For most of us, the upcoming Memorial Day celebrations mark the beginning of summer. But doctors see something less auspicious: the height of Lyme disease season. Now, scientists may be better able to predict the risk of Lyme disease thanks to a study of Ixodes ticks, which transmit the disease. The keys to prognostication are acorns, mice, and chipmunks.
Lyme disease is caused by a bacterium called Borrelia burgdorferi. Ixodes ticks become infected when they feed on animals that carry the bacterium, such as deer and chipmunks, and can spread the bug when they bite humans. In 2002, there were almost 24,000 reported cases of Lyme disease--which causes a characteristic circular rash, fever, arthritis, and occasional neurological symptoms--mostly in the Northeastern United States. Experts suspected that the risk for Lyme disease was greatest in years when deer were abundant and when cool, rainy days favor juvenile ticks. But there was no evidence for this theory.
To get a better handle on the predictors of Lyme disease outbreaks, researchers led by Richard Ostfeld, a senior scientist at the Institute of Ecosystem Studies in Millbrook, New York, estimated the number of infected ticks in six plots of land in Dutchess County, New York, over 13 years. The team also recorded several factors it suspected might be important predictors of Lyme disease risk, including average temperature, precipitation levels, acorn quantity, and various animal populations.
Chipmunks and acorns appear to hold the key to forecasting Lyme disease. The abundance of infected ticks in any given Lyme disease season strongly correlated with mouse and chipmunk populations the year before and acorn abundance 2 years prior, the team reports in the June issue of PLoS Biology. Taken together, these factors explained close to half of the annual variation in Lyme disease risk in the study, says Ostfeld. Juvenile forms of the Ixodes tick feed on mice and chipmunks, which in turn feed on acorns; mice and chipmunks have bigger litters the year after a good acorn season. This may account for the strong correlation seen in the study, Ostfeld says.
The results "very sharply show" that ecological conditions have important effects on the pattern of human diseases, says Andrew Dobson, an ecologist at Princeton University in New Jersey. But other experts say more work is needed. "There are a lot of places where there aren't any acorns, and yet patterns of tick risk and Lyme disease cases are similar to those reported in this study," says Thomas Mather, a tick biologist at the University of Rhode Island in Kingston. "Data from our group would suggest that, in some places, environmental factors, especially moisture levels, are very important determinants of Lyme disease risk as well."
As for the upcoming Lyme disease season? According to Ostfeld, the risk should be lower than average this year throughout eastern New York and western New England thanks to a low rodent population last summer.