The Epidemiology of Newcastle Disease in Village Chickens

Newcastle disease virus (NDV) infects most avian species. The serious consequence is the disease (frequently fatal) that occurs when virulent strains of the virus infect domestic chickens. The epidemiology of Newcastle disease (ND) in commercial chickens is fairly well understood. Infected chickens are the usual source of virus, which can be transported mechanically by fomites or by people. Preventive measures include vaccination and attention to biosecurity. Village chickens are also susceptible to ND, which, in developing countries, is the most important constraint to rural poultry production. Similar epidemiological factors probably apply to the spread of NDV in commercial chickens and village chickens. Until recently, thermostable ND vaccines were not available for use in village chickens. Biosecurity is extremely difficult to practice at the village level. Both epizootic and enzootic ND are recognised in village chickens. Epizootics occur when virus is introduced to a susceptible population. Spectacular outbreaks with high mortalities result. Enzootic ND occurs when the virus transmits slowly in a partially immune population. In this case, there are too few susceptible birds to maintain an outbreak and the occasional birds that die do not come to veterinary attention. Possibly a breeding population of as few as 500 birds can sustain the virus, as indicated by computer modelling. Anecdotal evidence suggests that seasonal conditions favour the initiation of outbreaks. More likely, it is movement of poultry, especially through markets, that initiates and fuels an outbreak. Salvage sales increase the dissemination of virus once the disease is recognised. All village chickens are sold as live birds, for consumption or for breeding. It does not seem feasible to control this aspect of the epidemiology of the disease. Suitable vaccines seem to be the only answer to the control of ND in village chickens. IN MANY countries, ND is an important infectious disease. Control is possible, but this requires efficient applications of vaccines, and rigorous biosecurity. The epidemiology of ND is fairly well understood in commercial chickens. Factors that influence spread between flocks can be addressed with physical and chemical barriers. Vaccination hinders spread within a flock. Aspects of the epidemiology of ND in commercial chickens have been reviewed by Alexander (1988) and Arzey (undated). Some of their observations are applicable to ND in village chickens.