Force of Infection and Variation in Outbreak Size in a Multi-Species Host-Pathogen System

The size of annual outbreaks in seasonally forced host-pathogen systems is poorly understood. We studied contributing factors to the six-fold observed variation in the number of human cases of West Nile virus in New York City in the years 2000–2008. Sampling error and intrinsic noise (demographic stochasticity) explain roughly half of the observed variation. To investigate the remaining sources of variation, we estimated the monthly force of infection from data on the distribution and abundance of mosquitoes, virus prevalence, vector competence, and mammal biting rate at two spatial scales. At both scales, the West Nile virus force of infection was remarkably consistent from year to year. We propose that fine scale spatial heterogeneity is the key to understanding the epidemiology of West Nile virus in New York City.