Invasive plants increase risk of tick-borne disease in suburbs as well

"You don't have to go out into the woods anymore," says tick expert Brian F. Allan, PhD, who just completed a postdoctoral appointment at Washington University in St. Louis. "The deer are bringing tick-borne disease to us."

So, it stands to reason that anything deer like, might increase the risk of tick-borne disease for people.

The invasive plant bush honeysuckle, for example.

Yes, that leafy shrub with the lovely egg-shaped leaves on arching branches, fragrant white or yellow flowers and the dark red berries so attractive to birds.

Called bush or Amur honeysuckle, Lonicera maackii derives from the borders of the Amur River, which divides the Russian Far East from Manchuria. Its Latin name honors Richard Maack, a 19th-century Russian naturalist.

"I've spent a lot of time in honeysuckle," Allan says, "and I can tell you there are deer tunnels through it. So if you get down low, you can actually move through honeysuckle pretty efficiently. And you pick up a lot of ticks while you're back in there."

An interdisciplinary team made up of ecologists, molecular biologists and physicians from Washington University in St. Louis and the University of Missouri-St. Louis tested Allan's suspicions by experiment in a conservation area near St. Louis.

In this part of the country, the tick of concern is Amblyomma americanum, called the lone star tick because the adult female has a white splotch on her back. The tick-borne diseases are the ehrlichioses, caused by bacteria in the genus Ehrlichia, named for the German microbiologist Paul Ehrlich.

As Allan and his colleagues report this week in the Proceedings of the National Academy of Sciences, 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.

Hard as it may be to believe, given the long chain of interactions needed to get there, the presence of bush honeysuckle substantially increases the risk of human disease.

"But that's exactly what is happening," says Jonathan M. Chase, professor of biology in Arts & Sciences and a collaborator on the project. The big question now, says Chase, who is also director of Washington University's Tyson Research Center, 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."