At the drop of a tick: a corps of Lyme-disease fighters meets its match in an army of arthropods - includes related article

Science News, March 25, 1989 by Ingrid Wickelgren, Rick Weiss

Ixodes dammini has been found on 12 mammalian species and 18 bird species, according to Fish and entomologist John E. Anderson of the Connecticut Agricultural Experiment Station in New Haven. In the West, entomologist Robert S. Lane and his associates at the University of California, Berkeley, found 80 species of vertebrates hosting California's and Oregon's most important Lyme-carrying tick, I. pacificus. While engorging an animal's blood, the tick unknowingly may receive or donate a spirochete, but not all animals are susceptible to infection.

White-tailed deer are likely the primary host of the adult tick in the eastern United States. Ticks lay their eggs in the spring and the emerged larvae feed in late summer. The larvae remain quiet throughout the winter and develop into nymphs in the spring. The nymphs feed in May and June, just before the peak appearance of human infection in early July. Scientists believe tick larvae acquire the infection from white-footed mice and then transmit it to humans as nymphs. The adults, which feed any time from late fall to late spring, probably do not account for many human cases because they are large enough to be detected and removed before the spirochete is transmitted, Fish says.

I. dammini often eats its main larval meal on the foot of a white-footed mouse. At several Massachusetts sites, Spielman and his co-workers found that 80 to 90 percent of the larvae dropping off mice just after their early autumn feeding were infected with B. burgdorferi, says parasitologist Sam Telford, who works in Spielman's laboratory. "We've taken ticks off of all the other animals in these areas, [and although] many animals have ticks, few others produce infected ticks," Telford says.

Checking for adult I. dammini on various mammals living on Long Island, N.Y., Spielman found 93 percent of the ticks on deer and the remaining 7 percent on dogs and other animals. Deer provide all the nourishment for the adult tick and its typical brood of about 2,500 eggs. However, the deer don't seem to carry the spirochete. Spielman found that only about 1 percent of ticks dropping off the backs of deer were infected. This is no more than the tick's natural infection rate -- the precentage of young that contract the spirochete from an infected parent -- so it appears deer do not increase the proportion of infected ticks.

Ground-feeding birds also serve as important hosts of I. dammini, but how important is hard to determine because birds are hard to track, Fish says. Scientists do know that birds transport the tick long distances and so contribute to the spread of the disease. In addition, Anderson has shown that birds carry the spirochete and are able to infect ticks.

In the West, I. pacificus is most often found on lizards and the black-tailed jackrabbit, Lane says. Lizards aren't easily infected and so help dilute the prevalence of the disease, but jackrabbits can infect ticks. Probably because more lizards than jackrabbits live in the West, the infection rate of I. pacificus is low, Spielman says. Only 1 to 2 percent of I. pacificus are infected, compared with 30 to 60 percent of I. dammini. This difference in infection rates probably explains the lower incidence of the disease in the West, Lane says.