Invasive shrubs increase spread of tick-borne disease

For a hungry tick, bush honeysuckle is as good as a drive-through.

The common invasive shrub is a popular habitat for deer, which in turn are ticks’ favorite blood source. As the deer move through the honeysuckle, loitering ticks can easily grab hold of a passing meal — and in the process become infected with pathogenic bacteria.

New research reported this week in the Proceedings of the National Academy of Sciences shows that the presence of bush honeysuckle substantially increases the risk of human disease.

The study found that the density of white-tailed deer in honeysuckle-invaded areas was roughly five times that in areas without honeysuckle, and the density of nymph life-stage ticks infected with bacteria that cause human disease was roughly 10 times higher. The researchers confirmed these large-scale results with an experiment that removed honeysuckle in some areas but not in others. When honeysuckle was removed, deer activity was greatly reduced and the density of infected ticks dropped.

The research was led by tick expert Brian F. Allan, who just completed a postdoctoral appointment at Washington University in St. Louis, and conducted by an interdisciplinary team of ecologists, molecular biologists and physicians from Washington University and the University of Missouri-St. Louis.

“One of the really exciting things about this study is the finding that an invasive plant alters deer behavior in a way that changes how deer and ticks interact, and in a way that promotes spread of disease,” says John Orrock, a co-author of the study and new professor of zoology at the University of Wisconsin–Madison.

“The deer used the open areas less than the honeysuckle patches and we don’t think it’s because they’re eating the honeysuckle; we think they’re using it for physical structure,” says Allan. “They like to bed in it because it’s the densest thing out there, the best structure in town. No native species comes close to achieving the same density.”

Moreover, he says, bush honeysuckle retains its leaves longer than most native species do. It’s the first thing to leaf out in the spring, and it’s the last thing in the understory to drop its leaves in the fall, so it creates structure for a large portion of the year — including the critical windows when larval ticks emerge in search of their first blood meal.

“The larval ticks become infected when they take their blood meal from an infected host, usually a deer, and the next life-stage, the nymphs, may spread disease to people if they grab onto them for the next blood meal,” Allan says.

The researchers collected lone-star ticks from experimental plots with and without honeysuckle in a conservation area near St. Louis and applied an innovative DNA assay developed by Washington University scientists Robert E. Thach and Lisa S. Goessling to analyze what they were eating and whether they were infected.

The ticks present a much greater challenge than other bloodsuckers like mosquitoes, which dine frequently over a short period of time. “It’s much harder to get blood from a tick, which usually takes only one blood meal per life stage,” Thach says. “By the time we capture the tick, eight months to a year may have elapsed. The tick has had a long time to digest that blood, so there may be only a tiny amount of DNA left — if there’s any.”

The team did two assays on tick DNA: one to identify pathogenic bacteria and the other to identify the animal that provided the tick’s last blood meal. The ticks sometimes bite coyotes, foxes and other animals, but their favorite hosts are wild turkey and white-tailed deer.

The results showed that more blood meals were taken from deer in honeysuckle-intact plots.

Bush honeysuckle is already common in Wisconsin, and Orrock says it is likely that a similar relationship exists among the shrubs, deer, and ticks in this state.

The current study did not assay for Lyme disease, which is also transmitted by ticks, but did identify bacteria that can cause a less well-known tick-borne disease, ehrlichiosis. Ehrlichiosis begins with symptoms typical of bacterial infection, such as fever, headache, fatigue and muscle aches. More serious symptoms, such as joint pain and confusion, may occur and in rare instances the disease is fatal.

“Many studies around the world are showing an increase in the risk of infectious disease as a result of the loss of biological diversity,” says Allan. “This may be a case of win-win ecology. Honeysuckle control would benefit native species but it would also benefit human health.”

The big question now, says Washington University professor of biology Jonathan M. Chase, is whether what holds for honeysuckle holds for other invasive plants as well. “This may be something that’s occurring quite broadly, but we’re really just starting to look at the connection between invasive plants and tick-borne disease risk.”