Title: Invasive shrub increases risk of human disease (via ticks, deer and bacteria)
[Discover Magazine Oct 11, 2010 by Ed Yong] --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."
