Household-level risk factors for Newcastle disease seropositivity and incidence of Newcastle disease virus exposure in backyard chicken flocks in Eastern Shewa zone, Ethiopia.

A cross-sectional study with repeated sampling was conducted to investigate potential risk factors for Newcastle disease (ND) seropositivity and for incidence of ND virus (NDV) exposure in household flocks of backyard chickens in Eastern Shewa zone, Ethiopia. Data were collected from 260 randomly selected households in 52 villages in Adami Tulu Jido Kombolcha and Ada'a woredas (districts) using a structured questionnaire, and serum samples from chickens were tested for NDV antibodies using a blocking enzyme-linked immunosorbent assay (ELISA). Sampling took place during September 2009 and the same households were again sampled in May 2010. Household-level seroprevalence and incidence of NDV exposure were estimated in various ways using serological results from the two samplings, flock dynamics, and farmers' reports of ND in their flocks. The risk factors were assessed using multivariable mixed-effects logistic regression models. Household-level seroprevalence at the two sampling times was 17.4% and 27.4%, respectively, and the estimated incidence of household-level NDV exposure during the intervening period ranged between 19.7% and 25.5%. At the first sampling, reduced frequency of cleaning of poultry waste was associated with increased odds of seropositivity (OR=4.78; 95% CI: 1.42, 16.11; P=0.01) while hatching at home vs. other sources (buying in replacement birds, receiving as gift or buying fertile eggs) was associated with lower odds of seropositivity, both at the first sampling (OR=0.30; 95% CI: 0.11, 0.82; P=0.02) and the second sampling (OR=0.23; 95% CI: 0.10, 0.52; P<0.001). The risk of NDV exposure was shown to be higher with larger flock size at the beginning of the observation period (OR=3.6; 95% CI: 1.25, 10.39; P=0.02). Using an open water source (pond or river) for poultry compared to closed sources (tap or borehole) was associated with increased risk of NDV exposure (OR=3.14; 95% CI: 1.12, 8.8; P=0.03). The use of a grain supplement (OR=0.14; 95% CI: 0.03, 0.69; P=0.03) and hatching at home for flock replacement (OR=0.23; 95% CI: 0.10, 0.52; P=0.005) were associated with a lower risk of NDV exposure. Newcastle disease seroprevalence and incidence of NDV exposure were more heterogeneous between villages than between kebeles (aggregations of villages) and woredas in the study area. Further investigation of village-level risk factors would likely improve our understanding of ND epidemiology in backyard chickens.