Fluctuating rodent populations and risk to humans from rodent-borne zoonoses

The fluctuations in abundance of a wildlife reservoir are an attractive explanation for temporal variation in primary human cases of a zoonosis. This is because high abundance may lead to more contact between humans and animals, but also to outbreaks of disease within the reservoir population. We propose a mathematical framework that sets out the consequences of correlation between reservoir abundance and reservoir prevalence for how numbers of human cases are related to reservoir abundance. The fluctuations of rodent populations are well studied and often dramatic. A review of field studies of rodent reservoirs for plague, hantaviruses, and other zoonoses shows that, at a seasonal time scale, a positive correlation between host abundance and host prevalence is rarely observed. More commonly, there is an inverse relationship or negative correlation such that a seasonal increase in rodent abundance is not accompanied by a corresponding increase in the abundance of infectious animals. Seasonal changes in rodent abundance are hence unlikely to fully explain seasonal variation in primary human cases. The few longer field studies (>5 years) show a positive but delayed relationship between reservoir abundance and reservoir prevalence.