate the dynamics of special action areas and lacks the inherent flexibility required to work in such areas. It is now realized that approaches that use local community-based participation are more likely to succeed. The participatory-based approach to the elimination of disease and the provision of animal health services promotes decentralized, community-based and privatized delivery of vaccination and other animal health services. These should be under the general supervision of official veterinary services. To carry out a successful disease eradication programme in a special action area, a thorough understanding of the complexities of the area and positive interaction and dialogue with a substantial cross-section of the local community are required. The use of thermostable vaccines, which are less reliant on refrigeration, is preferred, if these are available.
Wildlife or feral animal involvement in epidemic livestock disease outbreaks. This situation complicates emergency disease responses. The actual role of wild or feral animals in the epidemiology of the disease should first be considered. In some diseases they may act as a reservoir for the disease and be a genuine threat for transmission of infection to domestic animals, but in others they may simply be acting as an indicator of infection that is already occurring in livestock in the area.
Reduction programmes for susceptible wild or feral animals may be possible in infected areas, but may be precluded on ecological or environmental grounds. If attempted, care must be taken to ensure that such programmes do not simply act to disperse potentially infected wildlife to new areas. Wildlife vaccination has been extremely successful in eliminating fox rabies from some regions, but as yet has very limited application in other diseases.
It may be possible to limit contact between susceptible wild and domestic animals and thereby reduce the chances of transfer of infection from one to the other. This could be done by fencing, livestock-free buffer zones or removing livestock from epidemiologically important wildlife. In the case of epidemic poultry diseases such as highly pathogenic avian influenza (HPAI) and virulent Newcastle disease, poultry sheds can be wire-netted or otherwise sealed to prevent direct access of wild birds. Steps should also be taken to prevent faecal contamination of poultry feedstuffs. In the case of HPAI, faecal contamination of water supplies by wild water-birds is an important source of infection for chickens and other domestic poultry. This may be prevented by using water from town-water or underground water supplies. Alternatively, water drawn for poultry farms from dams, lakes or rivers where water-birds congregate may be treated by chlorination to remove any HPAI virus contamination.
If none of these measures is likely to be practicable and/or successful, it will probably be necessary to mount ring or blanket vaccination programmes for livestock in those areas where infection in wildlife constitutes a continuing threat.
As has already been stated, surveillance activities should be extended to wild and feral animal populations, in collaboration with wildlife authorities.
The end game-verified freedom from infection
This is often the most critical phase of the eradication campaign and occurs when the clinical disease has apparently disappeared. If the wrong actions are taken at this stage and undetected pockets of infection are left, many of the benefits that have accrued from the eradication campaign may eventually be lost.
Governments may make one of two potentially bad decisions at this stage unless they are properly advised. The first is that they may decide that since the clinical disease has waned or disappeared, the socio-economic losses are over and the scarce financial and other resources expended might be better diverted elsewhere. If disease control activities are prematurely wound down leaving undetected infection, the disease is likely to flare up into further serious outbreaks as immunity levels in animal populations decline.
The second, at the other end of the spectrum, is that routine disease control programmes such as annual vaccinations may be maintained indefinitely because of the fear of the political consequences if vaccination is stopped and there is another outbreak. In this case there will be a lasting economic burden from the control costs.
In both cases the export trade opportunities that may flow from having an internationally recognized disease-free status will not be available.
When the clinical disease appears to have disappeared from either a region of a country or the whole country it is time to take stock of the situation and to carry out a thorough epidemiological and economic assessment of future options.
It may prove desirable to maintain strategic vaccination if there is still a high risk of a new incursion of the disease from a neighbouring country, for example. On the other hand it is often advantageous to change direction completely by stopping vaccination programmes altogether and moving to a disease search-and-destroy policy. This does not necessarily mean that fewer resources will be devoted to combating the disease in the short term. Rather, they will be directed away from routine vaccination to increased activities directed to early warning and early response. There must be a willingness to enhance active disease surveillance activities. and to maintain preparedness against the disease at a high level. In this way any disease breakdowns can be detected and eliminated quickly before they have done much harm by either a short, sharp targeted vaccination campaign or by eradication procedures. If the latter strategy is followed, it should be possible to declare provisional freedom from the disease after a suitable period following the cessation of vaccination. After further periods, declarations of freedom from the disease and finally from infection may be made to OIE. This is subject to demonstrated evidence of a high level of clinical surveillance and the carrying out of well-planned serological surveys giving negative results. At the stage where searches are being made for the last possible pockets of infection, consideration could be given to offering monetary or other forms of reward to persons reporting a clinical episode of what might be the disease in question or for actually finding the disease. However, the advantages and disadvantages should be carefully evaluated before embarking on this course.
Recommended standards for epidemiological surveillance in order to make declarations of freedom have been laid down by OIE for both rinderpest and CBPP (commonly known as the OIE "pathways"). These pathways are shown in Appendix 3.
It is of course possible to foreshorten considerably the periods for declarations of freedom to be made if a “stamping out” policy has been followed.
Much of the discussion in this chapter has been based on the presumption that an emergency disease outbreak has been detected relatively early and is still only present in one or a few separate pockets. Many countries are not in this fortunate position and have to contend with an epidemic livestock disease that has become well established in the country, and may well have been present for a number of years. In these circumstances, commencing a national disease eradication campaign that covers the whole country at once may be neither practical nor wise. The spreading of resources too thinly over too large an area may result in overall setbacks and frustrations.
It may be more effective in the long term to tackle the eradication in a step-by-step progression moving from one region to the next. In this case regions should be defined and selected on the basis that once eradication has been achieved in one region, and the campaign moves on to the next, there can be confidence that the disease will not re-enter the first region. Geographic barriers should be utilized wherever possible. In this respect, archipelago countries are fortunate in that the eradication can take place as an island-hopping campaign. Otherwise, utilization should be made of any epidemiological or livestock production and marketing patterns that tend to make an area a discrete unit in terms of disease spread.
Next is the question of prioritization - which region(s) to tackle first. There is merit in selecting the major livestock breeding areas in the country since they are often important source areas for the disease, and livestock movements (and possibly infection) tend to spread centrifugally from there. The other advantage of tackling these areas first is that, when free, they will act as a valuable source of disease-free animals for restocking other areas.
Further prioritization should also be based on an understanding of epidemiological factors and livestock production and marketing systems which influence how the disease spreads and to where. A policy could be to follow the spread of the disease, starting regional campaigns at its source and ending where it finishes. In tropical and semi-tropical countries, livestock movements and direct contact among animals are often overwhelmingly the most important method of spread of infection. Therefore a thorough understanding of livestock movement patterns and routes is often vital for effective prioritization within epidemic disease eradication campaigns.