Ticks of domestic animals directly cause poor health and loss of production to their hosts. Ticks also transmit numerous kinds of viruses, bacteria, and protozoa between domestic animals. These microbes cause diseases which can be severely debilitating or fatal to domestic animals, and may also affect humans. Ticks are especially important to domestic animals in tropical and subtropical countries, where the warm climate enables many species to flourish. Also, the large populations of wild animals in warm countries provide a reservoir of ticks and infective microbes that spread to domestic animals. Farmers of livestock animals use many methods to control ticks, and related treatments are used to reduce infestation of companion animals.
Variety of ticks affecting domestic animals
Ticks are invertebrate animals in the phylum Arthropoda, and are related to spiders. Ticks are in the subclass Acari, which consists of many orders of mites and one tick order, the Ixodida. Some mites are parasitic, but all ticks are parasitic feeders. Ticks pierce the skin of their hosts with specialized mouthparts to suck blood, and they survive exclusively by this obligate method of feeding. Some species of mites may be mistaken for larval ticks at infestations on animal hosts, but their feeding mechanisms are distinctive. All ticks have an incomplete metamorphosis: after hatching from the egg, a series of similar stages (instars) develops from a six-legged larva, to eight-legged nymph, and then a sexually developed, eight-legged adult. Between each stage is a molt (ecdysis), which enables the developing tick to expand within a new external skeleton. Ticks are grouped in three families, of which two have genera of importance to domestic animals, as follows.Sonenshine, D.E. (1993), Biology of ticks (vol 1). Oxford University Press, New York. The family Argasidae contains the important genera Argas, Ornithodoros, and Otobius. These genera are known as soft ticks because their outer body surfaces lack hard plates. The family Ixodidae contains 14 genera, including Amblyomma, Dermacentor, Haemaphysalis, Hyalomma, Ixodes, Margaropus, and Rhipicephalus. Also, the important boophilid ticks, formerly of the genus Boophilus, are now classified as a subgenus within the genus Rhipicephalus. These genera are known as hard ticks because their outer surfaces have hard plates. Within these genera are, very roughly, 100 species of importance to domestic animals. Some of these species are also important to humans. The only countries that do not have some kind of problem with ticks on domestic animals are those that are permanently cold. An outline classification of the Acari, including the two families of ticks of importance to domestic animals is in the article Mites of livestock.
Typical ticks of domestic animals
Amblyomma and Rhipicephalus ixodid ticks
Amblyomma species are widespread on domestic animals throughout tropical and subtropical regions. Typical Amblyomma species are: Amblyomma americanum, the lone star tick of the Southern and Eastern US; Am. cajennense, the Cayenne tick of South America and Southern US; Amblyomma variegatum, the bont tick of Africa and the Caribbean (see Gallery below for photograph of female and male). A typical Rhipicephalus species is Rhipicephalus sanguineus, the tropical dog tick, specialized to feed only on dogs. It is distributed globally throughout the warm countries, wherever humans with their dogs live. Typical Rhipicephalus species that feed on cattle in Africa are R. appendiculatus, the brown ear-tick, and R. evertsi, the red-legged tick. Rhipicephalus (Boophilus) microplus (or now simply Rhipicephalus microplus) is the most important tick of cattle in many tropical and subtropical countries to which it spread from Southeast Asia on transported cattle.
Boophilid ticks, a subgenus within Rhipicephalus
These ticks, commonly known as cattle ticks or blue ticks, have a highly characteristic morphology and one-host lifecycle. They have high specificity for cattle as hosts and their morphological characteristics used for identification are less distinct than those of three-host rhipicephalids such as R. appendiculatus. They are economically important to the cattle-rearing industry by causing direct parasitic losses and by transmission of microbes. In addition to Rhipicephalus microplus, species of most importance to domestic animals are R. annulatus, which is widespread in tropical and subtropical countries, and R. decoloratus which occurs in Africa.
One-host lifecycle of R. microplus
These ticks are adapted to the advantages of specialising to feed on cattle and with all the feeding stages occurring on one individual host in a rapid sequence. They also can survive by feeding on deer or some wild bovid hosts. Infestation starts when larvae on vegetation attach to a new host. When a larva feeds, it molts at the site where it feeds and emerges as a nymph. The nymph feeds at the same site or close by and molts where it feeds. It emerges from molt as either an adult female or male. The female's single large blood meal is converted into a batch of 2000 eggs. The males take several small meals of blood to support their repeated attempts at mating. The molts are rapid and the next stage remains in the hair coat to start feeding again. The combined feeding and molting periods take about 21 days. The engorged female drops from the host, hides under leaf litter on soil surface, lays one batch of eggs, and then dies. When eggs hatch, the larvae crawl up grass stems and wait until they can attach to passing cattle.
Hyalomma ticks
This genus contains many species of hard ticks important to domestic animals in hot, dry regions in Africa, the Mediterranean basin, the Middle East, Pakistan, India, and through to China. Typical species are Hyalomma anatolicum, Hy. rufipes, Hy. truncatum, and Hy. detritum, which feed as adults on cattle, sheep, and goats. Hyalomma dromedarii is specialized to feed on dromedary camels. Hyalomma ticks are adapted to live in regions with large seasonal variation of temperature and low rainfall. Diapause is an important mechanism to adjust to these climates. Another adaptation is to have a lifecycle within one species that can be two-host or three-host. For example: Hy. anatolicum may feed on a hare, molt on the hare, and feed again on the same individual hare, detach and molt to an adult and then feed on a cow—that is a two-host lifecycle. Or it may feed as a larva on a gerbil, then as a nymph on a cow, and then as an adult on another cow in a three-host lifecycle. Furthermore, this tick commonly feeds as a three-host tick with larvae, nymphs, and adults feeding on separate individual dairy cows confined to cattle housing in zero-grazing systems.
Argas and Ornithodoros soft ticks
Argas persicus, the fowl tick, is a major pest of poultry birds. The tampan ticks within the Ornithodoros moubata complex of species infest domestic pigs and also feed on humans. Ornithodoros savignyi is often found in large numbers at enclosures where camels and cattle are herded. Many species of argasid soft ticks are adapted to live in the nest or regular resting sites of their hosts, often waiting for months or even years for the host to return and enable the tick to feed. This nest-dwelling behavior is described as endophilic or nidicolous.
Multihost lifecycle of O. moubata
Argasidae soft ticks have different lifecycles from Ixodidae hard ticks, and these are very variable between species. Typically, in Ornithodoros, a larva hatches from an egg laid in the nest or resting place of the host. The larva does not feed, but directly molts into the first nymph stage. This stage feeds, then molts into the next nymph stage. Feeding by soft ticks is generally completed within minutes rather than days, as with hard ticks. Depending on circumstances, four or five nymph stages occur, each progressively larger. Finally, a molt produces an adult female or male. The female takes repeated blood meals that are small compared to a female hard tick. Each blood meal is converted to a small batch of eggs. The male feeds sufficiently to support its mating. The lifecycle of Argas persicus is similar, but the larva feeds on blood of its bird host, remaining attached around 7 days.
Other groups of ticks
Other genera with species that are often of high local importance to domestic animals include the following examples, some of which are illustrated in the gallery below. Ixodes (Ixodes ricinus, the deer tick of Europe; Ixodes scapularis, the black-legged tick of North America; Ixodes holocyclus, the paralysis tick of Australia). Haemaphysalis (Ha. leachii, the yellow dog tick of the tropics). Dermacentor (Dermacentor andersoni, the Rocky Mountain wood tick; Dermacentor variabilis, the American dog tick; D. reticulatus, the ornate dog tick of Europe). D. nitens, the tropical horse tick of the Americas, has a one-host lifecycle similar to the boophilids. Margaropus winthemi, the beady-legged tick, infests horses and cattle in South Africa. The soft tick Otobius megnini, the spinose ear tick, has its nymphs feeding within the ear canal of many species of domestic animals. Adults of Ot. megnini do not feed. This tick occurs in the Americas and has spread to Africa and Asia.
Negative impacts to health
Biting stress and lost production
When a hard tick pierces the skin of its host, initially little or no pain is caused. Later, during the prolonged feeding of ticks, inflammation is caused at the wound, followed by acquired immune reactions in the skin (dermal hypersensitivities types 1 and 4) to the foreign proteins in tick saliva. This defense by the host is generally effective, but at the cost of pruritus (itch) and pain at the feeding site. Infestations of ticks on certain individual animals of a herd of livestock animals can build up to very high levels. This occurs on a minor proportion of individuals in the herd, whilst most individual animals have low infestations. On a herd basis, the accumulated effect of this biting stress can cause loss of appetite and loss of blood. These two losses result in reduced feed intake and anemia; combined, they cause a lower rate of growth or of milk production compared to hosts without tick infestation. The feeding of soft ticks can cause severe biting stress because of the pain whilst they feed; Ornithodoros savignyi feeding on livestock that are herded in enclosures during nighttime is one notorious example.
Physical damage
At each feeding site of hard ticks, granuloma and wound healing produce a scar that remains for years after the tick has detached. When the skin of livestock animals is made into leather, these scars remain as blemishes that reduce the value of the leather. Larger ticks cause obstructive and painful damage, such as Amblyomma variegatum adults, which often feed on udders of cattle and reduce suckling by the calves. Hyalomma truncatum adults feed on the feet of sheep and goats, causing lameness. Wounds caused by dense clusters of adult ticks can make the host susceptible to infestation with larvae of flesh-eating myiasis flies, such as the screw-worm, Cochliomyia hominivorax.
Poisoning