ANIMAL VACCINATION PROGRAMME IN CASE OF DISEASE OUTBREAKS
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 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.
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:
- The purposes of the vaccination programme should be carefully defined and the programme targeted to meet the desired objectives. If the national goal is eradication on a regional or countrywide basis, vaccination should not be allowed to become merely a routine activity of government veterinary services.
- Having selected the target animal population and area, the vaccination should be carried out as comprehensively as possible, with the target as close to 100 percent vaccination cover as practicable.
- Different vaccination teams should be used for herds/flocks that are known or thought to be infected and those that are thought to be free. This is to minimize the possibility of spread of the disease.
- For the same reason, groups of animals from different herds should not be congregated
together for vaccination. - Vaccinated animals should be permanently identified as such, even if this involves
something as simple as earnotching.
Compiled & Shared by- Team, LITD (Livestock Institute of Training & Development)
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Reference-On Request.
