UC Davis

Although wildfires are increasingly common in California, information regarding their effect on tick populations and pathogen prevalence is limited. This study explores the questing tick population and potential human disease risk in two hiking areas, Stebbins Cold Canyon (SCC) and Skyline Wilderness Park (SKY), after varying wildfire exposure. Stebbins Cold Canyon experienced two wildfires over the past ten years, the Wragg Fire in 2015 and the LNU lightning complex fire in 2020, whereas SKY was exposed solely to the Atlas Fire in 2017. Questing ticks were collected monthly over three seasons at each location. Tick species were identified and screened for known and emerging human pathogens using quantitative PCR. Human foot traffic, provided by park directors, was recorded for each location based on trailhead counters (SCC) and admission fees (SKY). Foot traffic (mean + SD) at SCC for spring, summer, and fall was 3,236 + 1,238, 1,743 + 522, and 2,557 + 449, respectively. Foot traffic (mean + SD) at SKY for spring, summer, and fall was 5,861 + 834, 5,030 + 70, and 4,435 + 263, respectively. Negative binomial regression was used to compare tick count data, with location and season as interaction terms. In total, 459 ticks were collected: 54 adult ticks from SCC and 405 from SKY, including 279 (68.9%) larvae, 2 (0.5%) nymphs, and 124 (30.6%) adults. At SCC, Ixodes pacificus accounted for 92.6% (50/54) of collected ticks. The remaining 7.4% consisted of Dermacentor occidentalis (4/54). At SKY, I. pacificus (16.8%, 68/405), D. occidentalis (12.3%, 50/405), D. similis (0.7%, 3/405), and D. albipictus (70.4%, 285/405) were collected. No statistical difference was seen in tick counts across locations when removing the large amount of D. albipictus larvae from the model (p = 0.352). These findings suggest that SCC’s plant, host, and tick communities are recovering after the most recent wildfire, likely expedited by increased rainfall. However, SKY had significantly more ticks than SCC (p < 0.001) when including larvae, likely due to the lack of leaf litter at SCC compared to SKY. Borrelia burgdorferi SL and Rickettsia rhipicephali were only found at SKY with a prevalence of 0.8% (1/121) and 0.7% (2/121), respectively. Rickettsia tillamookensis and Babesia duncani were found at both locations. Prevalence for R. tillamookensis at SCC and SKY was 3.7% (2/54) and 1.7% (2/121), respectively. Lastly, B. duncani was identified in 3.7% (2/54) and 1.2% (5/405) of ticks at both SCC and SKY, respectively.