The family Argasidae (the soft ticks) have a leathery dorsal surface and lack the highly sclerotized scutum that is characteristic of ixodid ticks, or hard ticks.
Male and female argasids appear to be much alike, except for the female's larger size and differences in external genitalia. The argasid capitulum (mouthparts) arises from the anterior of the body in larvae but from the ventral body surface in nymphs and adults.
Argasidae are highly specialized for sheltering in protected niches or crevices in wood or rocks, or in host nests or roosts in burrows and caves. Some argasid species are known to survive for several years between feedings.
Most argasid ticks inhabit tropical or warm, temperate environments with long dry seasons. Hosts are animals that either rest in large numbers near the argasid microhabitat or return from time to time to rest or breed there.
Soft ticks can be a serious pest in poultry and pig operations in tropical and subtropical countries. Blood loss and subsequent anemia can be considerable and substantially affect weight gains and egg-laying performance. Massive infestations can cause numerous fatalities.
An argasid population typically parasitizes only a single kind of vertebrate and inhabits its shelter area. Argasid ticks use multiple successive hosts: larvae feed on one host and drop to the substrate to molt; the several nymphal instars each feed separately, drop, and molt; adults feed several times but do not molt.
Argasid nymphs and adults feed rapidly (usually for 30–60 minutes). Larvae of some argasids also feed rapidly; others require several days to engorge fully. Adult argasids mate off the host several times; afterward, females deposit a few hundred eggs in several batches and feed between ovipositions.
The world’s argasid tick fauna comprises approximately 190 species in 4 genera, namely Argas, Carios, Ornithodoros, and Otobius.
Argas spp of Ticks
Most of the 57 described Argas spp parasitize birds that breed in colonies in trees or against rock ledges; others parasitize cave-dwelling bats. Few feed on reptiles or wild mammals, and none on livestock. A few parasitize wild terrestrial mammals or Galapagos giant tortoises.
A persicus (the fowl or poultry tick) is an important poultry pest worldwide in warm climates. A persicus thrives in poultry houses and chicken coops. Heavy infestations and frequent bites of A persicus can lead to secondary infections, anemia, weakness, and weight loss in birds. A persicus are most active during warm, dry weather. The adults and nymphs feed at night and hide in cracks, crevices, or under debris during the day; larvae remain attached to birds for 2–7 days.
Likewise, A reflexus can cause severe blood loss in pigeons.
A miniatus (the South American chicken tick) and A radiatus (the North American bird tick) can present a problem for traditional or outdoor poultry operations from the Caribbean to Central America and from the Caribbean to North America, respectively.
Several species have become important pests of domestic fowl and pigeons; among these are vectors of Borrelia anserina (avian spirochetosis) and the rickettsia Aegyptianella pullorum (syn Anaplasma pullorum). The species of importance in transmitting Aegyptianella pullorum and Borrelia anserina to poultry are Argus persicus (many tropical and subtropical areas of the world), A arboreus (much of Africa, including Egypt), A africolumbae (tropical Africa), A walkerae (southern Africa), and A miniatus (South and Central America). Other species that infest poultry appear to transmit both A pullorum and B anserina.
Tick paralysis is caused by feeding A persicus, A arboreus, A walkerae, A miniatus, A radiatus, and A sanchezi (US). These and other Argas spp can cause great irritation when feeding on humans.
Argas spp also transmit Pasteurella multocida (agent of fowl cholera).
Many Argas spp are vectors of a variety of arboviruses, some of which cause human arbovirus infections.
Carios spp of Ticks
The genus Carios includes 88 species, most of which are species-specific parasites of mammals, especially bats and rodents. Depending on the species, Carios inhabit dens or roosts of bats in caves, tree holes, or rodent burrows.
Several species parasitize colonial birds nesting in rocks and caves and dwell in the substrate or under stones and debris in ground-level bird colonies.
Many of these ticks parasitize only a single host species or a group of closely related species. However, some Carios ticks will feed on humans and domestic animals if the primary host is not available.
These ticks normally live alongside their hosts in caves, hollow trees, and rock crevices, and therefore rarely come in contact with domestic animals. However, in locations where bats occupy roof cavities, their parasites may present a problem for humans and their pets.
Tick species C kelleyi in North America and C vespertilionis in Europe, which in nature feed almost exclusively on rock- and tree-roosting bats, have been found in massive numbers in homes with associated bat colonies and have been reported to attach to humans. C kelleyi, a tick associated with bats and bat habitats, has been reported to carry a novel spotted fever group Rickettsia and a relapsing fever spirochete closely related to Borrelia turicatae.
Nest parasites (eg, C amblus, C capensis, and C denmarki) of colonial birds may pose a distinct threat to breeding colonies and are known to cause chick death.
The seabird tick C capensis has been shown to transmit West Nile virus to ducklings. The American C puertoricensis and C talaje are potential vectors of African swine fever virus.
Ornithodoros spp of Ticks
Most of the 37 species belonging to the genus Ornithodoros inhabit protected niches (in animal burrows, caves, dens, cliffsides, lairs, and bird colonies) in hot, arid climates and feed on almost any potential hosts that enter their habitat.
Larvae in this nidicolous (spending its life cycle within the host's burrow) genus do not feed, which may be related to the fact that these ticks dwell in burrows that may house hosts irregularly.
A few species that are also pests to humans have adapted to living in wall crevices and under fences where livestock are confined.
Certain species are vectors of relapsing fever spirochetes (Borrelia spp) and African swine fever virus.
Some species cause toxicosis. Numerous Ornithodoros-transmitted salivary toxins and arboviruses cause irritation and febrile illnesses in humans.
Among the few species that parasitize livestock, O savignyi and O coriaceus are exceptional, because they have eyes and they rest just below or just above ground level, under the shade of trees and rocks, where livestock and game animals rest and sleep.
O savignyi (the sand tampan) lives in semiarid areas from Namibia to India and Sri Lanka and is often tremendously abundant. It can cause fatal tick paralysis in calves. Humans and tethered livestock suffer severe irritation, allergy, and toxicosis from sand tampan bites, and paralysis and death of animals have been recorded.
O coriaceus (the pajaroello tick), of hillside scrub oak habitats from northern California and Nevada to Chiapas, Mexico, occupies deer beds under trees and near large rocks. It is well-known for irritating deer and cattle, and its bite causes a severe skin reaction in humans. Epizootic bovine abortion, caused by an unnamed spirochete bacterium (in the order Myxococcales), is transmitted by O coriaceus in the western US.
O gurneyi shelters in tree-shaded soil in arid zones of Australia where kangaroos and humans rest; livestock are rare or absent in these habitats.
Among the numerous Ornithodoros spp that inhabit burrows, several species are either naturally infected with African swine fever (ASF) virus in Africa or have the laboratory-confirmed ability to harbor and transmit the agent in Europe and the Americas. The natural reservoir and vector of ASF virus is O porcinus, which is abundant in burrows of tropical African pigs and also of aardvarks (antbears) and porcupines. It has secondarily adapted to human dwellings and domestic animal shelters, where it lives in wall and floor cracks.
Domestic pig populations in the vicinity of infected wild pigs can be decimated by ASF. Wild and domestic pigs are not involved in the epidemiology of Borrelia duttoni, the agent of human African relapsing fever, which is transmitted by O moubata.
ASF virus has been transported in infected meat to Spain where O marocanus, an inhabitant of rodent burrows and pig sties, is an efficient vector. O marocanus is also a reservoir and vector of Borrelia hispanica, the agent of Spanish-northwest African human relapsing fever.
ASF has likewise been introduced in Brazil, Haiti, the Dominican Republic, and Cuba. The American O turicata, O dugesi, and O coriaceus are potential vectors of ASF virus.
Ornithodoros species that are vectors of tickborne relapsing fever Borrelia spp include the following:
North America: O hermsi, O parkeri, and O turicata
Central and South America: O dugesi, O talaje, and O rudis
Africa: O moubata and O sonrai
Mediterranean: O eraticus complex
Southern Europe: O verrucosus
Asia: O tholozani, O tartakovskyi, and O sawaii
O tholozani (syn O papillipes; also O crossi) infests burrows, caves, stables, stone and clay fences, and human habitations in semidesert, steppe, and long dry-season environments from China, the southern former Soviet Union, northwestern India, and Afghanistan to Greece, northeastern Libya, and the eastern Mediterranean islands. Numerous rodents, hedgehogs, porcupines, and domestic animals support O tholozani populations.
Humans develop severe, sometimes fatal, Persian relapsing fever when bitten by O tholozani infected with Borrelia persica.
O lahorensis, originally a parasite of wild sheep resting in the lee of cliffsides, is an important pest of stabled livestock in lowlands and mountains of Tibet, Kashmir, and the southern former Soviet Union to Saudi Arabia and Turkey, Greece, Bulgaria, and Yugoslavia. The two-host life cycle and long wintertime attachment of O lahorensis is biologically remarkable. It is deleterious to livestock held for much of the winter in heavily infested stables.
O lahorensis may cause paralysis, anemia, and toxicosis, and it transmits the agents of piroplasmosis, brucellosis, coxiellosis, tularemia, and possibly Borrelia persica, the agent of Persian relapsing fever.
O turicata parasitizes rodents that live in burrows, crevices, or caves; owls; snakes; tortoises; and also domestic pigs and other livestock in the southern US and Mexico. Contrary to most Ornithodoros feeding patterns, immature O turicata engorge in < 30 minutes, but adults may attach for up to 2 days.
O turicata has been associated with diseases of pigs, and serious toxic reactions and secondary infections can result when humans are bitten.
O furucosus parasitizes humans and livestock in houses and stables in northwestern South America. Other South American pests of livestock and humans, probably originally parasites of peccaries, are O braziliensis and O rostratus.
Otobius spp of Ticks
The unique argasid genus Otobius has three species, of which O megnini and O lagophilus are of veterinary importance. Ticks of the genus Otobius do not feed in the adult stage.
O megnini (the spinose ear tick) is exceedingly specialized biologically and structurally.
O megnini infests the ear canals of pronghorn antelope, mountain sheep, and Virginia and mule deer in low rainfall biotopes of the western US and in Mexico and western Canada. Cattle, horses, goats, sheep, dogs, and humans are similarly infested.
This well-concealed parasite has been transported with livestock to western South America, the Galapagos Islands, Cuba, Hawaii, India, Madagascar, and southeastern Africa.
Notably, adults have nonfunctional mouthparts and remain nonfeeding on the ground but may survive for almost 2 years. Females can deposit as many as 1,500 eggs in a 2-week period. Larvae and two nymphal instars feed for 2–4 months, mostly in winter and spring. There can be two or more generations per year.
In contrast with most other ticks, O megnini bites are painful for the hosts. Humans and other animals may suffer severe irritation from ear canal infestations, and heavily infested livestock lose condition during winter.
Tick paralysis of hosts and secondary infections by larval screwworms have been reported.
O megnini can carry the agents of coxiellosis, tularemia, Colorado tick fever, and Rocky Mountain spotted fever, although its ability to transmit these pathogens to the hosts is unknown.
O lagophilus feeds on the heads of jackrabbits (hares) and rabbits in the western US.
For More Information
Walker AR, Bouattour A, Camicas, et al. Ticks of Domestic Animals in Africa: A guide to Identification of Species. University of Edinburgh; 2014.
Barker SC, Walker AR. Ticks of Australia: the species that infest domestic animals and humans. Zootaxa 2014;3816(1):1-144.
Estrada-Peña A, Mihalca AD, Petney TN, eds. Ticks of Europe and North Africa: A Guide to Species Identification. Springer; 2017.
