Impact of routine Newcastle disease vaccination on chicken flock size in smallholder farms in western Kenya

BACKGROUND: Poultry represent a widely held economic, nutritional, and sociocultural asset in rural communities worldwide. In a recent longitudinal study in western Kenya, the reported mean number of chickens per household was 10, with increases in flock size constrained principally by mortality. Newcastle disease virus is a major cause of chicken mortality globally and hypothesized to be responsible for a large part of mortality in smallholder flocks. Our goal was to determine the impact of routine Newcastle disease virus (NDV) vaccination on flock size and use this data to guide programs to improve small flock productivity. METHODS: We conducted a factorial randomized controlled trial in 537 households: in 254 households all chickens were vaccinated every 3 months with I-2 NDV vaccine while chickens in 283 households served as unvaccinated controls. In both arms of the trial, all chickens were treated with endo- and ecto parasiticides every 3 months. Data on household chicken numbers and reported gains and losses were collected monthly for 18 months. RESULTS: Consistent with prior studies, the overall flock size was small but with increases in both arms of the study over time. The mean number of chickens owned at monthly census was 13.06±0.29 in the vaccinated households versus 12.06±0.20 in the control households (p = 0.0026) with significant gains in number of chicks (p = 0.06), growers (p = 0.09), and adults (p = 0.03) in the vaccinated flocks versus the controls. Household reported gains were 4.50±0.12 total chickens per month when vaccinated versus 4.15±0.11 in the non-vaccinated controls (p = 0.03). Gains were balanced by voluntary decreases, reflecting household decision-making for sales or household consumption, which were marginally higher, but not statistically significant, in vaccinated households and by involuntary losses, including mortality and loss due to predation, which were marginally higher in control households. CONCLUSION: Quarterly NDV vaccination and parasiticidal treatment resulted in an increase in flock size by a mean of one bird per household as compared to households where the flock received only parasiticidal treatment. While results suggest that the preventable fraction of mortality attributable to Newcastle disease is comparatively small relatively to all-cause mortality in smallholder households, there was a significant benefit to vaccination in terms of flock size. Comparison with previous flock sizes in the study households indicate a more significant benefit from the combined vaccination and parasiticidal treatment, supporting a comprehensive approach to improving flock health and improving household benefits of production in the smallholder setting.