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Overview of Ticks

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

Ticks are obligate ectoparasites of most types of terrestrial vertebrates virtually wherever these animals are found. Ticks are large mites and thus are arachnids, members of the subclass Acari. They are more closely related to spiders than to insects. The ~850 described species are exclusively bloodsucking in all feeding stages. Ticks transmit a greater variety of infectious organisms than any other group of arthropods and, worldwide, are second only to mosquitoes in terms of their public health and veterinary importance. Some of these agents are only slightly pathogenic to livestock but may cause disease in people; others cause diseases in livestock that are of tremendous economic importance. In addition, ticks can harm their hosts directly by inducing toxicosis (eg, sweating sickness [see Sweating Sickness], tick paralysis [see Tick Paralysis] caused by salivary fluids containing toxins), skin wounds susceptible to secondary bacterial infections and screwworm infestations, and anemia and death. International movement of animals infected with the tick-transmitted blood parasites Theileria, Babesia, and Anaplasma spp and Ehrlichia(Cowdria) ruminantium is widely restricted.

Movement of tick-infested livestock over great distances is an important factor in the extensive distribution and prevalence of many tick species and tickborne disease agents. A number of introduced tick species thrive in the vast grazing and browsing environments established during recent centuries of human and livestock population explosions. Conversely, introduction of livestock into areas with exotic tick species and tickborne agents to which they have no immunity or innate resistance often results in significant losses.

Two of the three families of ticks parasitize livestock: the Argasidae (argasids, “soft ticks”) and the Ixodidae (ixodids, “hard ticks”). Although they share certain basic properties, argasids and ixodids differ in many structural, behavioral, physiologic, ecologic, feeding, and reproductive patterns. Tropical and subtropical species may undergo one, two, or rarely three complete life cycles annually. In temperate zones, there is often one annual cycle; in northern regions and at higher elevations in temperate regions, at least 2–4 yr are required by most species. There are four developmental stages: egg, larva, nymph, and adult. All larvae have three pairs of legs; all nymphs and adults have four. Adults have a distinctive genital and anal area on the ventral body surface. The foreleg tarsi of all ticks bear a unique sensory apparatus—Haller’s organ—to sense carbon dioxide, chemical stimuli (odor), temperature, humidity, etc. Pheromones stimulate group assembly, species recognition, mating, and host selection.

Certain tick species that parasitize livestock can survive several months, and occasionally a few years, without food if environmental conditions permit. Tick host preferences are usually limited to a particular genus, family, or order of vertebrates; however, certain ticks are exceptionally adaptable to a variety of hosts, so each species must be evaluated separately. The larvae and nymphs of most ixodids that parasitize livestock feed on small wildlife such as birds, rodents, small carnivores, or even lizards.

In the Argasidae, the leathery dorsal surface lacks a hard plate (scutum). Male and female argasids appear to be much alike, except for the larger size of the female 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.

In the Ixodidae, the male dorsal surface is covered by a scutum. The scutum of the ixodid female, nymph, and larva covers only the anterior half of the dorsal surface. The ixodid capitulum arises from the anterior end of the body in each developmental stage.

Argasid Parasitism

The world’s argasid tick fauna comprises 185 species in four genera, namely Argas, Carios, Ornithodoros, and Otobius, in the family Argasidae. The 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 of these leathery parasites inhabit tropical or warm, temperate environments with long dry seasons. Hosts are those 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 significant 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. Argasids use multiple hosts, ie, the 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 30–60 min). 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.

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. Several species have become important pests of domestic fowl and pigeons; among these are the vectors of Borrelia anserina (avian spirochetosis) and the rickettsia Anaplasma (Aegyptianella) pullorum (aegyptianellosis). Argas spp also cause tick paralysis and transmit Pasteurella multocida (agent of fowl cholera), and many are vectors of a variety of arboviruses, some of which infect people.

Genus Carios includes 88 species, most of which are parasites of mammals, especially bats and rodents. Depending on the species, they inhabit dens or roosts of bats located in caves or tree holes or rodent burrows. Several species parasitize colonial nesting birds 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 hosts. However, some Carios ticks will feed on people and domestic animals if the primary host is not available. 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. 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.

The majority of nearly 37 species belonging to the genus Ornithodoros inhabit animal burrows and lairs in hot, arid climates and feed on most any potential hosts that enter their habitat. Larvae in this nidicolous 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 have adapted to living in crevices of walls and under fences where livestock are confined and also are pests of people. Certain species are vectors of relapsing fever spirochetes (Borrelia spp) and African swine fever virus; some species cause toxicosis, and one species (O coriaceus) transmits a spirochete causing epizootic bovine abortion in the western USA. Numerous Ornithodoros-transmitted salivary toxins or arboviruses cause irritation or febrile illnesses in people.

The unique argasid genus Otobius (see Otobius spp) has three species, which do not feed in the adult stage. O megnini (spinose ear tick) is exceedingly specialized biologically and structurally. It infests the ear canals of pronghorn antelope, mountain sheep, and Virginia and mule deer in low rainfall biotopes of the western USA, Mexico, and western Canada. Cattle, horses, goats, sheep, dogs, various zoo animals, and people are similarly infested. This well-concealed parasite has been transported with livestock to western South America, Galapagos, Cuba, Hawaii, India, Madagascar, and southeastern Africa. Notably, adults have nonfunctional mouthparts and remain nonfeeding on the ground but may survive for almost 2 yr. Females can deposit as many as 1,500 eggs in a 2-wk period. Larvae and two nymphal instars feed for 2–4 mo, mostly in winter and spring. There can be two or more generations per year. People and other animals may have severe irritation from ear canal infestations, and heavily infested livestock lose condition during winter. Tick paralysis of hosts and secondary infections by larval screwworms are reported. O megnini is infected by the agents of coxiellosis/Q fever, tularemia, Colorado tick fever, and Rocky Mountain spotted fever. Another species, O lagophilus, feeds on the heads of jackrabbits (hares) and rabbits in western USA.

Ixodid Parasitism

The Ixodidae number >600 species, occupy many more habitats and niches than do argasids, and parasitize a greater number of vertebrates in a wider variety of environments. Most ixodid species have a three-host life cycle, others have a two-host cycle, and a few have a one-host cycle. Each ixodid postembryonic developmental stage (larva, nymph, adult) feeds only once but for a period of several days. Males and females of most species that parasitize livestock mate while on the host, although some mate off the host on the ground or in burrows. Males take less food than females but remain longer on the host and can mate with several females. During inactive seasons, few or no females are found feeding, even though males may remain attached to the hosts. Such males may contribute to transmission of pathogens to new susceptible animals by serial interhost transfer. Larval and nymphal population activity generally peaks during the “off seasons” of adults, although in some species there is overlap in the seasonal dynamics of immatures and adults.

The ixodid males, except those in the genus Ixodes, become sexually mature only after beginning to feed, after which they mate with a feeding female. Only after mating does the female become replete and proceed to develop eggs. She then detaches, drops from the host, and over a period of several days deposits a single batch of many eggs on or near the ground, usually in crevices or under stones, leaf litter, or debris. Depending on species and quantity of female nourishment, the egg batch usually numbers 1,000–4,000 but may be >12,000. The female dies after ovipositing. Notably, ixodids (except one- and two-host species, which use vertebrate host animals as habitat for much of their life cycle) spend >90% of their lifetime off the host, a fact of utmost significance in planning control measures. The several-day feeding process progresses slowly; the balloon shape characteristic of engorged larvae, nymphs, and females develops only during the final half day of feeding and is followed by detaching. The dropping time at certain hours of the day or night is governed by a circadian rhythm closely associated with the activity cycle of the principal host.

It is also important, especially in understanding the epidemiology of tickborne pathogens, to know whether immatures of an ixodid species feed on the same host species as do the adults, or on smaller vertebrates. Where acceptable smaller-sized hosts are scarce, immatures of some ixodid species can feed on the same livestock hosts as adults; immatures of other species seldom or never do so.

The proximity of acceptable hosts, air temperature gradients, and atmospheric humidity during resting and questing periods are among the factors that regulate the development of each stage and, in the case of females, oviposition.

Three-host Ixodids:

Most ixodids have a three-host cycle. The recently hatched larvae quest for a suitable host, usually from vegetation, feed for several days, drop, and molt to nymphs, which repeat these activities and molt to adults. Of the three-host species that parasitize livestock or dogs, a few have immatures and adults that parasitize the same kind of host; these often develop tremendous population densities. The success of ixodid species that require smaller-size hosts for immatures depends on the availability of those hosts in the livestock browsing and grazing grounds. The natural hazards inherent in the three-host cycle have been compensated for by the benefits afforded adaptable tick species by animal husbandry practices. Only certain ixodids specific for herbivores have adapted to coexistence with livestock, and therein lies the answer to numerous livestock tick problems in Africa, where hosts for adults and immatures are abundant.

Two-host Ixodids:

Some ixodids, especially those that parasitize wandering mammals (and also birds in certain cases) in inclement environments of the Old World, have developed a two-host cycle in which larvae and nymphs feed on one host, and adults on another. As in three-host species, both hosts may be different or may be the same species. Two-host parasites of livestock thrive in both inclement and clement environments and are difficult to control. This is especially true of two-host species that feed in the ears and anal areas of livestock.

One-host Ixodids:

Among the most economically important ticks are several one-host species. These parasites evolved together with herbivores that wandered in extensive ranges in the tropics (Rhipicephalus [Boophilus] spp, Dermacentor nitens, etc) or in temperate zones (D albipictus, Hyalomma scupense). Larvae, nymphs, and adults feed on a single animal until the mated, replete females drop to the ground to oviposit.

Feeding Sites:

Each species has one or more favored feeding sites on the host, although in dense infestations, other areas of the host may be used. Some feed chiefly on the head, neck, shoulders, and escutcheon; others in the ears; others around the anus and under the tail; and some in the nasal passages. Other common feeding sites are the axillae, udder, male genitalia, and tail brush. Immatures and adults often have different preferred feeding sites. Attachment of the large, irritating Amblyomma spp is regulated by a male-produced aggregation-attachment pheromone, which ensures that the ticks attach at sites least vulnerable to grooming.