Influence of spatial heterogeneity and vertebrate species assemblage on the ecology and risk of Lyme disease in the far-western United States

The role of habitat heterogeneity and vertebrate reservoir assemblage on Lyme disease distribution and risk was assessed in oak woodland forests in northern California. The influence of habitat heterogeneity on Lyme disease ecology was examined with a natural experiment that utilized the impacts of an introduced forest pathogen, Phytophthora ramorum that causes sudden oak death (SOD). SOD has fragmented the forest, reduced canopy cover, and altered the structure and species composition in oak woodlands in California. Lyme disease is endemic in oak woodland habitat because the pathogen (Borrelia burgdorferi), the tick vector (Ixodes pacificus nymphs), and vertebrate reservoirs of the pathogen (e.g. Neotoma fuscipes and Peromyscus maniculatus ) occur there. Structural equation modeling of I. pacificus populations found that nymphal ticks are limited by high temperatures in the dry season, a situation likely to increase under scenarios of advanced SOD. In addition, the impact of SOD on vertebrate densities and relative abundances of species was assessed empirically, with particular emphasis on vertebrates important in the maintenance of B. burgdorferi or tick hosts. I found that the main pathogen reservoir of B. burgdorferi in California, the dusky-footed woodrat (Neotoma fuscipes), is not currently affected by the impacts of SOD but that the less competent reservoir P. maniculatus and the most common tick host S. occidentalis are responding positively to forest-level changes caused by SOD. Because P. maniculatus and S. occidentalis do not contribute significantly to tick infection prevalence, a static model predicted that SOD is expected to decrease risk of Lyme disease, as measured by nymphal tick infection prevalence. S. occidentalis has important impacts on tick abundance and infection prevalence because they host up to 98% of all juvenile ticks but are not a reservoir of B. burgdorferi. An experimental removal of lizards showed that when lizards are not available, I. pacificus larvae switched to feeding on the reservoir host, N. fuscipes. Collectively, these studies demonstrate the importance of habitat heterogeneity on vertebrate species composition and abundance and, in turn, the importance of vertebrate species composition and abundance on the spatial distribution and prevalence of Lyme disease in the far-western United States.