A cross-sectional survey of Australian chicken farms to identify risk factors associated with seropositivity to Newcastle-disease virus.

Several outbreaks of virulent Newcastle-disease occurred in Australia in 1998-2000. We conducted a cross-sectional survey of 753 Australian chicken farms to identify risk factors associated with the seroprevalence of chicken flocks with Newcastle-disease virus (NDV). We had a 99.7% response rate to the survey and the overall prevalence of NDV seropositive farms was 39.8%. Associations were analysed for the layer, chicken-meat and breeder production sectors in sector-specific logistic-regression models using 187, 198 and 146 farms, respectively. In the layer sector, increased risk of seroprevalence was associated with increasing age of the chickens, and decreased risk when the nearest-neighbour poultry farm was >10 km distant (odds ratio (OR)=0.30). In the chicken-meat sector, increased risk of seroprevalence was associated with location in the Sydney basin (OR=13.67), eastern Victoria (OR=26.10) or western Victoria (OR=5.43), and decreased risk when the nearest-neighbour poultry farm was greater than 0.5 km distant (OR=0.34). In the breeder sector, increased risk of seroprevalence was associated with increasing age of the chickens, the presence of wild birds on the farm (OR=5.28) and location in eastern Victoria (OR=16.19). A conditional logistic-regression for 112 pairs of farms matched for age, survey region and production sector identified a distance of >1.0 km to the nearest-neighbour poultry farm (OR=0.24) and ownership by owner 2 (OR=0.02), owner 5 (OR=0.11) or owner 9 (OR=0.25) as significant in reducing the risk of NDV seroprevalence. Our survey found that high levels of biosecurity and hygiene practices had been adopted by most farms.