Analytical models of zoonotic pathogens in agricultural watersheds

Zoonoses can dominate reported human diseases in countries with agricultural economies. For example, 65–80% of New Zealand's reported notifiable disease cases in recent years are zoonoses, much of which is campylobacteriosis, a major cause of enteric illness worldwide. Commonly, the efficacy of BMP (Best Management Practices) interventions in freshwater environments is assessed using a faecal indicator, most notably E. coli. But in so doing an important question arises: “If the interventions result in a reduction in E. coli loads by X%, what is the concomitant reduction in the zoonotic pathogens?” We will present two simple mathematical models to examine this question: one for E. coli and one for Campylobacter (as a typical zoonotic pathogen). They predict the relative removal efficacy for these bacteria after implementation of BMPs, and also the prevalence of Campylobacter infection among farm animals. Dimensionless numbers and simple graphs are developed to assess how prevalence is influenced by a number of factors including animal density and farm spacing.