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Rhipicephalus spp

By Michael L. Levin, PhD, Rickettsial Zoonoses Branch, Centers for Disease Control and Prevention

Approximately 60 of the 81 rhipicephalid species are found in sub-Saharan Africa. The other rhipicephalid species have their origins in Eurasia and northern Africa, with R sanguineus and R (Boophilus) microplus being spread by human activities into Asia, Australia, and the Americas. Adults of most species parasitize wild and domestic artiodactyls, perissodactyls, or carnivores. Immatures feed mostly on smaller mammals; however, of those that parasitize rodents or hyraxes, and of those that parasitize artiodactyls, a few feed on the same host as the adults. The rhipicephalid life cycle is typically three-host, but in the Mediterranean climatic zone (long, warm summer with low rainfall) R bursa has a two-host cycle. In sub-Saharan Africa with long dry seasons, R evertsi and R glabroscutatum also have two-host cycles. In contrast, each of the five species in subgenus Boophilus has a one-host life cycle that may be completed in 3–4 wk (see below).

A number of Rhipicephalus spp have long been difficult to identify or have been incorrectly identified. Current concepts of tick phylogeny, taxonomy, and nomenclature are based on molecular analyses.

Subgenus Boophilus spp

Each of the five Rhipicephalus (Boophilus) spp has a one-host life cycle that may be completed in 3–4 wk and results in a heavy tick burden. Under these conditions, acaricide resistance becomes a major problem in control efforts. Zebu cattle, which have served for centuries as hosts of R (B) microplus in the Indian region, have developed resistance to feeding by large numbers of ticks and are used (purebred or crossbred) in integrated control programs. R (B) microplus, considered the world’s most important tick parasite of livestock, has been introduced from the bovid- and cervid-inhabited forests of the Indian region to many areas of tropical and subtropical Asia, northeastern Australia, Madagascar, the coastal lowlands of southeastern Africa to the equator, and much of South and Central America, Mexico, and the Caribbean. R (B) microplus and R (B) annulatus were eradicated from the USA after a long, costly control program. Constant surveillance is maintained to prevent their reintroduction. R (B) annulatus of southern former USSR, the Middle East, and the Mediterranean area, was introduced with livestock of the early Spanish colonialists into northeastern Mexico but has not spread into Central America. In Africa, south of the Sahara and north of the equator, cattle movements probably account for the many R (B) annulatus populations.

R (B) decoloratus, which ranges from southern Africa to the Sahara, is being replaced in the southeastern part of this area by R (B) microplus. In more humid West African zones, R (B) annulatus mixes with or is totally replaced by R (B) geigyi. Scattered R (B) geigyi populations are found as far east as southern and central Sudan. The only boophilid restricted to sheep and goats (and occasionally horses) is R (B) kohlsi of Syria, Iraq, Israel, Jordan, western Saudi Arabia, and Yemen. R (B) microplus is an experimental vector of Babesia equi and has been collected from the nasal passages of equids in Panama. This tick and R (B) annulatus are major vectors of Babesia bigemina, B bovis, and Anaplasma marginale. R (B) decoloratus is an efficient vector of B bigemina and A marginale but does not transmit B bovis or B equi.

Subgenus Rhipicephalus spp

Tropical Asia is the home of five Rhipicephalus (Rhipicephalus) spp; adults of two species parasitize domestic animals. R haemaphysaloides infests all types of livestock, and wild antelope, deer, carnivores, and hares in continental southeast Asia (and Taiwan and the Philippines) westward to India, Sri Lanka, Nepal, Pakistan, and western Afghanistan. R pilans infests livestock and wildlife in Indonesia and Borneo. Immatures of both species feed chiefly on rodents, also on shrews, hares, and smaller carnivores.

From central Europe to Kazakhstan, R rossicus, R schulzei, and R pumilio are of medical and veterinary importance. In southwestern Europe, R pusillus infests dogs as well as European rabbits, foxes, and wild pigs. R turanicus, as presently recognized, ranges from China, southern former USSR, India into southern Europe, and Africa as far south as South Africa. A member of the taxonomically difficult R sanguineus group, “R turanicus” and its various populations, which may represent separate species, requires further studies of its abilities as a vector.

An easily recognized two-host species, R bursa, ranges from the western Mediterranean area of Europe to Iran and Kazakhstan. Adults and immatures parasitize livestock, hares, deer, wild sheep and goats, people, and infrequently dogs. It causes ovine paralysis and transmits Crimean-Congo hemorrhagic fever virus and other viruses to people, and numerous microbial diseases of livestock such as various species of Babesia, Anaplasma, and Theileria (notably Theileria parva, the agent of East Coast Fever, often fatal for cattle), Ehrlichia (Cowdria) ruminantium (the agent of heartwater), and Trypanosoma vivax (an agent of sleeping sickness).

The best known African rhipicephalid, R sanguineus, the kennel tick or brown dog tick, has spread worldwide with domestic dogs. It is now established in buildings as far north as Canada and Scandinavia and as far south as Australia. In Africa, the Near East, and parts of southern Europe, adults parasitize wild and domestic carnivores, sheep, goats, camels, other livestock, and various wild mammals, especially hares and hedgehogs. Immatures in nature in this area feed on small mammals. However, in urban situations everywhere, dogs are virtually the only hosts of immatures and adults. People are attacked infrequently, more often in situations when children play and sleep in close contact with heavily infested dogs. Strains of adult R sanguineus that feed on cattle are recorded in parts of Mexico and in Tahiti. This tick is active throughout the year in the tropics and subtropics but only from spring to fall in temperate zones. Newly active adults and nymphs are frequently seen climbing walls from floor-level cracks. R sanguineus is a vector of Babesia canis, Ehrlichia canis, Rickettsia conorii, R massiliae, R rickettsii, and R rhipicephali, Crimean-Congo hemorrhagic fever virus, and Thogoto virus. In southcentral USA, R sanguineus is associated with scattered foci of Leishmania mexicana. Certain American populations have become resistant to insecticides. The hymenopteran (chalcid) parasite of ticks, Hunterellus hookeri, frequently infests nymphal R sanguineus in East Africa.

R appendiculatus, the brown ear tick, is a major pest in cool, shaded, woody and shrubby savannas from southern Sudan and eastern Zaire to Kenya and South Africa. Adults and immatures feed in the ears of cattle, other livestock, and antelope, but also on other areas when the infestation is massive. Immatures may infest small antelope and carnivores, and occasionally rodents. Engorged females lay as many as 5,000 eggs. The life cycle lasts 3–9 mo. Seasonal activity is closely associated with temperature and rain periods. As many as three generations a year can follow in regions with two rainy seasons. R appendiculatus is the major vector of the Theileria parva group of diseases (East Coast fever, Corridor disease, Zimbabwe malignant theileriosis) and Nairobi sheep disease virus, and is also a vector of T taurotragi, Ehrlichia bovis, R conorii, and Thogoto virus. Heavy infestations on susceptible Bos taurus cattle cause a sometimes fatal toxemia, loss of resistance to various infections, and severe damage to the host’s ears.

The closely related R zambeziensis, with similar host preferences, is found in drier lowland savannas in Tanzania, Zimbabwe, Zambia, Botswana, and Transvaal; it also is a vector of East Coast fever. Other species closely related to R appendiculatus include R nitens in the Cape Province of South Africa and R duttoni in Angola and Zaire.

The ivory-ornamented R pulchellus, a parasite of zebras, also infests livestock and game animals in savanna habitats east of the Rift Valley from southern Ethiopia to Somalia and northeastern Tanzania. Adults and immatures generally infest the same host; however, immatures also feed on hares, and larvae (“seed ticks”) are notoriously annoying pests of people. R pulchellus feeds in the ears and on the lower abdomen, chiefly during wet seasons. This tick is a vector of Babesia equi (among zebra), Theileria spp, Trypanosoma theileri, Rickettsia conorii, several Bunyaviridae (Crimean-Congo hemorrhagic fever virus; Nairobi sheep disease; and Kajiado, Kismayo, and Dugbe viruses), and Barur virus.

The two-host African rhipicephalids are R evertsi subspecies and R glabroscutatum. R evertsi evertsi, a large, beady-eyed, red-legged tick, a parasite of the East African zebra, parasitizes all types of herbivorous wildlife and livestock (but seldom pigs). Immatures and adults infest the same hosts; immatures are also recorded from hares. It ranges from South Africa through eastern Africa east of the Nile to southern Sudan and is established in the mountains of Yemen. Scattered foci, introduced by domestic animals, are found west of the Nile. Immatures feed in the ear canal; adults feed mostly around the anus and under the tail but also in the axillae and groin and on the sternum. Large numbers on a single host are common on Equidae and are difficult to control because of their concentrations in difficult-to-reach feeding sites. Adult females lay as many as 7,000 eggs. The life cycle takes 36 mo to complete, depending on weather conditions. R evertsi evertsi transmits Babesia equi, Theileria parva (secondary vector), Borrelia theileri, Rickettsia conorii, and Kerai, Wad Medani, and Thogoto viruses. The banded-legged (Hyalomma-like) western subspecies, R evertsi mimeticus, found from western Botswana to Namibia, Angola, and Zaire, is like the nominate subspecies in host preferences, feeding sites, and life cycle.

The tiny R glabroscutatum has become a common pest of sheep, goats, and other livestock in the arid, small-shrub savanna of southeastern Cape Province, South Africa. Kudu and other small antelope are also infested. The few records of immatures are from rodents.

The R pravus group, currently under taxonomic study, consists of four or more species of which the adults feed on livestock and herbivorous wildlife (including hares); immatures feed on elephant shrews (insectivores), hares, and other small mammals. R pravus, a brown, convex-eyed tick, is found in shrubby and wooded savannas in east Africa. It is infected by Kadam virus. The closely related R occulatus, a parasite of hares, and another related, unnamed parasite of livestock are found in southern Africa.

The difficult-to-classify R punctatus group of parasites of livestock and wild artiodactyls consists of R punctatus (Angola, Mozambique, Tanzania), R kochi (neavi) (Botswana to Kenya and Zaire), and an as yet unnamed species from Zimbabwe and South Africa.

The R capensis group is also under study. Originally parasites of the Cape buffalo, these species now parasitize livestock and wildlife in Namibia and South Africa (R capensis and R gertrudae), East Africa (R compositus and R longus), and West Africa to southwestern Sudan (R pseudolongus).

Above 5,900 ft (1,800 m) altitude in East African forest and shrub zones, R hurti and R jeanelli infest livestock and Cape buffalo and other large game animals. R hurti also inhabits mountains in Zaire. Both species feed chiefly in the hosts’ ears; R jeanelli also feeds in the tailbrush.

R simus, the prototype of the R simus group and long considered to be a well-established species, is divided into several species. R simus sensu stricto is found through central and southern Africa, roughly south of latitude 8°S, where it is a competent experimental vector of Anaplasma marginale and A centrale. In eastern and northern Africa, R simus is replaced by a less punctate species, R praetextatus, which ranges from central Tanzania to Egypt. Adults of both species parasitize livestock, dogs, wild carnivores, large and medium-sized game animals, and people. Occurrence and densities on livestock are inexplicably erratic. Immature stages feed on the common burrowing rodents in savannas. Both species cause tick paralysis of people and transmit Rickettsia conorii and Coxiella burnetii. In Kenya, R praetextatus is a vector of Thogoto virus and may be a secondary vector of Nairobi sheep disease virus. West of the Nile, these species are replaced by R senegalensis and R muhsamae.

Much literature regarding R tricuspis (Tanzania to South Africa) and R lunulatus (West Africa to Ethiopia and Tanzania) has been incorrect. The chief feeding site of both on livestock and wildlife is the tailbrush, but other parts of the host may also be feeding sites.

R sanguineus and R turanicus of the R sanguineus group are described above. Related species are R camicasi and R bergeoni of northeastern Africa, R guilhoni and R moucheti of West Africa, and two widely distributed “forms” of R sulcatus, which are under study.

Two quite distinctive species often confused with R appendiculatus are R supertritus (Natal to southern Sudan) and R muhlensi (Kenya and southern Sudan to Central Africa). Adults of both species parasitize cattle, Cape buffalo, antelope, and big game animals; R supertritus also is found on carnivores.