highly “competent” reservoir of B. burgdorferi, transmitting infection to most ticks that bite it. If the immune response is strong and effective, damping down the level of blood-borne spirochetes, that species will be a relatively less competent reservoir. Studies by Ostfeld’s group, involving captive animals and the ticks feeding on them, showed white-footed mice to be the most competent of reservoirs for the Lyme disease spirochete. Chipmunks were a distant second in reservoir competence, with shrews close behind them.
Further complication: Besides being very competent as reservoirs, white-footed mice are also inefficient groomers, poor at clearing off the ticks, which target especially their faces and ears, so that a high percentage of their ticks survive into later stages. Shrews are also inefficient self-groomers, unfortunately for them, and therefore mice and shrews contribute disproportionally to the feeding, infecting, survival, and successful metamorphosis of larval ticks. By this standard, chipmunks were third in overall importance.
What matters perhaps less than their relative rankings is the more general point that these four little mammals together weigh so heavily in the system. Summary statistics compiled by Ostfeld and his gang indicate that up to 90 percent of the infected nymphal ticks “questing” for their next hosts, in a typical forest patch near Millbrook, New York, had taken their larval blood meal from (and therefore been infected by) either a white-footed mouse, a chipmunk, a short-tailed shrew, or a masked shrew. Those four hadn’t fed 90 percent of all blacklegged nymphs but, because of the differences in reservoir competence and grooming efficiency, they had fed 90 percent of those that became infected and dangerous to people. Should I repeat that? Four kinds of small mammal fueled nine-tenths of the disease-bearing ticks.
So forget about deer abundance. White-tailed deer are involved in the Lyme disease system, yes, but involved like a trace element, a catalyst. Their presence is important but their numerousness is not. The littler mammals are far more critical in determining the scale of disease risk to people. Adventitious years of big acorn crops, yielding population explosions of mice and chipmunks, are more likely to influence the number of Lyme disease cases among Connecticut children than anything that deer hunters may do. Beyond helping the blacklegged tick (infected or uninfected) to survive, white-tailed deer are almost irrelevant to Lyme disease epidemiology. They don’t magnify the prevalence of infection in the forest. They don’t pass the spirochete to humans or to newly hatched ticks. They’re dead-end hosts, Ostfeld told me.
Then again, he said, “We also happen to be dead-end hosts, in that, once we’re infected, the infection goes nowhere. It stays in our body. It doesn’t go back into ticks. So we’re an incompetent reservoir.” Mice and shrews make the ticks sick; the ticks make us sick; and we don’t make anybody sick. The Borrelia burgdorferi spirochete, if a person catches it, stops there. It doesn’t travel on a sneeze or a handshake. It doesn’t move downwind. It’s not an STD. This is interesting ecologically but probably cold consolation to anyone suffering from Lyme disease.
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Ostfeld is sensitive to the human toll, not just to the wondrous intricacy of Borrelia burgdorferi dynamics in American forests. He showed me some figures from Dutchess County, New York, which includes Millbrook and the Cary Institute, between 1986 and 2005. The twenty-year trend of human infections was steeply upward, with especially high peaks in 1996 and 2002. People were suffering. In 1996 there were 1,838 reported cases of Lyme disease. After that came a sizable decline until, in 2002, again almost two thousand new cases were reported.
Still, it’s best understood as an ecological phenomenon, not just a medical problem. “Lyme disease in humans exists because we are sort of unwitting victims of a wildlife-tick interaction,” Ostfeld said. “We’re interlopers into this system where ticks and these hosts—the reservoir hosts—pass bacterial infections back and forth.” One way of construing those peaks in 1996 and 2002, he explained, is that they reflect autumns of bounteous production in the local forests. White-footed mice love acorns and, because the mice reproduce quickly and mature quickly, responding to food abundance with bursts of heightened fecundity, big masting events are often followed (after a two-year lag) by big increases in the mouse population. One pair of mice, given circumstances of plentiful food, could produce a net gain of fifty to seventy-five mice within a year. More acorns, more mice, more infected ticks, more Lyme.
Dutchess County is a halcyon Yankee getaway just east of the Catskills and only two hours