LYME DISEASE

    The Blacklegged Tick, Deer Tick or Ixodes scapularis(= I. dammini)

    The genus Ixodes contains almost 250 species worldwide with at least 35 species reported from North America. Pathogens transmitted by these ticks include viruses, bacteria, rickettsia, protozoans and nematodes. In Europe, the sheep tick (I. ricinus) has been implicated in the transmission of at least eight disease-causing organisms, while the Eurasian I. persulcatus transmits four infectious agents. In the northeastern and midwestern United States, the blacklegged tick (I. scapularis) has been shown to transmit the causative agents of both human babesiosis and Lyme disease. In the western and southern United States the western black-legged tick (I. pacificus) and the blacklegged tick (I. scapularis), respectively, have been implicated in the transmission of the Lyme disease spirochete to humans.

    Life Cycle

    Like all ticks in the genus Ixodes, I. scapularis is a three host tick, i.e. each feeding stage (larva, nymph and adult) requires one vertebrate blood meal for its development. Each stage attaches to a vertebrate host, feeds to repletion, detaches, drops from the host (usually into the leaf litter) and molts to the next stage. In Massachusetts, the life cycle of I. scapularis may range from two to four years and appears to be regulated by host abundance, physiological mechanisms of the tick, and environmental factors. Typically, I. scapularis takes about two years to complete one life cycle. The adults appear to exhibit two breeding periods. Adults, resulting from spring nymphs, first emerge in the early fall and undergo a fall breeding period. While on the host animal (primarily white-tailed deer), the female tick feeds to repletion and the male mates repeatedly with several females. The females then fall to the ground and lay a cluster of several thousand eggs before dying. The second breeding period results from unengorged adults which have overwintered and by adults which have developed from overwintering nymphs. The occurrence of two breeding seasons results in a bimodal spring-summer distribution of larvae.

    The first larval activity peak is seen in May and results from females that successfully mated and deposited their eggs the previous fall. The second and much larger larval activity peak is seen in August. This activity peak results from the more successful spring breeding period. The larval stage is the only six-legged stage in the life cycle. They usually feed on small mammals for three to five days before dropping from the host to metamorphose to the nymphal stage in the leaf litter. The eight-legged nymphs are most active in the months of May, June and July. These nymphs have developed from larvae of the previous summer which have successfully fed and overwintered. Furthermore, there is a small amount of nymphal activity in the early fall, most likely resulting from the early spring larvae.

    Host Preferences

    Ixodes scapularis has been reported from a total of 80 different species of wild mammalian and avian hosts. The adults appear to prefer medium to larger sized mammals and are only moderately host specific, as they have been recorded from at least thirteen different mammalian host species. The white-tailed deer (Odocoileus virginianus) is the most commonly infested mammalian species. Populations of I. scapularis appear most abundant in areas where deer are also numerous. However, adult blacklegged ticks have also been recorded from black bears, Ursus americanus, in Wisconsin, opossums, Didelphis virginiana; raccoons, Procyon lotor, and skunks, Mephitis mephitis, in New York and woodchucks, Marmota monax; gray squirrels, Sciurus carolinensis and red fox, Vulpes vulpes.

    Immature I. scapularis are less host specific. They have been reported from at least 31 mammalian and 49 avian species. In some instances the immatures can be found on the same hosts as the adults, for example both larvae and nymphs can be found on white-tailed deer. However, in most instances the immatures prefer smaller mammalian hosts, as well as ground-nesting and ground-foraging birds. The white-footed mouse, P. leucopus is an important host for larval I. scapularis. In addition, mammals such as meadow voles (Microtus pennsylvanicus), pine voles (Microtus pinetorum), southern red-backed voles (Clethrionomys gapperi), short-tailed shrews (Blarina brevicauda), meadow jumping mice (Zapus hudsonicus), chipmunks (Tamais striatus), and eastern cottontail rabbits (Sylvilagus floridanus) may also serve as hosts to immature I. scapularis. Other animals (dogs, cats, humans, etc.) may be acceptable hosts for any of the three I. scapularis life stages. However, these hosts are usually accidental hosts and are not normally successfully parasitized.

    Habitat

    Island habitats containing dense brush and characterized by vegetation such as bayberry, poison ivy, green-briar, and scrub oak appear to be ideal for populations of I. scapularis. Questing nymphs and larvae are very abundant in wooded areas, particularly the associated leaf litter, rather than on open grassland habitats. Adult ticks are more common on high shrubs (>1 meter) and in wooded areas as opposed to grassy or low shrub (<1 meter) habitats and are more active during the spring and fall. Inland populations of I. scapularis may be extremely abundant in heavily forested areas that have a high canopy. Ixodes scapularis populations can be found in dense brushy or heavily forested areas and in any associated ecotonal habitats. They are not usually found in open or grassy areas.

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