There are many ways of fighting disease, but Brian Allan from Washington University has suggested a most unusual one – a spot of weeding. Allan’s research shows that getting rid of a plant called the Amur honeysuckle might be one of the best ways of controlling an emerging human disease called ehrlichiosis. The plant, however, doesn’t cause the disease. The connection between the two is far more complicated than that.
The Amur honeysuckle is an Asian plant that’s naturally alien to American shores. But, like many species that are brought to new habitats, it has become an invader. It forms thick growths that deprive native plants of light, causing local diversity to plummet in the face of an expanding blanket of honeysuckle. This story has been repeated all over the world with different species cast as invasive villains, and different communities cast as suffering victims. But the true consequences of these invasions often go unnoticed.
The honeysuckle doesn’t just crowd out local plants; Allan has found that it also attracts white-tailed deer. Where the deer go, so do their parasites, and these include the lone star tick, the animal that spreads ehrlichiosis. Through their blood-sucking bites, the ticks spread five species of bacteria that infect and kill white blood cells. This weakens the immune systems of their hosts and causing the flu-like symptoms that accompany a bout of ehrlichiosis.
More honeysuckle means more deer, which means more ticks, which means more bacteria, which means more potentially infected humans. This invasive shrub might help to explain why cases of ehrlichiosis have gone up by around 6 times in the early part of the 21st century. In 1999, the Centers for Disease Control and Prevention recorded just 100 or so cases of ehrlichiosis in the United States. By 2006, that number had risen to just under 600.
Allan discovered this remarkable chain of events by carrying out a series of field surveys in the Missouri woodlands. He found that compared to honeysuckle-free areas, invaded zones had 18 times as much plant matter and around five times as many deer (which presumably are drawn to areas with more shelter and potential food). As a result, they also had 10 times as many ticks that were infected by ehrlichiosis-causing bacteria. Clearly, the risk of catching the disease is higher in areas that contain honeysuckle.
To test his hypothesis, Allan removed the honeysuckle from selected patches of woodland. The result: far fewer signs of deer and far fewer infected ticks. Allan also found that the presence of honeysuckle didn’t affect the odds of a tick being infected with the problematic bacteria, or their odds of survival. This suggests that the removal of the honeysuckle was indeed lowering the numbers of ticks by driving away the deer, rather than simply creating conditions that are more hostile towards ticks.
This is a good example of an invasive species increasing the burden of human disease and it’s unlikely to be the only one. Other studies have found that in the northeastern United States, the honeysuckle and the Japanese barberry (another invasive shrub) might increase the risk of Lyme disease, another tick-borne bacterial disease.
To Allan, these domino effects mean that removing invasive species isn’t just an environmental cause – it’s a public health issue too. Honeysuckle might repress local plants but through a convoluted chain of events, it could end up repressing the immune systems of local people. That should provide even more incentive to deal with these invaders. As Allan himself writes,
“Our finding that removal of the invader mitigates disease risk, coupled with the benefits of invasive plant removal to wildlife communities, suggests a potential “win-win” scenario for biodiversity conservation and human health.”
Reference: PNAS http://dx.doi.org/10.1073/pnas.1008362107
More on invasive species: