The “deer tick”, officially known as the black-legged tick, is essentially the sole vector of the Lyme disease spirochete, Borrelia burgdorferi, as well as the agents of anaplasmosis, babesiosis, and deer tick virus. This tick’s life cycle requires at least two years for completion.
Following its first appearance in southern Maine in the 1980’s, this tick advanced along the coast and then inland, and may now occasionally be encountered in northern Maine. A mated adult female deer tick, having obtained a blood meal from a white-tailed deer, dog, cat, or other large mammal in the fall or early spring, may deposit up to 3000 eggs in late May and early June.
The Lyme spirochete is not passed from an infected female deer tick to her eggs. Uninfected larvae emerge in mid-summer and soon seek a blood meal, primarily from mice, other small mammals and certain songbirds. Many of the animals they feed on, particularly mice and chipmunks, will have been previously infected with the agents of Lyme and other tick-borne diseases; it is from these “reservoir hosts” that they become infected.
After over-wintering, larvae molt to nymphs which seek a second blood meal in the spring, passing on the infections they acquired as larvae to the next year’s crop of small mammal/avian hosts.
Nymphs also feed on humans, dogs, and horses, and other hosts. Their tiny size and painless bites may allow them to remain undetected through the ~36 hours it takes for spirochetes to be transmitted from a feeding tick.
Most human Lyme disease results from the bite of undiscovered nymphs in the summer. Dogs and other furred animals are more frequently infected by adult ticks which escape detection. In Maine, nymphs peak in late June and July, which is when ~65% of the human cases of Lyme disease are reported. After repletion, deer tick nymphs drop to the leaf litter, and in early fall molt to adult males and females.