Tick sampling at Sedgwick Reserve, Santa Ynez Valley, CA.
Summary: Human cases of tick-borne diseases, most notably Lyme disease, have been increasing in incidence and geographic distribution across the northern hemisphere. This increase is likely being driven by a number of interacting factors including climate and land use change, changes in host community composition and human behavior, along with a concomitant increase in both our ability to detect tick-borne pathogens and in reporting efforts. In North America, there are an estimated 300,000 cases of human Lyme disease each year with geographically distinct disease foci, or regions where the disease is considered to be endemic, including in the northeastern, upper midwestern and far western United States. While the northeastern and upper midwestern foci have experienced geographic expansion and increases in incidence, the far western United States has experienced low and consistent annual incidence in the human population. Understanding this distinct and unique epidemiological pattern is the primary driver of my work on tick-borne disease, informing the questions I have addressed in the field in California:
1. Vector Phenology:
Timing of seasonal activity of different tick life stages is critically important to pathogen transmission from infected vectors, to susceptible hosts and again to uninfected vectors. This project investigates seasonal patterns of activity of the western blacklegged tick (Ixodes pacificus), the primary vector of human Lyme disease in the western US, across California's latitudinal and climate gradient to better understand pathogen transmission pathways and overall human risk of tick exposure in a region with low, but heterogenous human incidence of tick-borne disease. We have found periods of seasonal tick activity to be much shorter in southern than northern California, with key implications for reduced human exposure risk, host-feeding and pathogen transmission in this region (studies published in Ticks and Tick-Borne Diseases and PLoS One). We have also employed species distribution models to understand the seasonal ecological niche of the western blacklegged tick in California. We find our model reasonably predicts empirical patterns of monthly tick activity across the state, and uncover key environmental drivers of tick phenology that will likely lead to shifts in activity patterns under future global change scenarios, including temperature and precipitation (study in collaboration with the California Department of Public Health published in Journal of Applied Ecology). We have also investigated the response of vector phenology and distribution to projected climate and land use change in California using machine learning approaches (study published in Global Change Biology). |
2. The Effect of Wildfire disturbance on Tick and Host Ecology, and Human Risk: under projected climate and land use change in the western US, wildfire risk is expected to increase. This project investigates the effects of wildfire disturbance on tick populations and host communities in California oak woodlands using recent wildfires, including the 2013 White Fire in Los Padres National Forest in Santa Barbara County and the 2017 Tubbs and Nuns Fires in Sonoma and Napa Counties as experimental manipulations. We have found that wildfire initially amplifies risk of tick encounter in southern California, with host-seeking tick abundance higher in the year following fire, but that the long-term effects of fire are to reduce risk through reductions in reservoir host (woodrats) and reproductive host (deer) abundance, as well as reductions in tick populations over the long-term (study published in Ecosphere).
3. Environmental Drivers of Tick Populations and Pathogen Infection Prevalence Along a Latitudinal and Climate Gradient: large scale empirical studies of the effects of climate, or climate change, on vector-borne disease risk are logistically challenging to undertake. In this project, I make use of the significant latitudinal and climate gradient captured by the UC Natural Reserve System (NRS), and the wealth of existing data and logistical support the NRS provides, to investigate the environmental drivers of vector population dynamics and infection prevalence at various spatial scales along a climate gradient in California.
In local scale studies in Santa Barbara County, we find: 1) host abundance, micro-habitat structure and temperature to be key drivers of the western blacklegged tick, 2) tick diversity to be strongly associated with rates of infection with Borrelia burgdorferi in tick communities, and 3) different tick vector species exhibit different environmental drivers and will likely respond differently to global change (studies published in Parasites and Vectors, PLoS One and Journal of Medical Entomology). This work has been supported by the UC Institute for the Study of Ecological Effects of Climate Impacts (ISEECI). Ongoing efforts across the UC Natural Reserve System are identifying the effects of drought and climate variability on tick population dynamics and infection prevalence across California's latitudinal gradient. |