Tela Zembsch, entomology graduate student at the University of Wisconsin-Madison, uses an antenna to track radio-collared mice in a study on tick presence in mouse nests. (Photo credit: Ryan Larson)

By Melissa Mayer

Scientists know a great deal about how blacklegged ticks (Ixodes scapularis) feed on hosts, such as whitefooted mice (Peromyscus leucopus) and woodland deer mice (Peromyscus maniculatus), but what happens after the ticks drink their fill is more mysterious. Lab studies suggest these ticks may detach from mouse hosts inside their nests. However, no field studies have confirmed the presence of blacklegged ticks in naturally occurring mouse nests—until now.

Melissa Mayer

In a new study published last week in the Journal of Medical Entomology, researchers tracked Peromyscus mice and examined their nests for evidence they may serve as habitat for blacklegged ticks as well as other tick species, such as American dog ticks (Dermacenter variabilis).

Baiting the Trap 

Susan Paskewitz, Ph.D., professor and chair of the of the Department of Entomology at the University of Wisconsin-Madison and senior author on the study, says checking out mouse nests was a logical choice. “We were developing an agent-based model that explored mouse behavior and blacklegged tick numbers on the mice,” says Paskewitz, who conducted the research alongside Wisconsin graduate students Ryan Larson and Tela Zembsch and research associates Xia Lee, Ph.D., and Gebbiena Bron, Ph.D. “The model suggested that mice spend so much time in nests during the day that ticks should be detaching and ending up in that environment at greater rates than we had suspected. So, we decided to look in nests, which turned out to be more difficult than you might imagine.”

To uncover mouse nests, the research team used peanut butter bait to trap mice in Wisconsin’s Chequamegon National Forest at monthly intervals between April and August of 2018. The researchers removed ticks from the trapped mice, tagged the mice, and took ear biopsies for DNA analysis since the two species of mouse look so similar. Then, they fitted the mice with radio telemetry collars and tracked the released mice for between one and five days.

Susan Paskewitz, Ph.D.

So Many Ticks, So Much Time

The team located 28 total nests used by 10 mice over the study period. Of these, 11 nests were suitable for collection and dissection for ticks, a labor-intensive process that took up to 40 hours per nest. The research team recovered a total of 69 detached blacklegged and American dog ticks in nest materials, spread among 9 of the 11 Peromyscus nests they examined—and the team suspects those numbers are low.

When it came to juvenile specimens, the researchers found blacklegged tick larvae in 64 percent of nests and American dog tick larvae in 55 percent of nests. They also saw blacklegged tick nymphs in 18 percent of nests. After excluding damaged ticks, 85 percent of recovered blacklegged tick specimens and 92 percent of American dog tick specimens were blood-fed.

Inside the Nest

Some of the nesting habits observed over the tracking period were unexpected. “We were surprised by the proportion of mice that were using arboreal sites for nests, as we had thought most white-footed mice were using underground sites,” says Paskewitz.

Researchers at the University of Wisconsin-Madison tracked mice such as this one to discover the locations of their nests and study the presence of ticks therein. (Photo credit: Ryan Larson)

Tela Zembsch, entomology graduate student at the University of Wisconsin-Madison, places a radio-tracking collar a mouse during a study on tick presence in mouse nests. (Photo credit: Susan Paskewitz, Ph.D.)

Ryan Larson, entomology graduate student at the University of Wisconsin-Madison, collects a mouse nest from several feet up in a tree in the course of a study on tick presence in mouse nests. (Photo credit: Tela Zembsch)

Researchers at the University of Wisconsin-Madison tracked mice to discover the locations of their nests (such as this one) and study the presence of ticks therein. (Photo credit: Ryan Larson)

“We were also surprised to see two [mouse] species in the same nest at the same time and by the number of nests used by a single mouse,” Paskewitz adds. “They are not restricted to a single nest but instead tended to move and use different nests each day.”

This is important because understanding nest selection may help scientists figure out mouse-tick interactions within nests. Nests used frequently or by multiple mice likely contain more ticks. And nests used by mice over multiple years may enable overwintering ticks to encounter hosts in the spring. And overwintering may play a role in the spread of tick-borne disease.

“If ticks are overwintering in nests, then they will be in close proximity to a major reservoir for Lyme disease and may play an important role in infecting mice early in the season, helping to start the disease cycle anew each year,” Paskewitz says.

The Bigger Picture

The relationship between the nests and tick-borne disease is compelling. After all, blacklegged ticks are the primary vector for Lyme disease in the eastern United States. “I’m intrigued by the idea that the ticks in these nests might be important in disease dynamics,” says Paskewitz. “If the larvae in the nests are able to molt to nymphs in that environment, then they will be more likely to contact the same host type, which are excellent disease reservoirs, and to spread disease agents.”

Figuring out what happens to ticks at various life stages after they detach in the nest and piecing together the ecological significance of ticks in Peromyscus nests are likely areas for future study. One possibility is deploying nest boxes in the field to make the process simpler.

Paskewitz and her team are already exploring biological differences between the two species of mice to better understand disease transmission. They are also working with Tick App, a crowd-sourced research platform gathering data on human tick exposures.

Melissa Mayer is a freelance science writer based in Portland, Oregon. Email: melissa.j.mayer@gmail.com.