ARTHROPOD MUSEUM NOTES
Number 28 • September 17, 2004 • Jeffrey K. Barnes
Genus and species: Ixodes scapularis Say
Adult blacklegged ticks are about the size and shape of a sesame seed when not fed, increasing to about 0.13 – 0.25" (3 - 6 mm) when engorged with a blood meal. They are dark reddish brown, and hard dorsal plate (scutum) is uniformly colored. The American dog tick is larger, and its scutum bears white markings. Male deer ticks are smaller than females, and the scutum completely covers the upper surface.
Black-legged ticks, also known as deer ticks, occur in the eastern half of the United States, southern Canada, and northern Mexico, where they live in shrubby and wooded areas and adjacent grasslands. They have a two-year life cycle, and they take their meal of blood only three times. In spring and summer, six-legged tick larvae hatch from eggs in the soil, and they feed on blood in summer and early fall. The next spring they molt into eight-legged nymphs, which feed on blood in late spring and summer. Mice and and other small mammals, some ground-frequenting birds, skinks, and glass lizards are important hosts for larvae and nymphs. In the fall, nymphs molt into adults, which feed and mate on large mammals, especially deer, cattle, dogs, and other medium to large sized mammals, in the fall and early spring. A relationship has been observed between the abundance of deer and the abundance of blacklegged ticks. Female ticks drop off their hosts and lay eggs on the ground. Ticks are known to be active at temperatures above 35°F (2°C).
Tick populations can be detrimental to livestock, and females can cause tick paralysis in dogs. However, this species is best known as the carrier of Lyme disease. Lyme disease is now the most common arthropod-borne illness in the United States. It is a non-contagious, inflammatory condition caused by the spirochete bacterium Borrelia burgdorferi, transmitted primarily by the bite of the tick in the Northeast. Lone star ticks and American dog ticks also transmit the disease in Arkansas. Ticks become infected with Lyme disease bacteria in their younger stages, when they feed on small animals, especially the white-footed mouse in the Northeast. Nymphs and adults transmit the disease to other animals, including humans, dogs, and cats. Most Lyme disease cases are caused by bites of nymphal ticks from May through August, and the bitten individuals may be unaware of them because the nymphs are so small and their bites are painless. The autumn bites of larger adult ticks are more easily spotted. Only a small percentage of individuals reporting bites by known tick vectors in established endemic areas become infected with the disease. Early stages of Lyme disease are marked by fatigue, chills and fever, headache, muscle and joint pain, swollen lymph nodes, and a characteristic skin rash, erythema migrans, which appears within a few days to a month after the bite of an infected tick. The characteristic skin lesion of acute infection occurs in up to 80% of Lyme disease patients. It is a ring of redness that slowly expands from the site of the tick bite, and it often shows central clearing, producing a bull's eye appearance. Later stages of the disease can involve cardiac, arthritic, or neurological symptoms.
Borrelia burgdorferi is widespread in Arkansas, where it has been found infecting ticks, lone star ticks, and American dog ticks, but at the present time infection rates are low. Wild rodent reservoirs include the mouse (Peromyscus maniculatus), marsh rice rat (Oryzomys palustris), hispid cotton rat (Sigmodon hispidus), cotton mouse (Peromyscus gossypinus), white-footed mouse (Peromyscus leucopus). Arkansans are at low risk of contracting Lyme disease. In the 10-year period 1994 through 2003, cases of Lyme disease reported to the Arkansas Department of Health numbered only 110. The peak years were 1996 and 1997, with 27 cases each. In 2003 no cases were reported in Arkansas. Further information on the incidence of Lyme disease in Arkansas and other states can be found at the American Lyme Disease Foundation web site. Further information on Lyme disease can be found on the Centers for Disease Control web site.