The deleterious effects of ticks on their host are all associated directly or indirectly with their feeding activity. Each stage in the ixodid life-cycle takes a single blood meal over a period of several days. The volume taken is relatively small except in the case of the adult female, those of some species imbibing 300 times their own body-weight. Even after surplus water has been pumped back into the host as saliva, she is typically 2–3 times her unfed size. This large meal provides the protein needed for the production of a single clutch of between 2000 and 20 000 eggs.

In contrast, argasid ticks take multiple small feeds and excess water is expelled through a gland behind the first pair of legs. The female argasid produces a small batch of eggs after each meal.

Hard ticks have three developmental stages: larva, nymph and adult. Apart from size and sexual maturity, they are similar except that the larva has six legs and no spiracles. When not on the host, most are hidden in the debris that collects at the base of thick vegetation. This provides the humid microclimate essential for their survival, as well as protection from predators. Ticks tend to have very specific environmental requirements and these determine the geographical distribution and seasonal abundance of each species. In temperate and cold climates, tick development is halted during winter. In warmer climates, no break occurs if year-round rainfall maintains a favourable microclimate. If rainfall is seasonal, tick activity will be governed by plant transpiration rates and their effect on relative humidity within the vegetation mat.

When ticks seek a host, they climb to the top of nearby foliage. A motionless posture is adopted with the front legs, which carry sensitive sensory organs, testing the air for chemical signals that might indicate the proximity of a potential host. This ensures they are ready to scramble onto a passing animal as soon as it comes in contact. This behaviour is called ‘questing’.

Ixodid species are categorised as three-, two- or one-host ticks, depending on the number of animals used by an individual tick of that species during its life-cycle. Note that this refers to the number of host individuals each tick feeds on – not the number of host species involved.

Soft ticks undergo one larval and several nymphal stages before moulting to become an adult. They take small feeds from many animals, as and when the opportunity arises. They can survive long periods between meals. Mating takes place on the ground. They are more drought-tolerant than hard ticks and are generally found in drier environments, often close to their host’s lair, pen or nest. Most species are active mainly at night.

The size of the tick lesion depends on the depth of the bite and the array of biologically active substances injected into the host. Some genera, such as Rhipicephalus, attach superficially while others, like Amblyomma, penetrate deeply into the dermis. Typically, the cement that has been secreted becomes surrounded by a zone of oedema, inflammatory cell infiltration and epithelial hyperplasia.

Extra information box 3.1

The damaging effects caused by ticks can be summarised as:

1. blood losses (heavy infestations can cause anaemia);
   
2. tick worry (discomfort and rubbing, which restrict time for feeding and ruminating);
   
3. production losses (reduced weight gain, milk yield and fleece weight/quality); fleece and hide damage (due to puncture wounds and self-inflicted trauma); secondary infection of tick bites by microorganisms, blowfly and screwworm larvae;
   
4. tick paralysis (by species producing salivary neurotoxins);
   
5. disease transmission (ticks are vectors for a range of protozoal, bacterial, rickettsial and viral agents).

Extra information box 3.2

Ticks are very efficient vectors of disease. Reasons for this include:

1. many tick species feed on more than one animal during their life-cycle (the exception being one-host ticks);
   
2. ixodid ticks remain attached to the skin for lengthy periods while feeding, during which time they pump large quantities of saliva, which may contain pathogens, into the wound;
   
3. many tick-borne pathogens multiply within the tick;
   
4. some organisms invade the tick ovary to become incorporated into developing eggs.

There are two mechanisms involved in the transmission of pathogens by ticks:

1. Trans-stadial: the pathogen is taken up by an immature tick (larva or nymph) and, usually after a series of intermediate steps, invades the salivary gland so it can be transmitted to a new host when a subsequent life-cycle stage (nymph or adult) feeds. This is also known as ‘stage-to-stage’ transmission.
   
2. Transovarian transmission: this occurs when pathogens invade the ovary of a female tick and become incorporated into developing eggs, so infection can be subsequently spread by the next generation of ticks. This explains how one-host ticks are able to transmit tick-borne diseases. In some cases, the organisms can be passed down through several tick generations.

Pathogens that congregate or multiply in the tick salivary gland may be released at particular points in the feeding cycle. For example, Borrelia (the bacterial agent of Lyme disease) is transmitted from 48 hrs after attachment, whereas Theileria (a protozoan parasite) does not appear in tick saliva before the 5th day of feeding.

Ixodes species are mostly found in temperate or cooler climates. The most widely distributed member of the genus is I. ricinus, which acts as vector for a number of life-threatening protozoal and microbial diseases, including babesiosis and Lyme disease. Other species are more localised such as I. scapularis (also known as I. dammini) and I. pacificus which transmit Lyme disease in eastern and western parts of the USA, respectively. I. canisuga has a narrower host range and is associated with dog kennels and foxes in Europe. I. holocyclus seems relatively innocuous when feeding on its natural host, the Australian bandicoot, but its saliva is capable of paralysing or even killing a dog, calf or foal.

This one-host tick is the scourge of cattle production in many hot, humid climates. It is a vector for babesiosis (see Section 4.8.1) and anaplasmosis (a rickettsial infection producing debilitating anaemia, jaundice and abortion). In the wet tropics, these diseases curtail the use of highly productive European breeds, which have to be cross-bred with, or replaced by, tick-resistant Bos indicus-type or other indigenous cattle (see Section 8.2.4). The main Boophilius species are B. annulatus, which has a widespread distribution, although mostly eradicated from the USA, and B. microplus in Australia, South Africa and Latin America. They spend about three weeks on the host and the generation time can be as little as two months, so large numbers of host-seeking ticks can build up on pastures during the wet season.

Amblyomma is perhaps the most attractive of the ticks as the scutum is highly ornate. The long mouthparts, however, penetrate deeply causing a painful wound prone to secondary infection. Most species infest a range of mammals, including cattle, preferring the head, ears and neck. They are of greatest significance in Africa, where they transmit rickettsial diseases such as heartwater and Q fever.

Species of Dermacentor important in the USA include: the Rocky Mountain wood tick, D. andersoni, which parasitizes cattle and other herbivores in the Northwest, the American dog tick, D. variabilis, which is mostly found on the eastern side of the country, and D. nitens, the tropical horse tick, found in Central America and neighbouring areas. In southern Europe and parts of Africa, D. reticulatus plays a similar role in transmitting a long list of protozoal and rickettsial diseases in cattle, horses and dogs. The American species are the principal vectors of Rocky Mountain spotted fever and can sometimes produce paralysis, particularly in calves.

Otobius megnini is known as the ‘spinose ear tick’ which gives a clue to both its appearance and life-cycle, the parasitic stages being spent deep in the external ear canal. O. megnini is different from other soft ticks as it uses only one individual animal during its life-span. The larva seeks a host and stays on it for several weeks, by which time it has moulted twice to the second nymphal stage. When this is fully engorged, it is up to 1 cm long, blue-grey, covered in spines and ready to leave the animal. All further development takes place off the host and the adults, which do not feed, complete their biological functions hidden in cracks and crevices.

The feeding activities of larvae and nymphs invoke inflammatory responses and a waxy exudate. Secondary bacterial infection and myiasis may follow. The resulting distress can lead to further self-inflicted damage.

The poultry tick, Argas persicus, is a common parasite of poultry in warmer climates. They are straw coloured when unfed and spend the day hiding within crevices in poultry houses. They take a blood meal at night, usually selecting a site beneath the wing. Although generally of minor clinical significance, they can reduce productivity. Large numbers cause anaemia and even death, especially in young birds. They transmit spirochaete and rickettsial infections and some species invoke tick paralysis. They are sometimes found on migratory birds visiting temperate regions.