In 2002, a record 18,000 cases of the disease, which is caused by being bitten by an infected deer tick, were reported to the U.S. Centers for Disease Control and Prevention.
New research suggests the weather two years ago plays a role in how active the tick population will be this year.
"Given that the moisture index was somewhat above average in the summer of 2001, we can expect that the number of cases [of Lyme disease] will be high this summer," predicts study author Susan Subak, an environmental scientist at the University of East Anglia in Norwich, U.K. Her report appears in the current issue of the American Journal of Epidemiology. "The 2002 winter was warm, but this weather factor is less important than moisture."
Ticks feed off rodents such as mice and chipmunks, but also small animals such as dogs. Once fed, the ticks nest, and a year later produce eggs that may go on, in the following year, to infest humans with Lyme disease.
The black-legged tick that causes Lyme disease is most commonly found in the northeastern, mid-Atlantic and upper north-central regions of the United States.
Lyme disease is an acute inflammatory disease characterized by a skin rash, joint inflammation and flu-like symptoms. If untreated, it can lead to serious illness, such as painful arthritis or coronary problems.
A high moisture index is the ideal condition for those ticks that have fed on a host (such as a mouse or chipmunk) and are rendered immobile for a while to survive, Subak explains. They thrive in moist soil. "By wetter and moister I don't mean necessarily more rainfall," she says. "The moisture index is a measure of soil measure" and other factors that take into account such things as cumulative precipitation.
Subak also believes warm weather contributes to higher incidence of Lyme disease by allowing the white-footed mouse, which is often the host, to survive more easily.
To add credence to her hypothesis, Subak analyzed the June moisture index in seven U.S. states for two years previous to her analysis of the incidence of Lyme disease reported in those states, and compared the two.
For all seven states, there was a significant positive correlation between moist conditions and higher rates of the disease two years later.
"The surprising finding is that the two-year lag between moisture conditions and Lyme disease is present in all seven states," Subak says. "The presence of the lag suggests that the ticks die after their blood meal rather than before, but this is not an obvious result because a tick is less likely to become dehydrated when it is engorged and filled with fluids." However, when a tick falls off the host, it is relatively immobile and its chances of survival improve if it lands on a moist spot, she says.
A theory suggested by other researchers is that Lyme disease incidence varies with acorn production, which is a rodent host food supply.
However, recent studies have shown that two years after the best acorn production in central Massachusetts, Lyme disease there was at a record low. "Acorn availability is probably not the leading cause of Lyme disease variability," Subak notes.
David Weld, executive director of the American Lyme Disease Foundation, contends that neither acorns nor Subak's theory are absolutes. "There are probably a million different factors that affect the life cycle of a tick," he says.
Weld says there can be thousands of ticks in one area, but only a few found just a half mile away, so variation is great and "probably not all tied in with the weather."
While he would like to see the tick population eradicated, that is not likely to happen, he says, but he adds that advances in reducing the population are promising. Bait boxes that trap the hosts and treat them with pesticides before releasing them have shown great promise, Weld says.