The effects of latitudinal environmental variables on the seasonality of host-attached juvenile ticks in California

Climatic differences influence the variation of Ixodes ticks's seasonality, which is hypothesized to influence the spatially varying rates of human Lyme disease in the US. However, collecting appropriate climate and tick data to test this hypothesis requires time-sensitive and costly resources. With prior studies data on juvenile Ixodes pacificus tick burden data on western fence lizards (Sceloporus occidentalis; a major host for Ixodes ticks in California), we matched with remotely sensed climate data and analyzed using a generalized time series model that accounts for series dependence of non-Gaussian count data (i.e. GLARMA model). We find that the seasonality of juvenile I. pacificus varies along a latitudinal gradient, with southern, drier locations experiencing earlier and shorter seasons. Yet our analysis found that tick counts were more sensitive to climate metrics such as cumulative precipitation in the prior rain year, mean minimum temperature in winter quarter, mean maximum temperature in summer quarter, and weekly Palmer Drought Severity Index, compared to other variables included in traditional tick-climate models such as daily temperature or relative humidity. We confirm prior studies that juvenile I. pacificus seasonality spatially varies along a latitudinal gradient. However, our findings suggest that additional climate variables besides daily temperature and humidity should be included in climate-tick models. With this knowledge, we can better understand where and when juvenile I. pacificus may be active, which is important for better understanding tick-borne disease risk.