For viral diseases, vaccination is the only form of prevention. A proper vaccination campaign can rapidly and significantly minimize losses due to disease.
In Indonesia, after an Newcastle Disease vaccination campaign, mortality in village flocks dropped from 50 to 8 percent and the population of chickens increased from 900 to 3 500, representing a 250 percent increase (Moerad, 1987). Newcastle Disease vaccines are available in either “live” or “dead” forms:
Live vaccines are fragile and have very precise rules for use, requiring a cold chain up to the point of application to the bird.
Their effectiveness is reduced if there are residual antibodies in the chickens. This is especially important with maternal antibodies, which are retained by the newborn chick and protect it for up to ten days.
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Even a low level of maternal antibody reduces the effectiveness of gaining immunity from the vaccine.
Group vaccination can be administered in very clean drinking water in very clean drinkers, or by aerosol (in enclosed buildings). The conventional live vaccine, Hitchner B1, cannot be given in drinking water to village flocks, but can be given using the eye-drop method, which has the advantage that each bird receives its dose individually.
This has been successfully carried out in Morocco, where it led to a considerable reduction in mortality (Bell et al., 1990a). The eye-drop method should be used only if there are veterinary personnel available for training vaccinators.
Killed vaccines give good immunity but require priming with a live vaccine for best results, unless a natural infection has already served this purpose. They have been used successfully in Burkina Faso (Verger, 1986, and Ouandaogo, 1990).
Killed vaccines have two disadvantages:
- They must be administered individually by intramuscular injection, which requires some veterinary training,
- As with live vaccines given by eye-drops – the birds must be caught, a cumbersome task which cannot be avoided with the techniques presently available.
- Killed vaccines have the advantage that they do not require as rigid a cold chain as do live vaccines, and, because they have a consequently longer shelf life, they can be used in more remote locations.
They appear to be most effective in birds that have already acquired some degree of immunity from natural NDV exposure or an initial live vaccine inoculation.
2.The virus-killing chemical used in its preparation also acts against all other possible vaccine pollutants, such as unwanted viruses, bacteria and other micro-organisms.
3.They are usually cheaper than live vaccines because the product is more durable, but this is only viable for large flocks.
Evidence from Burkina Faso and Niger indicates that because each vial contains at least 100 individual doses, there was a high degree of wastage, as the villagers only managed to vaccinate a few dozen birds a day at best.
Much of the advantage gained in efficient manufacturing, packaging and dispatching can be lost at this final stage if the contents of the vial are not fully utilized.
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