Ixodes pacificus, known as the western blacklegged tick, is the primary vector of the bacteria that causes Lyme disease in the western United States. A new study by the U.S. Centers for Disease Control and Prevention examined tick abundance and variability in California and found that cold-season temperature and rainfall are the two leading factors that determine an area’s suitability for ticks—deeming the state’s far northern coast and the western Sierra Nevada foothills as the most likely habitat for I. pacificus. (Photo credit: James Gathany, CDC Public Health Image Library)

By Andrew Porterfield

Ticks that contribute to the spread of Lyme disease, anaplasmosis, and other diseases have been subject to various tracking methods to prevent the spread of disease over the past several decades. However, most tracking methods that are cost effective depend on county-wide reporting, which isn’t precise enough to predict disease prevalence, especially in counties in the American west, many of which are larger than some eastern states.

Andrew Porterfield

County-wide reporting not only fails to accurately narrow down tick prevalence, but it also can overlook disease reservoirs in small areas of western counties. Researchers for the U.S. Centers for Disease Control and Prevention have long called for more precise spatial epidemiological data collection, but most methods are much too expensive to adopt.

In a paper published in April in the Journal of Medical Entomology, researchers led by CDC scientist Rebecca Eisen, Ph.D., reviewed tick collection and tracking models used to detect Ixodes pacificus, the western blacklegged tick that is largely responsible for the spread of Lyme disease in the Pacific United States. Looking at tick collection records from 1980 to 2014 showed a great deal of spatial variability of the tick, and that variability—in California—was largely due to higher temperatures and more rainfall during the coldest part of the year. Climate suitability maps pointed to California’s far northern coast and the western Sierra Nevada foothills as the most likely habitat for I. pacificus and could more precisely determine the highest-risk regions for Lyme disease. In addition, the researchers observed that the models used are now widely available on the internet at low to no cost.

According to state sources, about 1-2 percent of California adult I. pacificus ticks and 2-15 percent of nymphs carry Borrelia burgdorferi, the bacterium that causes Lyme disease. About 2,600 cases of Lyme disease were reported in California between 1989 and 2014. Meanwhile, cases of human granulocytic anaplasmosis have been increasing since its discovery in 1990 and may infect up to 36 percent of endemic tick populations, according to the CDC. While the tick has been reported in 56 of 58 of California’s counties, this new study indicates that the tick is concentrated in certain areas that should be better defined for disease control.

The researchers reviewed data on locations where tick larvae, nymphs, and adults were collected in California, focusing on records between 1980 and 2014. They then matched the tick population reporting with climate data, looking at 35 climate variables, including precipitation, maximum temperature, minimum temperature, vapor pressure and day length. A total of 4,546 I. pacificus locations were entered into the researchers’ models; about 77 percent of those ticks were collected from forest, grass, scrub, or riparian land, the most prevalent habitat for the tick.

The team’s models consistently showed that rainfall and average temperatures during the year’s coldest quarter determined the suitable habitat of I. pacificus in the state. Areas that were wet and warm during the winter were most suitable, pointing to the northwest coast (including Mendocino, Humboldt, and Del Norte counties), and the western slope of the northern Sierra Nevada foothills. Other parts of those counties were not as suitable to the tick.

“Suitable habitat for vectors to survive and reproduce are not uniform within counties. In these areas, sub-county information on where vectors are present can aid in targeting limited vector surveillance and prevention resources. Overall, our models identified suitable habitat for the tick in 55 of 58 California counties, but there was considerable variation in the percentage of land in each county that was considered suitable for the tick. For example, less than 1 percent of Lassen county (in northeastern California) was considered suitable, whereas over 90 percent of Mendocino county (in north-coastal California) was classified as suitable,” Eisen says. “Knowing where ticks are likely to be found is key to avoiding tick habitat when ticks are active and empowers people to protect themselves. In our study, we modeled the potential range of Lyme disease vectors in California, and this information improves on our knowledge of where Lyme disease vectors are likely to be found in California.”

While the study did not focus on any effects of climate change, Eisen notes that “based on our models, we assume that forest, grass, scrub-shrub, and riparian areas in California that become warmer and wetter during the winter would become more suitable for this tick to survive and reproduce.” Because tick surveillance and reporting aren’t standardized among counties and cities, there are as yet no long-term data available to determine if climate suitability has changed over time.

Andrew Porterfield is a writer, editor, and communications consultant for academic institutions, companies, and nonprofits in the life sciences. He writes frequently about agriculture issues for the Genetic Literacy Project. He is based in Camarillo, California. Follow him on Twitter at @AMPorterfield or visit his Facebook page.