Ticks in Maine
Ixodes scapularis (dammini), the "deer tick," also known as the "black-legged tick", is the principal vector of the Lyme disease spirochete in the northeastern United States. Not all deer ticks are infected, however. Limited studies in Maine have shown that although in some sites over half of the adult ticks sampled contained spirochetes, rates may vary considerably, even in adjacent areas. Infection rates in questing nymphs are typically somewhat lower. Immature stages feed on small mammals such as mice, and adults prefer deer, but all stages may feed on humans and domestic animals. Although rare, the agents of two other infectious diseases, human granulocytic erlichiosis (HGE), also called anaplasmosis, and babesiosis have also been found in this species of tick. Below is a chart, explaining the two-year life cycle of the deer tick.
The deer tick's life cycle requires at least two years to complete. Each consecutive stage of the deer tick - larva, nymph, and adult - feeds only once. Sometime during the summer, the eggs hatch into larvae. The larvae feed on small mammals, particularly mice, and birds in the summer and early autumn. Then they become dormant for the winter until the following spring when they molt into nymphs. The nymphs then feed on small mammals such as mice and chipmunks, and subsequently molt into adults. The adult ticks feed and mate on larger animals, particularly deer, in the fall and early spring. Engorged female ticks drop off of these animals and lay their eggs on the ground, completing the life cycle. It is during the larval and nymphal stages of life that ticks become infected with Lyme disease.
The seasonal appearances of the larval, nymphal and adult stages of the deer tick in Maine are shown in a bar graph developed from ticks submitted for identification over the last ten years. A map demonstrating the emergence of the deer tick in Maine since 1989 has been generated from this same data.
Ixodes cookei, the "woodchuck tick" is widely distributed in Maine and is the second most common species of Ixodes found on people. It has not been associated with Lyme disease transmission. It usually feeds on wild animals, such as woodchucks and raccoons, but will also feed readily on humans and domestic animals. This tick is known to be a vector of Powasson virus. Rare cases of encephalitis have occurred in Maine in people with Powasson virus.
Ixodes marxi, the "squirrel tick," has not been associated with Lyme disease. It is commonly found on squirrels but will occasionally bite humans.
Ixodes muris is occasionally found in Maine. Usually it is found only on voles and mice, but it may bite humans, cats, dogs, and birds. A recent report indicates that it is a weak vector of Lyme disease, but no human cases of Lyme disease have been attributed to it. We have associated its bite with a reaction in cats, dogs and other domestic animals characterized by pain, swelling, fever, lethargy, and loss of appetite. If this reaction is observed we are very interested in receiving the tick, alive, if possible, and relevant information.
Ixodes angustus is usually found only on voles and mice and is common in many parts of Maine, but it is very rarely found on humans or domestic animals.
Dermacentor variabilis, the "American dog tick," is not a vector of Lyme disease. This tick is particularly abundant in southwestern Maine but its range has been expanding in recent years. Immature stages feed on voles and other small rodents, but adults are often found on humans, dogs, and other domestic animals. The adults, found from May through July and rarely later in the season, are larger than Ixodes ticks and can be distinguished by characteristic white markings. The tick is the vector of Rocky Mountain Spotted fever in the eastern United States. There have not been cases of Rocky Mountain Spotted fever reported in Maine. A bar graph showing the seasonal abundance of dog ticks is shown. Dog ticks are not active in October and November when adult deer tick activity is at it's peak.
Dermacentor albipictus, the "winter tick," or "moose tick" is found on moose and deer and occasionally on horses, cows, dogs and humans particularly in central and northern Maine. Large numbers of the tiny larvae may be encountered in the fall, particularly in habitat where moose are found. This tick has not been associated with Lyme disease.
Haemaphysalis leporispalustris, the "rabbit tick," is usually found only on rabbits and birds. Although it has been reported to be rarely infected with Lyme disease bacteria, it does not transmit Lyme disease to humans.
Amblyomma americanum, the "Lone Star tick," is becoming more frequently found in Maine, most often on people traveling from states to the south where it is very common. It has been shown to carry a different spirochete, which in humans may produce a rash and some symptoms similar to Lyme disease.
Rhipicephalus sanguineus, the "brown dog tick" or "kennel tick," is widely distributed over the world, but only rarely found in Maine. Dogs are the principal host. It has not been associated with Lyme disease transmission.
Other species of Ixodes, I. brunneus (found on migratory birds), I. dentatus (found on rabbits and hares), I. uriae (found on marine birds), and Ixodes gregsoni (found on mink, weasel and marten) have occurred in Maine, although very infrequently. The bird tick Haemaphyhysalis chordeilis and Ixodes banksi (found on beaver and muskrat) may occur in Maine as well. There is no record of soft ticks, Family Argasidae, being collected in Maine.
Elsewhere in the country ticks may carry other diseases, such as Rocky Mountain spotted fever, tularemia, Powassan virus, and Q fever.
Top Row: Ixodes scapularis (dammini), the deer tick which transmits Lyme disease. Left to right: nymph, adult male, adult female, engorged adult female. Nymphs are most common May through July. Adults appear in the fall and early spring (graph).
Bottom Row: Dermacentor variabilis, the American dog tick, which is not thought to transmit Lyme disease. Left to right: adult male, adult female, engorged adult female. Adults are most common in May, June, and July. Note that the adult dog ticks are somewhat larger than adult deer ticks, and have characteristic white markings on the dorsal (top) side.
Photo by Jack Milton, Portland Press Herald