Ixodes spp, the largest genus of the family Ixodidae, contains 249 species and is highly specialized both structurally and biologically. So far as is known, all Ixodes spp have a three-host life cycle. Almost all inhabit temperate or tropical forest zones or wooded or shrubby grasslands; fewer are adapted to humid areas in semideserts or to arctic or sub-Antarctic nesting colonies of marine birds. Hosts are a wide variety of birds and mammals and a few reptiles. Most species parasitize burrowing hosts or those that return regularly to caves, dens, or terrestrial or arboreal nesting colonies. The few Ixodes spp that parasitize wandering artiodactyls or perissodactyls are exceptionally adaptable; they also parasitize livestock and are important pests or vectors of agents that infect livestock and people.
The I ricinus group of Eurasia, northwestern Africa, and North and South America is especially important. I ricinus, the so-called sheep tick and prototype of this group, inhabits relatively humid, cool, shrubby and wooded pastures, gardens, windbreaks, floodplains, and forest through much of Europe to the Caspian Sea and northern Iran, and also northwestern Africa. Its life cycle is 2–4 yr, depending on environmental temperature. (In drier, warmer, eastern Mediterranean biotopes, I ricinus is replaced by I gibbosus, which completes its life cycle in 1 yr.) I ricinus larvae feed on small reptiles, birds, and mammals. Nymphs feed on small and medium-sized vertebrates, and adults feed chiefly on herbivores and livestock. All stages, especially nymphs and adults, parasitize people. Male I ricinus take little or no food and can mate either on or off the host. Unfed adults often mate while on vegetation. Adult activity peaks in spring; in some populations, there is a lower peak of adult activity in the fall. Chief among the numerous arboviral diseases transmitted by I ricinus are louping ill, tickborne encephalitis, and Crimean-Congo hemorrhagic fever. Other agents transmitted to livestock are Coxiella burnetii, Anaplasma marginale, Babesia divergens, and A phagocytophilum, various strains of which cause bovine, ovine, and human granulocytic anaplasmoses.
I persulcatus, the taiga tick, is closely related to I ricinus and has similar host preferences. It ranges from the central and eastern mountains of Europe through the lowland forests from the Baltic Sea and Karelia eastward through the Siberian taiga to the Seas of Japan and Okhotsk and the northern islands of Japan. The life cycle can be completed in 3–4 yr but can last up to 7 yr in regions with a short summer season. It is one of the main vectors of Russian spring-summer encephalitis virus and Borrelia burgdorferi. In addition, it transmits Babesia spp, Ehrlichia muris, and the agents of human and ovine anaplasmoses and tularemia.
Other Asian representatives of the I ricinus group are I sinensis of China; I kashmiricus of mountainous northern India, Pakistan, and Kyrgyzstan; I pavlovskyi of the southern Siberian mountains of Russia; and I kazakstani of mountain taiga and deciduous forest in Kazakhstan, Kyrgyzstan, and Turkmenistan.
In the Americas, representatives of the I ricinus group include I scapularis, I pacificus, I affinus, I jellisoni, I minor, and I muris. I scapularis is distributed throughout the eastern and northcentral USA and southern Canada. It is a vector of Borrelia burgdorferi, the agent of Lyme disease, and A phagocytophilum, which causes granulocytic anaplasmosis in people, horses, and dogs. It also transmits Babesia microti, the agent of human babesiosis in coastal areas from Rhode Island to Virginia, and an E muris–like agent causing granulocytic ehrlichiosis in people and rodents in the midwestern USA. The chief hosts of adult I scapularis are deer; livestock seldom graze in the wooded zones inhabited by this tick. Adults of I pacificus parasitize livestock from Baja California to British Columbia and in inland pockets of Idaho, Nevada, and Oregon. I pacificus and I neotomae transmit the agents of Lyme disease, tularemia, and a rickettsia of the Rocky Mountain spotted fever group; I pacificus also transmits A phagocytophilum. The tick bites cause slowly healing ulcers. A related species, I affinis, ranges from South Carolina and Florida to Argentina. It is recorded chiefly from wildlife and has not been shown to be a vector.
In Africa, only four Ixodes spp have adapted to livestock. Chief among these is the South African paralysis tick, I rubicundus, of humid hill and mountain karoo vegetation in South Africa. Its salivary toxins cause a flaccid tetraplegia in livestock, people, dogs, and jackals. Immatures parasitize the rock hare, other hares, and elephant shrews. Other parasites of livestock in African highlands are I drakenbergensis (Natal), I lewisi (Kenya), and I cavipalpus (southern Sudan to Zimbabwe and Angola).
I holocyclus, the Australian paralysis tick, is considered the most medically important of the Australian tick fauna. Although most cases of tick bite are uneventful, some can result in life-threatening illnesses, including paralysis, tick typhus, and severe allergic reactions. The species is found throughout the year in a variety of habitats, particularly wet sclerophyll forests and temperate rainforest areas, across the humid coastal regions of eastern Australia. Adults are more abundant in the spring and early summer, larvae in mid to late summer, and nymphs during winter. Natural hosts include bandicoots, kangaroos, possums, birds, and sometimes even reptiles, but it regularly attaches to domestic animals, including dogs, cats, cattle, and horses, and occasionally people. The tick injects a neurotoxin that causes progressive motor paralysis, respiratory depression, and death in animals that have no immunity to the toxin. Most cases of tick paralysis seen by veterinarians are in dogs and cats, but other species can be affected. The condition has been recorded in sheep, cattle, goats, llamas, and Muscovy ducks. Fatal cases have occurred in people. In the larger species, the younger and smaller animals are likely to be affected.
I rubicundus, the karoo paralysis tick, may cause paralysis in sheep and goats in southern Africa. The immature stages of I rubicundus can be found on scrub hares and elephant shrews. The life cycle of the tick is ~2 yr. The ticks can often be found along the lower line of the neck, chest, and belly. The active period for adult ticks begins usually late in summer and reaches peak levels in autumn and early winter (April/May) and declines thereafter. The paralysis commonly occurs from February and reaches a peak in April and May.