- Created on Tuesday, 15 April 2014 11:10
Lyme Disease in Vermont: An Occupational Hazard for Birders
November 1 was a warm, sunny day in Vermont a few seasons ago. As I walked out of the woods at the LaPlatte River Natural Area in Shelburne, I looked down at my pant legs and saw them-ticks-a dozen or more on each leg. As I brushed the tenacious ticks away I thought, “Vermont-we’ve got a problem”.
Lyme disease is the most common tick-borne disease in the country, accounting for over 30,000 new cases each year. In recent years Lyme disease cases in Vermont have skyrocketed. In 2013 nearly 900 probable or confirmed cases were reported to the Vermont Department of Health, placing Vermont first in the country in terms of the incidence of this disorder. Other states with a high incidence of Lyme disease included Maine and New Hampshire. What is going on here?
In North America Lyme disease is caused by a spirochete called Borrelia burgdorferi. This microorganism is transmitted by the bite of black-legged ticks (Ixodes scapularis), formerly known as deer ticks. These ticks are commonplace in our forests and are increasing, due in part to reforestation of the landscape and perhaps the effect of climate change. But this is not the whole story. Ixodes ticks have a complicated life cycle. These ticks do not live forever, thank God. Rather, they live for two years during which they undergo three successive molts, that is transformation to a larger size and different form. After hatching from an egg these stages of development are called larva, nymph, and adult. At each one of these stages the tick must ingest a blood meal from an appropriate host or else it dies. Many hosts can serve as blood donors-mice, chipmunks, deer, even birds, especially birds that nest or forage on the ground like sparrows, robins, thrushes, and ovenbirds. Once the ticks ingest a blood meal they drop off the host, hide in the leaf litter while they undergo a molt to the next stage of development, and lurk in the brush waiting to attach like a burr to their next victim. Ultimately, adult male and female ticks meet on a host and reproduce. White-tailed deer appear to be the preferred host for adult ticks. At this point the pregnant female, after ingesting a third blood meal, drops off the host and lays her eggs on the ground.
If the ticks are infected with B. burgdorferi they can transmit this spirochete to their blood donors. However, not all of their hosts are “competent” to serve as a reservoir for this infection in nature. To qualify as a reservoir three conditions must be met-availability, poor grooming behavior, and an ability to tolerate chronic infection with B. burgdorferi. The mammals that best fit these qualifications are white-footed mice, chipmunks, and shrews. White-footed mice, in particular, are the most abundant and competent reservoir host.