The Ecology of Lyme-Disease Risk
Complex interactions between seemingly unconnected phenomena determine risk of exposure to this expanding disease
Life Cycle of the Tick
Lyme disease is virtually restricted to forested landscapes, since these are the habitats in which Ixodes ticks find hosts and complete their life cycles. Several species of Ixodes ticks have recently been found to transmit Lyme disease, and each species has a somewhat different life cycle as well as a slightly different menagerie of vertebrate hosts. The following generalized life cycle pertains to I. scapularis in the northeastern and midwestern United States. Larval ticks hatch from eggs in midsummer, and newly emerging larvae seek a host at that time. Because the larvae are about the size of the period at the end of this sentence, they are virtually impossible to detect when they are crawling on or embedded in one's skin. These ticks are weak crawlers and cannot hop or fly; therefore a warm-blooded vertebrate must pass extremely close for a larval tick to climb aboard. Larval ticks have catholic tastes and will feed readily on a wide variety of species of small mammals and birds (and lizards in the southern and western United States). The ticks locate a suitable site on the host and anchor their mouth parts for a single two-to-three-day blood meal, after which they drop off the host. Within about a month, the larval tick molts into the second post-egg stage, the nymph, and remains quiescent on or just beneath the forest floor for all of the following winter.
Late the next spring or early in the summer, the nymphs, now about the size of a poppyseed, actively search for a host. They either remain on the surface or climb into low vegetation to await the close passage of almost any species of warm-blooded vertebrate. Again, the tick inserts mouth parts into the host and feeds for about three to five days. Once it has become engorged with blood, the nymph drops off, and after approximately three months, molts into the adult stage.
The peak season for adult-tick activity is midautumn. An adult tick, about the size of a sesame seed, is somewhat more mobile than a juvenile tick and typically climbs as high up as one meter on vegetation to seek hosts. The result of its upward mobility is that the adult tick usually encounters and parasitizes large mammals, particularly the white-tailed deer Odocoileus virginianus. The specificity of the adult-stage tick for white-tailed deer is the reason why I. scapularis was formerly called the deer tick.
Adult ticks use deer not only as their terminal host but also as their mating grounds. Male ticks, which wander about the deer and feed only intermittently, often mate with feeding females, whose mouth parts are embedded in the deer’s skin. Females feed steadily for four to five days, after which they drop off the deer and overwinter in an engorged state, seeking shelter from extreme weather conditions beneath leaf litter in the forest. The following spring the engorged females lay eggs in masses of several hundred to a few thousand, and these eggs hatch a few weeks later into a new cohort of larvae.
The vast majority of larval ticks—well over 99 percent—hatch from eggs free of the Lyme-disease bacterium, indicating that the disease agent does not pass readily from adult through the ova to her offspring. However, larval ticks may become infected if they feed on an infected host, and these acquired infections are retained through the molts from larva to nymph and from nymph to adult. Work by Thomas Mather at the University of Rhode Island, Joseph Piesman at the Centers for Disease Control and Prevention and their colleagues have revealed that the white-footed mouse Peromyscus leucopus is the host primarily responsible for infecting larval ticks with the Lyme spirochete, and thus the mouse is considered the principal natural reservoir for the disease agent.
In Lyme-disease-endemic areas, approximately 25 to 35 percent of nymphs and 50 to 70 percent of adult ticks are infected with Borrelia burgdorferi. Because nymphal ticks are much smaller than adults and hence more difficult to detect on one’s clothing or skin, and because their peak activity period is from May to July, when most people also spend a lot of time outdoors, most cases of Lyme disease are probably caused by bites from nymphs rather than from adult ticks. Research by Durland Fish at Yale University and colleagues indicates that the primary ecological risk factor in the Lyme-disease epidemic is the number of infected nymphs within areas people use recreationally or domestically from late spring to midsummer.