THE MERCK VETERINARY MANUAL
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Amblyomma spp

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More than half of the ~140 known Amblyomma species are endemic to the New World. Amblyomma ticks are large, three-host parasites. They have eyes and long, robust mouthparts. They are more or less brightly ornamented and generally confined to the tropics and subtropics. Adults and immatures of 37 species in this genus parasitize reptiles, which together with ground-feeding birds, are often hosts of immature Amblyomma ticks that have adapted, in the adult stage, to parasitizing mammals. Their long mouthparts make Amblyomma ticks especially difficult to remove manually and frequently cause serious wounds that may become secondarily infected by bacteria or screwworms and other myiasis flies.

Several African Amblyomma that infest livestock are vectors of Ehrlichia (Cowdria) ruminantium, the rickettsial agent that causes heartwater (see Heartwater), whereas New World Amblyomma spp carry agents of monocytic and granulocytic ehrlichioses as well as several Rickettsia spp, including R rickettsii, the agent of Rocky Mountain spotted fever.

A americanum, the lone-star tick, is abundant in the southern USA from Texas and Missouri to the Atlantic Coast and ranges northward into Maine. Southward, its distribution extends into northern Mexico. Because of the changing climate, the geographic range of this species continues to expand.

The scutum is distinctive because of pale ornamentation in males and a conspicuous, silvery spot (“star”) near the posterior margin in females. Larvae, nymphs, and adults are indiscriminate in host choice and parasitize a variety of livestock, pets, and wildlife as well as people. Activity in the USA continues from early spring to late fall. Feeding sites on domestic and wild mammals are usually skin areas with sparse hair; wounds at these sites predispose livestock to attack by the screwworm fly Cochliomyia hominivorax.

A americanum is a vector of Francisella tularensis, the etiologic agent of tularemia; Ehrlichia chaffeensis, which causes monocytic ehrlichiosis in people; E ewingii, which causes granulocytic ehrlichiosis in dogs and people; and a recently described Panola Mountain Ehrlichia closely related to the agent of heartwater, which is pathogenic to at least goats and people. This tick also transmits Rickettsia amblyommii, Borrelia lonestari, and the Heartland virus pathogenic to people. A Coxiella sp symbiontic bacteria, closely related to the agent of Q fever, can be present in the majority of ticks in a population but is unknown to cause infections in people or other animals. A americanum may cause tick paralysis in people and dogs. In addition, Lone star virus (Bunyaviridae) has been isolated from a single A americanum nymphal tick that had been removed from a woodchuck (Marmota monax) in Kentucky.

A cajennense, the Cayenne tick, ranges from South America into southern Texas. This species is found most commonly in dry tropical habitats and lower elevations of subtropical highlands. As with A americanum, each active stage is indiscriminate in host choice: livestock and a large variety of avian and mammalian wildlife serve as hosts. People are severely irritated by clusters of A cajennense larvae (“seed ticks”) in wooded and high-grass areas. Most adults attach on the lower body surface, especially between the legs; some feed elsewhere on the body. Activity continues throughout the year. A cajennense is a vector of R rickettsii in Central and South America from Panama to Argentina and has been experimentally shown to transmit Ehrlichia ruminantium. Wad Medani virus (an Orbivirus, Reoviridae), an African virus transported to Caribbean islands by A variegatum-infested cattle from Senegal, has been isolated from A cajennense in Jamaica.

A maculatum, the Gulf Coast tick, is an important pest of livestock, particularly cattle, from South America to southern USA. Optimal habitats are warm areas with high rainfall, near seacoasts. Immatures usually parasitize birds and small mammals; adults parasitize deer, cattle, horses, sheep, pigs, and dogs. Adult feeding activity is chiefly in late summer and early fall but may begin later after a dry summer. Most adults infest the ears, where the feeding wounds are initial sites of screwworm infestations. Clustered feeding adults also cause much irritation to the upper parts of the neck of cattle and to the humps of Brahman cattle. A maculatum is the primary vector of R parkeri, and in some tick populations as many as 50% of individual ticks may be infected with this pathogen.

A imitator parasitizes livestock from Central America to southern Texas; it has been shown to transmit R rickettsii in Mexico. Occasional pests of livestock in tropical America are A neumanni (Argentina), A ovale and A parvum (Argentina to Mexico), A tigrinum (much of South America), and A tapirellum (Colombia to Mexico).

A testudinarium inhabits Asian tropical wooded environments from Sri Lanka and India to Malaysia and Vietnam, Indonesia, Borneo, Philippines, Taiwan, and southern Japan. Adults are particularly abundant on wild and domestic pigs and also infest deer, cattle, other livestock, and people. Immatures parasitize birds and small mammals as well as people. In India and Sri Lanka, adult A integrum and A mudlairi also parasitize livestock, wild ungulates, and people.

A hebraeum, the southern Africa bont tick, inhabits warm, moderately humid savannas of South Africa, Namibia, Botswana, Zimbabwe, Malawi, Mozambique, and Angola. Immatures feed on various small mammals, ground-feeding birds, and reptiles. Adults infest livestock, antelope, and other wildlife. Adults, attached chiefly to body areas with relatively little hair, cause serious wounds that become secondarily infected by bacteria and the screwworm Chrysomyia bezziana. Like other African Amblyomma ticks (bont ticks) that parasitize livestock, A hebraeum is an important vector of Ehrlichia ruminantium and the principal vector of Rickettsia africae, the agent of African tick bite fever, in southern Africa.

A variegatum, the tropical African bont tick, is an easily visible, brightly colored parasite found throughout sub-Saharan savannas southward to the range of A hebraeum, and also in southern Arabia and several islands in the Indian and Atlantic Oceans and the Caribbean. An eradication program is in progress in the Caribbean; St. Kitts, St. Lucia, Montserrat, Anguilla, Barbados, and Dominica qualified for "provisionally free" certification by 2002, although St. Kitts was reinfested in 2004. Host preferences are similar to those of A hebraeum but also include camels. The bites of the tropical bont tick are severe. They may result in septic wounds and abscesses, inflammation of the teats of cows, and considerable damage to hides and skins. Adults feed chiefly during rainy seasons, immatures during dry seasons. Most adults attach to the underside of the host body, on the genitalia, and under the tail.

A variegatum injuries to hosts and transmission of E ruminantium are similar to those of A hebraeum but also include the spread of acute bovine dermatophilosis (see Dermatophilosis). This tick is not considered to be an effective vector of Nairobi sheep disease virus but is a secondary vector of Crimean-Congo hemorrhagic fever virus. Dugbe virus has been isolated from A variegatum in six countries north of the equator; the Thogoto and Bhanja viruses are also associated with this tick in various areas north of the equator. Notably, yellow fever virus has been isolated from A variegatum collected from cattle in the Central African Republic and has been demonstrated to be transovarially transmitted to the progeny of infected females. Jos virus infects A variegatum from Ethiopia to Senegal and has been transported in this tick to Jamaica.

A lepidum, the East African bont tick, inhabits xeric savanna environments from northern Tanzania to central Sudan. A gemma, the gem-like bont tick, occurs in similar environments of Tanzania, Somalia, Kenya, and Ethiopia. A small variety of the buffalo bont tick, A cohaerens, is abundant on cattle in Ethiopian highlands, but from Zaire to Tanzania the larger variety of A cohaerens parasitizes chiefly Cape buffalo. Other African Amblyomma ticks of Cape buffalo and various other large mammals, including livestock, are A pomposum of humid highland forests in Angola, Zaire, Uganda, southern Sudan, Kenya, and Zimbabwe, and A astrion of West Africa and Zaire.

In Central and South America, numerous Amblyomma spp parasitize livestock and dogs, often in large numbers. Among those, Amblyomma and A ovale adults feed primarily on carnivores and A parvum on carnivores and armadillos. A auricularium has been found on wild hosts of the families Myrmecophagidae and occasionally Didelphidae, Caviidae, Chinchillidae, Hydrochaeridae, Muridae, Canidae, Mustelidae, and Procyonidae; it transmits R rickettsii in Brazil. A pseudoconcolor has been found occasionally on wild hosts of the family Didelphidae. A naponense is common on peccaries, and A oblongoguttatum has been found on a variety of hosts in several South and Central American countries. The South American tapir (Tapirus terrestris) seems to be the primary host for the adult stage of A latepunctatum, A scalpturatum, and A incisum. A dissimile is a common parasite of reptiles and true toads of the genus Bufo, from Argentina northward to southern Mexico, the Caribbean islands, and southern Florida.

Last full review/revision May 2015 by Michael L. Levin, PhD

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