Acorns, and not deer, may be the key to how big a risk Lyme disease is, researchers reported Thursday.
How many acorns are produced by oak trees in a forest may eventually determine how many infected ticks are out there to spread the disease, which can cause fever and sometimes permanent physical damage to victims, they said.Writing in the journal Science, Clive Jones and Rick Ostfeld of the Institute of Ecosystem Studies in Millbrook, New York, said they hoped to eventually come up with a way to predict the risk of Lyme disease.
"What our data suggests is that the risk of Lyme disease might be higher two years after an acorn crop, so it is potentially feasible to risk-rate the forest," Jones said in a telephone interview.
Just as the U.S. Forest Service posted signs telling of a high, medium or low risk of forest fires, perhaps they could post signs warning of the relative risk of being bitten by a tick infected with Lyme disease, Jones suggested.
But not quite yet. "The final link between the number of infected nymphs (ticks) - the ones that get on you and bite you - we haven't tied that one down yet," he said.
What Jones's team did find was that acorns are important to populations of the mice that infect the ticks in the first place, and the deer that pick up the ticks and carry them to places from which they get onto people.
Oak trees produce large crops of acorns every two to five years, but produce few or none at all in between. These acorns are the major food source for white-footed mice, as well as white-tailed deer.
Mice and deer both carry the black-legged tick. Mice infect the ticks with the spirochete bacteria that causes Lyme disease - Borrelia burgdorferi.
"Adult ticks feed and mate on white-tailed deer before dropping to the ground in autumn, laying eggs the following spring or early summer," they wrote.
The more acorns there are around, the longer the deer spend in the woods and the more likely the ticks are to get on them.
Jones' group did an experiment when the acorn crop was light in 1995, adding acorns to some areas and sticking some into mouse nests. Sure enough, the mouse population grew, too.
Furthermore, there were more ticks on the mice where acorns had been added.
So would removing mice solve the problem? Probably not, said Jones. Not only would it be hard to do, but the mice also eat gypsy moths.
"Our results provide strong support for the idea that a chain of events links acorns to gypsy moth outbreaks and Lyme disease risk," they wrote. As is often the case with real-life nature, these links are complex.
Lyme disease is spread to humans by ticks from deer and is prevalent in the northeastern United States and Canada. It usually starts with a rash, followed by fever and headache and in some cases can lead to heart, joint or neurologic disease.
The Centers for Disease Control and Prevention reported a record 16,461 Lyme disease cases in the United States in 1996, a 41 percent increase from 1995.
Its name is taken from the town of Lyme, Connecticut, where it was first identified in 1975.