Involvement of free-flying wild birds in the spread of the viruses of avian influenza, Newcastle disease and infectious bursal disease from poultry products to commercial poultry

It appears possible for free-flying native or feral wild birds to be responsible for direct or indirect spread of infectious agents from contaminated poultry product wastes to susceptible commercial poultry. Direct spread involves infected wild birds gaining access to commercial poultry. Indirect spread involves secondary spread to other free-flying birds that may subsequently gain access to commercial poultry. In analysing the level of risk of such spread consideration is given to three elements. These are the susceptibility of wild birds to infection by the agents concerned, the opportunity for susceptible species coming into direct or indirect contact with both poultry product wastes and commercial poultry, and the occurrence of actual cases of such spread. The scenario being considered is one in which poultry product wastes contain an infective dose of one of the agents of concern, namely avian influenza, Newcastle disease or infectious bursal disease. A review of the literature was undertaken to assist in assessing the likelihood of spread of these agents by wild birds. Exposure of wild birds, to any of the three viruses of concern at product waste disposal sites, is a potential threat but the likelihood of spread to commercial poultry appears to be low. More obvious means of spread occur in the poultry industry, as biosecurity is often rudimentary. Spread of infectious agent by fomites, live poultry and infected wild birds could well be involved in transmission of disease agents between farms. Where live bird poultry markets or semi-feral village poultry exist they provide an obvious means of spread of these agents and involvement of access of wild birds to poultry product wastes is probably of little importance. The likelihood of wild bird involvement in virus spread from poultry products could increase if a susceptible species with gregarious habits (flocking behaviour) leading to high population densities, were to frequent both poultry product waste disposal sites and commercial poultry premises. Some passerines are likely to frequent waste disposal facilities and poultry premises, but it appears that different species congregate at poultry premises than at waste disposal sites. There is little evidence of spread of disease from waste poultry products to commercial poultry premises having occurred.