Seasonal activity of the western blacklegged tick (Ixodes pacificus) and human Lyme disease risk in southern California

Background/Question/Methods

Conservation of biodiversity and protection of human health are central challenges for humanity in the face of anthropogenic change, and are increasingly recognized as fundamentally intertwined. Emergence of many zoonotic diseases, including Lyme disease (LD) in North America, is linked to land use and climate change through effects on reservoir hosts and/or vectors. While incidence of LD is increasing throughout North America, little is known about vector tick abundance or seasonal activity in southern California. A recent study suggests that in highly endemic northwestern (NW) California, potentially infectious ticks are active year round resulting in higher risk of human exposure than previously recognized. Here we were interested in determining whether activity patterns, and thus risk of tick exposure, in southern California mirror those in the north or are truncated leading to comparatively lower risk of exposure. We used standardized drag sampling techniques in oak woodland, as well as a range of other common habitats in southern California, to determine seasonal abundance of the primary vector (Ixodes pacificus) of the pathogen, and compared patterns in southern California to those observed in NW California.

Results/Conclusions

We found that seasonal activity of I. pacificus in southern California is significantly shorter, and relative abundance significantly lower, than in NW California. We also found significantly higher abundance of I. pacificus in oak woodland habitats than in other habitats. These results suggest that risk of tick exposure is lower in southern California than in NW California, and that oak woodland habitats pose the greatest risk. I. pacificus ticks are highly susceptible to environmental conditions, and under projected climate change endemic areas of NW California are expected to become hotter and drier, more closely approximating current climatic conditions in southern California, which may lead to a reduction in risk of tick exposure and tick-borne disease in western North America.