Impacts of an emerging forest pathogen on tick abundance and implications for Lyme disease in California

Sudden oak death (SOD) is an emerging forest pathogen that is altering California’s coastal oak woodlands. In this study, we address whether Lyme disease, a vector-borne zoonotic disease, is impacted by SOD by measuring two measures of Lyme disease risk, tick abundance and infection prevalence. These measures capture direct impacts of SOD on ticks through changes in habitat suitability as well as indirect effects of shifts in the composition and abundance of tick hosts in response to SOD. We sampled 48 randomly selected plots in Sonoma County, California. Our study utilized plots previously established to measure SOD impacts on forest composition and microclimate variables. Each plot was drag-sampled for ticks during the peak nymphal questing period. All ticks were collected, identified to species, and tested by nested PCR for the Lyme disease bacterium, Borrelia burgdorferi. We used principal components analysis to reduce 7 vegetation measures of SOD and 3 abiotic variables to attain 3 uncorrelated principal components. These components were then used in a multiple regression model to predict nymphal tick abundance. We found that SOD is positively correlated with tick abundance (R²=0.387, p < 0.0093) with the prior year’s indices of SOD on California bay laurel, Umbelluaria californica (the primary foliar host) having the highest influence. We will also discuss results on infection prevalence and impact on Lyme disease risk. This model predicted nymphal tick abundance better than abiotic factors alone (e.g. the prior year’s relative humidity and temperature), which suggests that SOD-caused vegetation changes play an important role in nymphal tick abundance, possibly through effects on tick host composition and abundance.