UC Davis

The current status of ticks and tick-borne diseases in California has been impacted by over a century of fire suppression and irruptions of catastrophic wildfires exacerbated by fire suppression and ongoing anthropogenic climate change. Important tick-borne diseases include Lyme disease caused by Borrelia burgdorferi, granulocytic anaplasmosis caused by Anaplasma phagocytophilum, and spotted fever rickettsioses caused by several rickettsial species. Their vectors in this ecological system are Ixodes pacificus, Dermacentor occidentalis, and D. similis ticks. Published research has focused typically on direct effects on tick populations immediately and shortly after fires. However, California fire management is transitioning from a regime of fire suppression to controlled burning, which will alter interactions among tick-borne pathogens, vectors, hosts, and those fire managers who conduct the burns, albeit in ways that are not yet fully understood. This dissertation comprises three chapters dedicated to exploring the indirect and long-term effects of fire on tick-borne diseases in California. Chapter 1 compares public awareness, knowledge, and preventive practices regarding tick-borne diseases with those of individuals engaged in fire-related forest management. Chapter 2 investigates the effects of ash on these tick species in the laboratory. Finally, Chapter 3 presents a predictive model analyzing longitudinal trends in nymphal I. pacificus abundance in a region affected by a severe wildfire, aiming to disentangle the effects of fire from broader population dynamics over time.

Individuals involved in managing forests to prevent wildfires, such as firefighters, foresters, inmate laborers, and indigenous burn crews, may be at heightened risk of acquiring tick-borne diseases. Blood samples from 55 forest workers and 58 members of the public near sites of documented elevated risk for Ixodes pacificus-transmitted pathogens were tested for the presence of bacterial DNA and antibodies for the three pathogens. Although more workers were positive for B. burgdorferi antibodies than the public (5.5% compared with 1.7%), the difference was not statistically significant. A questionnaire administered to 54 workers and 84 members of the public identified that tick bites were common (reported by 67.4% of participants) and that important gaps in knowledge and prevention practices exist including tick identification and safe removal practices, despite many workers having received training on tick-borne disease prevention. These findings underscore the risk of tick-borne diseases faced by fire management workers in tick-borne disease–endemic regions and highlight the need for enhanced training programs to minimize these risks.

The second chapter investigates the effects of ash produced from burning common California vegetation on the three tick species. Ticks may be exposed to ash when their wildlife hosts bathe in ash, after wildfires or controlled burns, and through the global ethnoveterinary practice of using ash to control ectoparasites. Effects on survival and behavior of ash from coast redwood, California bay laurel, and Tasmanian blue gum eucalyptus were compared to diatomaceous earth and kaolin clay (active controls), and a negative control. Bay and eucalyptus ash are lethal to adult I. pacificus and larval D. occidentalis, rivaling the tested active controls, while redwood ash influenced the ability of adult I. pacificus to orient. These findings provide an alternative management strategy to chemical acaricides and broaden our understanding of the effects of fire on tick-borne disease beyond direct impacts.

The final chapter addresses a critical gap in understanding the long-term effects of fire and other environmental dynamics on tick-borne diseases. Unlike studies that assess the impacts of fire in short intervals of time, this chapter considers longitudinal variations in tick abundance to understand the effects of wildfire and climatic variables such as drought on nymphal I. pacificus. Two long-term datasets documenting nymphal abundance and questing activity before and after a severe wildfire were analyzed. Nymphal tick activity exhibited significant year-to-year variation, with densities predicted by temperatures from early spring of the prior year, and the timing of questing predicted by late spring temperatures and relative humidity from the prior year. After adjusting for drought and climatic variables, the model was able to successfully predict that low nymphal abundances would occur in the years that ultimately followed a severe wildfire. Furthermore, the model helps quantify how much of the decrease in tick abundance that observed post-wildfire was likely due to the wildfire versus underlying climate conditions. This study underscores the necessity of considering long-term trends when evaluating the effects of fire on ticks and tick-borne diseases, and highlights limitations of short-term studies that may overlook long trends and other ecological factors.

Together the three studies advance our understanding of this monumental shift in the state’s fire policy on tick-borne disease. In particular, they can help inform those who conduct burns about their risk levels, where they have gaps in knowledge about prevention, and how specific features of their burn efforts can influence tick-borne disease.