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Jul 23, 2023

In Mississippi, Blacklegged Ticks' Seasonal Shift Deemed "Very Strange"

In decades of surveillance, adult blacklegged ticks (Ixodes scapularis, female shown here) have only been found in fall months in Mississippi. But in 2022, 13 adult blacklegged ticks were collected

In decades of surveillance, adult blacklegged ticks (Ixodes scapularis, female shown here) have only been found in fall months in Mississippi. But in 2022, 13 adult blacklegged ticks were collected between June and September in locations across the state, the time of year when previously only larvae or nymphs were found. Scientists sharing the findings admit: They’re quite puzzled. (Photo by Blake Layton, Ph.D., Mississippi State University Extension Service)

By Leslie Mertz, Ph.D.

Leslie Mertz, Ph.D.

Something strange happened in Mississippi last summer. Adult blacklegged ticks, the species that transmits Lyme disease, showed up when they shouldn’t.

“I have never caught any in the summer. Never. And all of a sudden, there they are, and not just in one spot, but in nine counties all over the state. It’s very strange,” says Jerome Goddard, Ph.D., extension professor of medical entomology at Mississippi State University and lead author of a new article published in July in the Journal of Medical Entomology that describes the surprising tick collections.

Jerome Goddard, Ph.D., extension professor of medical entomology at Mississippi State University, has studied ticks and other insect and arthropod vectors of disease in Mississippi for more than 30 years. “It was just stunning to me” that adult blacklegged ticks (Ixodes scapularis) were collected in multiple locations in the state between June and September 2022, the time of year when previously only larvae or nymphs were found. “And I think any tick person in the whole southern United States would say ‘Whoa!’ because we’ve never, ever seen the adults in the summer.”

The discovery came during an eight-month tick survey run by the Mississippi State Department of Health, which is particularly interested in those of the state’s 19 species that transmit diseases. That includes the blacklegged tick (Ixodes scapularis), which can carry not only the bacteria for Lyme disease but also pathogens for other human diseases such as anaplasmosis, ehrlichiosis, and babesiosis.

During that survey, health department technician Sharon Sims scoured forests, fields, and other tick habitat using the tried-and-true dragging method, or pulling a light-colored cloth across and through vegetation and checking it for ticks that had latched on. Among more than 100 samples collected in June, July, and September 2022, she collected a total of 13 adult blacklegged ticks, the identity of which was initially made by state medical entomologist Wendy Varnado, Ph.D., and then confirmed both by Goddard and by tick specialist Richard Robbins, Ph.D., of the Smithsonian Institution’s National Museum of Natural History.

The surprising part of the finding is not that adult blacklegged ticks were found in Mississippi, Goddard says, but rather it was that they were collected well outside their normal season. “It was just stunning to me,” he says. “And I think any tick person in the whole southern United States would say ‘Whoa!’ because we’ve never, ever seen the adults in the summer.”

For the first time ever, adult blacklegged ticks (Ixodes scapularis)—the species that transmits Lyme disease—were discovered in Mississippi in the summer of 2022, and not just in one spot, but in nine counties (as indicated) throughout the state. Previously, adult-stage blacklegged ticks only showed up in the fall. (Map by Leslie Mertz, Ph.D.)

Since larvae, nymphs, and adults can transmit disease, the discovery of summertime adults does not pose an additional health threat, Goddard says. It does, however, put a big wrinkle in their well-studied, two-year life cycle. “What happens over the two years is a larva feeds on a host in the summer, fills up with blood, and then turns into a nymph sometime in the fall. Then the nymph survives the winter and feeds on a host the following summer, finally turning into an adult that second fall. The adult then feeds on a deer or other mammal in the fall or winter, and lays eggs that hatch into larvae in the spring or summer,” he says. “In other words, the immatures—the larvae and nymphs—are active in the summer and the adults are active in the winter. The adults don’t like hot; they are cool-weather ticks.”

So, why would adults suddenly appear in the summer? It’s a mystery, Goddard says. “Perhaps it’s better tick habitat, maybe because of new methods of farming, cropland turning back into forests, fields no longer being plowed, or other changes. Perhaps it’s host availability, because there are more deer now than there have been, and the more deer you’ve got, the more hosts are available for ticks. It could be a lot of things.”

Speculation aside, Goddard hopes these findings encourage additional, systematic surveillance in Mississippi and other southern states so that seasonal activity and trends can be clearly compared. “I am concerned about what the 2022 finding says about the biology and ecology of this tick: What’s going on here? And what can we learn from it when what seemed to be something stable is all of a sudden turned upside down?” he says.

“That’s why I wrote up the finding for the journal. I called it enigmatic in the article, but if I was just talking to somebody, I’d say it was just plain weird,” he adds. “Now, I’m interested to see what response the article gets, because it really does throw the whole ecology and biology of the blacklegged tick into question.”

“Anomalous phenology of Ixodes scapularis (Ixodida: Ixodidae) in Mississippi ”

Journal of Medical Entomology

Leslie Mertz, Ph.D., writes about science and runs an educational insect-identification website, www.knowyourinsects.org. She resides in northern Michigan.

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