Evaluating potential sources of bovine tuberculosis infection in a New Zealand cattle herd

Abstract

In New Zealand, a major source of bovine tuberculosis (Mycobacterium bovis, Tb) infection in farmed cattle is thought to be transmission from infected wildlife, primarily the brushtail possum (Trichosurus vulpecula). At Hohotaka, central North Island, control work from 1988 to 1994 achieved a sustained mean reduction of 87.5% in the density of Tb‐infected possums. As expected, annual Tb incidence in local cattle herds consequently declined by a similar amount (83.4%). This decline was gradual, due to within‐herd transmission of residual infection and imperfect detection of infected animals. By fitting models to the observed rate of decline in Hohotaka Tb reactors, we are able to deduce the contributions to reactor rates of possums and cattle‐to‐cattle Tb transmission. Our results suggest that Tb transmission between cattle within the herd may have contributed around 20% to 32% of infections prior to the wildlife control programme. Nevertheless, our estimate for the rate of within‐herd transmission is well below that required to maintain cattle Tb in the absence of infectious wildlife. Our analysis supports the policy of wildlife control as the most effective method for reducing cattle Tb in areas such as Hohotaka.