This common parasite in the Caledonian Forest has become notorious for transmitting the bacteria that cause Lyme disease.
The deer tick is widely distributed in Europe and adjoining parts of northeast Asia and North Africa, covering much of what is known as the western Palaearctic Region. It occurs from Iceland and Britain eastwards to western Russia, from Scandinavia south to the Atlas Mountains in Morocco and Algeria, and also in Turkey, Armenia and northwestern Iran. However, because it requires moist conditions in which to live, it is not present in much of the Mediterranean area, due to the very dry summers there.
The deer tick has been recorded throughout Scotland, from the northern tip of the mainland to the Borders and Dumfries and Galloway, but it is most abundant in the north and west of the country, where it is wetter. It also occurs on many of the islands, including Orkney, Shetland, Lewis, Harris, Skye and Mull. Within its range, the density and distribution of the tick is related to the abundance of the host species it feeds on, and it is most common in woodlands and heaths.
The deer tick has not been assessed for its conservation status at an international level, so it does not feature on the IUCN Red List of Threatened Species. In some areas, such as northern Scandinavia, its range appears to be expanding, possibly due to climate change and the resultant northward spread of some of its host species. Elsewhere, tick numbers are growing due to increased populations of some of its host species such as wild boar (Sus scrofa) and red deer (Cervus elaphus), so it is not thought to be threatened in any way.
The deer tick is a member of the Ixodidae family, which includes all the hard-bodied ticks (ie those that have a scutum – a hard plate or shield – covering part or all of their body). It is not an insect, but belongs instead to the order of invertebrates called Acari, which also includes mites, and is part of the class, Arachnida, together with spiders, harvestmen and scorpions. Key features that differentiate ticks from insects are that they have only two body segments (whereas insects have three), adults have eight legs (insects have six) and they lack wings and antennae.
Other common names for the species include the sheep tick, because it is often found feeding on domestic sheep (Ovis aries), and the castor bean tick. The latter name is due to the resemblance of an engorged adult tick to the seeds of the castor bean plant (Ricinus communis), which is the source of castor oil, and whose genus name, Ricinus, is the Latin word for a tick.
Adult ticks range in size from 2.4 - 2.8 mm for males and 3.0 – 3.6 mm for females, before they have fed. After feeding, fully-engorged females are up to 11 mm long. Adult females are bright red, with a distinctive black scutum covering the front part of the body, whereas the males are a very dark blackish colour, because the scutum covers the whole of its body. The head and mouthparts of a tick are known as a gnathosoma. The mouthparts are black and tapering towards the front. They consist of two palps, one on each side, which protect the hypostome in the middle. This is a barbed, needlelike appendage, which is used to pierce the skin of the tick’s host and suck its blood. The palps play no part in feeding, moving to the side when the hypostome is inserted into the host.
The deer tick has no eyes, so is unable to see to find a host on which to feed. Instead, it relies on special organs, called Haller’s organs, near the tips of its front pair of legs, to detect potential hosts. The Haller’s organ consists of a pit and capsule containing hair-like sensory structures that detect odours including carbon dioxide, as well as humidity and temperature.
In order to feed, the tick will climb to the tip of some vegetation, such as a grass stem or a frond of bracken (Pteridium aquilinum), and will sit there with its front legs outstretched in a pose known as ‘questing’, in which it waits to detect the presence of a potential host. The tick needs moisture to live, and if it becomes desiccated it will descend to the soil to rehydrate before climbing up again. When an animal brushes past the vegetation, the tick crawls on to it – they do not jump or drop on to hosts. It will then find a suitable position and begin feeding.
The life cycle of a deer tick consists of four stages, beginning with an egg laid by a mature female. During its life, the tick will feed on three different hosts. About 8 weeks after being laid, the second stage of the life cycle begins when a larva emerges from each egg. The larva is very small, less than a millimeter in length, only has six legs and has no sexual characteristics. It requires a blood meal to grow and typically will feed on small mammals such as rodents or a hedgehog (Erinaceus europaeus). The tick’s saliva contains an anticoagulant, which prevents the host’s blood from clotting while it is feeding. When it is full, it drops off its host and moults, producing a nymph, which is the third stage of the life cycle. Like an adult, this has eight legs.
The nymph must also consume blood to grow, so it feeds on small and medium-sized mammals, and is the life cycle stage that most commonly bites humans. After feeding and dropping off its host the nymph moults to form an adult, which is the fourth life stage. Adult males feed occasionally, if at all, while the female will feed for five to seven days, becoming greatly engorged in size. Hosts for adults include red deer, wild boar and domesticated animals such as dogs (Canis lupus familiaris), cats (Felis catus), sheep, cows (Bos taurus) and horses (Equus caballus).
Mating between a male and female occurs on a host, and takes place with one tick facing the other. After mating the female continues feeding to reach her full size, while the male tick will mate with other females. A fully engorged female lays about 2,000 eggs before dying. The entire life cycle usually takes two or three years to complete, with the larva and nymph being dormant during the winter.
The deer tick is an ectoparasite, meaning that it is a parasite that lives on the outside of its host. In most cases the loss of blood has little if any apparent effect on the host. However the tick can have a major effect as a transmission agent for organisms that cause various diseases. These include protozoans (Babesia spp.) that causes redwater fever in cattle, bacteria (Anaplasma spp.) that cause anaplasmosis, and, most notoriously, another bacterium (Borrelia burgdorferi) that causes Lyme disease in humans.
Larval ticks do not carry the Borrelia bacteria, but gain them from the small mammals they feed on. The nymphs that are infected can then pass the bacteria on to humans when they bite. Because adult ticks are larger, they are more easily noticed by people and are quickly removed, and as a result the nymphs are the main transmission agent for Lyme disease.
Ticks themselves are food for other organisms, particularly some birds. Magpies (Pica pica) and starlings (Sturnus vulgaris) will eat ticks off the backs of sheep and cattle, reducing the parasite load on the animals while they feed. Ticks also modify the behaviour of animals, by causing them to use friction to remove their parasites. Wild boar, for example, remove parasites by rubbing against tree trunks, while red deer use mud wallows to roll in, and other species take dust baths.
Surprisingly, the tick could yet provide benefits to people, as its saliva contains a number of proteins of possible pharmacological use, and the saliva of a related tick species in Brazil has recently been demonstrated to have anti-cancer properties.
Alan Watson Featherstone
Sources and further reading:
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