THE MERCK VETERINARY MANUAL
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Dermacentor spp

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Of the 36 Dermacentor spp, 19 inhabit temperate zones. Of the tropical species, D nitens is of major veterinary importance, although others may transmit zoonotic infections, and adults may be common on wildlife such as pigs, deer, and antelope. Immatures infest chiefly rodents and lagomorphs. Dermacentor spp in cold areas and D (Anocentor) nitens in tropical America have specialized life cycles and seasonal dynamics of activity, each of which must be considered separately. With exception of D nitens, D albipictus, and D dissimilis, the Dermacentor life cycle is of the typical 3-host pattern.

D nitens, the 1-host tropical horse tick previously assigned to the genus Anocentor, is of considerable veterinary importance. It originally parasitized deer (Mazama) in the forests of northern South America. With the introduction of Equidae and other livestock into its habitat, it adapted to these animals. Spending its entire parasitic life deep in the hosts' ears, this parasite was easily spread by human activities to other areas of the Americas, including Florida and Texas. In addition to ear cavities, each active stage may infest nasal passages and the mane, ventral abdomen, and perianal area. D nitens transmits Babesia caballi transovarially to successive generations and is important in the horse-racing industry. It also is an experimental vector of Anaplasma marginale to cattle.

Another American 1-host species, D albipictus, the winter or moose tick, ranges from Canada and northern USA into western USA and Mexico. A brownish form, sometimes called D nigrolineatus, is distributed from New Mexico to southern and eastern USA and may merit subspecies if not full-species rank. The larval-nymphal-adult feeding period on a single host (moose, deer, elk, or domestic cattle or horses) extends from fall to spring. Heavily infested hosts may die. D albipictus causes the often fatal “phantom moose disease” of Canada, is a secondary vector of Colorado tick fever virus, and an experimental vector of B caballi; it is a natural vector of A marginale in Oklahoma.

In Mexico and central America, D dissimilis parasitizes a variety of equine and ruminant hosts and may be a 1-host tick on horses.

The Rocky Mountain wood tick, D andersoni, is found from Nebraska westward to the western mountains (Cascades and Sierra Nevadas), in northern New Mexico and Arizona, and in western Canada.

The American dog tick, D variabilis, is found west of the Cascades and Sierra Nevadas, in Mexico, from Montana to Texas and east to the Atlantic, and in eastern Canada. Both species may cause tick paralysis in livestock, wildlife, and humans. They are the primary vectors of Rickettsia rickettsii, the agent of Rocky Mountain spotted fever (see Rocky Mountain Spotted Fever). D andersoni is also the chief vector of Colorado tick fever virus and transmits Powassan virus, A marginale, A ovis, and the agents of tularemia and Q fever. D variabilis is an experimental vector of A marginale, Babesia caballi, and B equi. In addition, sawgrass virus, E chaffeensis, and E ewingii have been detected in questing D variabilis adults. Adults of both species parasitize livestock and wildlife including deer, bison, and elk, but those of D variabilis prefer skunk, raccoon, puma, etc, and domestic dogs. Immatures feed on rodents and other small wild mammals. A related, biologically similar species, D occidentalis, is restricted to the Pacific lowlands and foothills from Oregon to Baja California and is a natural vector of A marginale.

In western USA and Mexico, D parumapterus, D hunteri, and D halli parasitize various hares and rabbits, mountain sheep, and peccaries, respectively. These ticks seldom make contact with livestock. D hunteri is an experimental vector of A marginale and A ovis. In Costa Rica and Panama, D latus infests tapirs.

In Eurasian steppes, forests, and mountains, D marginatus, D reticulatus, and D silvarum, collectively, are vectors of numerous viruses and Babesia bovis, B caballi, B equi, B canis, Theileria ovis, and A ovis, together with the agents of tularemia and Q fever, and Russian spring-summer encephalitis. D marginatus is found in forests, marshes, semideserts, and alpine zones from France to southwestern Siberia, Kazakhstan, Xinjiang Uygur Autonomous Region of China, Iran, and northern Afghanistan. D reticulatus ranges from Ireland and Britain to northwestern Siberia and Xinjiang, China, in meadows, floodplains, and deciduous and deciduous-conifer forests. D silvarum ranges from central Siberia and northeastern China to Japan in marshes, meadows, shrubby and secondary forests, and farmlands in taiga forest areas. Some males in populations of each of these 3 species remain attached to the host during winter. Adults and immatures may overwinter on the ground. Greatest adult activity is from early spring to summer with a lower peak in fall. Larvae and nymphs are active from spring through fall. The life cycle may be completed in 1 yr or extended by one or more summer or winter diapauses to 2–4 yr.

About 12 other Dermacentor spp inhabit certain lowland, mountain steppe, and semidesert areas of temperate Asia. Their adults are commonly taken from camels, cattle, horses, sheep, and goats. In tropical Asia, the several species of the Dermacentor subgenus Indocentor are parasites of wild pigs; they also infest larger wildlife but seldom if ever feed on livestock.

Last full review/revision July 2011 by Michael L. Levin, PhD

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