Spring in the rural Northeast can be beautiful, but it has its drawbacks. Sure, the crocuses are blooming, I can see the ground for the first time in months, and the asparagus is starting to come up in the garden. But we’re also
inundated with ticks. Whenever I come inside from raking mulch or leaves, I spend some time pulling deer ticks off my pants, my face, and out from under my t-shirt. I paw through my kids’ hair like some sort of demented chimpanzee. Occasionally we find great swollen specimens on the kitchen floor after they fall off the dog (he’s a really hairy dog – sometimes we miss one. We’ve started shaving him in the summers.).
Although the ticks are nasty blood-sucking parasites on their own merits, I worry about them because they’re also the vector for an even nastier parasite: the spirochete bacterium Borrelia burgdorferi, which causes a little condition called Lyme disease.
Lyme disease first started to be a problem when I was a kid in New York. All while I was growing up, its spread was blamed on the growing white-tailed deer population. The growth of suburbs had produced a huge amount of deer-friendly forage and fewer people were hunting them, triggering a deer population explosion. More deer, more deer ticks, more chance of infection, QED. But a new study published Tuesday in PLoS Biology shows that we really should have been keeping an eye on the chipmunks instead.
Deer ticks eat three blood meals during their lifetime. They hatch in midsummer from eggs laid the previous fall. After hatching, the larvae go looking for a host. They’re not picky – they’ll happily take blood from mammals, birds, or even lizards. After they’ve engorged themselves, they molt, become nymphs, and go into hibernation until the following spring, when they reemerge and go looking for another meal. After the nymphs eat, they molt into adults and hide out until fall, when they find a large mammal for a final meal before they reproduce. If, during any of these meals, their host is carrying Borrelia burgdorferi, the tick sucks up the bacterium along with the blood and becomes a carrier for Lyme disease. Any animal they bite after that has a chance of getting infected, too.
For a long time, if a cool, rainy year followed a deer population spike, it was a “bad tick year.” Experts reasoned that larger deer populations produced lots of well-fed adult ticks, which would leave behind lots of eggs on the forest floor. If cool, rainy conditions followed, fewer larval ticks would dry out and die before they found hosts. Small rodents like mice and chipmunks would get heavier tick infestations. And because mice and chipmunks also happen to be one of the largest natural reservoirs for B. burgdorferi, more ticks would be carrying Lyme disease. It’s a pretty line of reasoning, but no one had ever tested it. Until now.
Richard Ostfeld and his team are based in Dutchess County, New York, which has one of the largest incidences of new Lyme cases each year. They spent more than a decade collecting data on local temperature, rainfall, the number of acorns oak trees produced, and the population densities of white-tailed deer, white-footed mice, chipmunks, and deer ticks. When they looked at their data, they found that the number of deer in the study area had almost no effect on the numbers of infected ticks. Rainfall and temperature also had little effect. The variables that mattered the most were, surprisingly, numbers of rodents and acorns. Years when ticks were abundant followed years with large rodent populations, which in turn followed years when oaks produced enormous numbers of acorns.
So ecology and life history matter. The oak trees aren’t dropping large numbers of acorns randomly. For them, it’s a reproductive strategy – they occasionally drop more seeds that their predators can eat, so some of those seeds will grow into new trees. But the seed predators – the small rodents – still get so much extra food that they can afford to have extra litters of babies, and their numbers soar. The rodent population boom is temporary, but while it lasts more larval ticks can find a bonanza of blood, and a bellyful of B. burgdorferi.
Maybe I ought to get another cat.
Ostfeld, R. S. et al. 2006. Climate, deer, rodents, and acorns as determinants of variation in Lyme-Disease risk. PLoS Biol 4(6): e145. DOI 10.1371/journal.pbio.0040145
Photo of adult deer tick by Scott Bauer for USDA