UNIVERSITY PARK, Pa. — The prevalence of the most abundant species of ticks found in Pennsylvania has shifted over the last century, according to Penn State scientists, who analyzed 117 years' worth of specimens and data submitted primarily by residents from around the state.
The researchers said understanding the spatial distribution patterns and host associations revealed by their analysis is important for assessing and reducing the risk of diseases caused by tick-borne pathogens.
The Centers for Disease Control and Prevention reported a 3.5-fold increase in vector-borne diseases in the United States between 2004 and 2016, with more than 76 percent of cases caused by tick-borne pathogens. The vast majority of these cases are Lyme disease, attributed to Borrelia burgdorferi. Pennsylvania has had the highest number of total Lyme disease cases in the U.S since 2000.
Since the early 1900s, people have submitted ticks for identification to the Penn State Department of Entomology and its forerunners, and these specimens have become part of Penn State's Frost Entomological Museum collection, noted Joyce Sakamoto, assistant research professor of entomology in the College of Agricultural Sciences.
"Then in the 1960s, the late entomology professor Robert Snetsinger ran a statewide media campaign to encourage tick submissions, and he received thousands from citizen scientists," she said. "He also conducted active surveillance, capturing and collecting ticks in the field."
Another Penn State entomologist, Steven Jacobs, a now-retired senior extension associate, led a similar public-service media campaign in the 1990s, resulting in another surge in tick submissions from citizens around the state.
The research team — Sakamoto, Jacobs and first author Damie Pak, doctoral candidate in biology — compiled data from more than 7,000 of these tick specimens dating from 1900 and analyzed the submissions for tick-community composition, host associations and spatio-temporal dynamics. Ticks were submitted from all 67 counties in Pennsylvania, and the specimens included 24 species.
Five species make up more than 90 percent of submissions, Amblyomma americanum — lone star tick; Dermacentor variabilis — American dog tick; Ixodes cookei —groundhog tick or woodchuck tick; Ixodes scapularis — blacklegged tick; and Rhipicephalus sanguineus — brown dog tick.
"We found that based on these collections, there have been several shifts in the dominant species of ticks over the last 117 years," Sakamoto said. "For example, before 1990, the majority of tick submissions were identified as Ixodes cookei (groundhog or woodchuck tick). Right now, the dominant species is Ixodes scapularis, or blacklegged tick, which is the primary vector of Lyme disease. But this tick was almost nonexistent in Pennsylvania in the 1960s."
The researchers reported their results in the May 3 Parasites & Vectors.