THE EPIDEMIOLOGY OF NEWCASTLE-DISEASE IN RURAL POULTRY - A REVIEW

Rural poultry are the dominant form of poultry kept in the developing world. They are a natural resource whose potential is not fully exploi ted for the welfare of rural populations. The most devastating disease of rural poultry is Newcastle disease (ND). ND is an epidemic disease in intensive poultry and is responsible for high economic losses. Its epidemiology in intensive poultry is well understood, but little has been published on its behaviour in village poultry populations. Some r esearch has been carried out during the past few years and it now appe ars that rural poultry are receiving increasing support for research a nd development from many government and international funding agencies throughout the world. Al strains of Newcastle disease virus (NDV) occ ur in rural poultry, but velogenic strains are reported to be more com mon. Serological surveys in conjunction with isolation studies have sh own that velogenic NDV strains are endemic in rural poultry population s even in isolated villages and possibly in isolated flocks. Although NDV is endemic in village poultry, the clinical disease usually follow s an epidemic pattern. ND outbreaks often occur once or twice a year a t regular intervals affirming the endemicity of the virus, however, 'm ini' outbreaks in individual flocks and sporadic cases in individual b irds may occur. Epidemics usually occur at times of climatic stress, l eading to seasonal occurrence. The spread of NDV within and between vi llage poultry populations is relatively slow due to a low contact rate . The major mode of transmission appears to be by the faecal-oral rout e. The respiratory route may also play a role in flocks where close bi rd-to-bird associations exist. Other poultry species, wild and feral b irds, wild animals, communal water reservoirs and domestic animals may play a role in transmission; however, their role has not been properl y investigated. In non-immune, intensively reared poultry, introductio n of a sufficient quantity of virulent NDV is sufficient to cause seve re outbreaks, but in rural poultry epidemic disease depends upon a num ber of factors. These include NDV pathotype, age structure and immunit y of the host population, breed susceptibility, concurrent disease and seasonal influences. Although ND is endemic in rural poultry, many as pects of the epidemiology and economics of ND have not been fully unde rstood. Well-designed, longitudinal studies made within a socio-econom ic framework are required to improve our understanding of the main pro blems of rural poultry systems.