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Sheep ticks are considered to be one of the worst disease transmitters among bloodsuckers in northern Europe.
The main disease transmitted to humans by ticks in Norway is Lyme disease (Lyme borreliosis). Tick-borne encephalitis is an infection of the central nervous system caused by a virus (TBE virus) transmitted through tick bites. Other diseases are anaplasmosis, tularemia (rabbit fever) and babesiosis.
Where are they found?
Also known as castor bean ticks, sheep ticks are common along the coast of Norway up to Helgeland, but are most prevalent in southern Norway. Some are also found inland. In western Norway and Trøndelag, the tick follows the coastline into the fjords.
Ticks can be carried by birds and may appear far outside their core area. Some inland places can have populations for a few years which then disappear.
In Sweden, the tick is prevalent in south-eastern parts of the country with a northern boundary from Iddefjorden to Gävle, but can also be found further north. The same applies in Finland’s northern boundary along a diagonal line from approximately 62°N in the west to approximately 64 °N in the east.
The sheep tick is common throughout most of Europe.
The tick is a mite, with eight legs and no clear distinction between the head, chest and abdomen (see Figure 1). All mites belong to the arachnid class and are not insects. They have no antennae.
The sheep tick has four stages of development; egg, larva, nymph and adult (see Figure 2). The larva is 0.5 mm long and has only three pairs of legs. Nymphs are approximately 1.5 mm. Adult males are 2-3 mm and grey-black, while adult females are 3-4 mm with a black head and legs and reddish-brown abdomen. When a tick is full of blood, the female may be up to 1.5 cm long, and the abdomen has a grey-blue colour.
After sucking the blood of a host animal, the adult female drops to the ground and burrows into the leaf litter to lay between 2,000 and 3,000 eggs.
After a few weeks the eggs hatch and the young larvae crawl into the ground vegetation to wait for passing birds, mammals and lizards. Due to their limited movement, the presence of larvae is sporadic. Larvae suck blood from small mammals and birds, and are commonly found in mouse ears and the nose and mouth of birds. Larvae can also suck blood from larger animals, including humans. They suck blood for 2-4 days until they are about 1.5 mm then drop to the ground. Depending on the temperature, it takes from one to several months before they shed their skin and reach the nymph stage.
Nymphs usually overwinter before trying to attach to a new host. They climb up plants and small shrubs to wait for passing hosts. Nymphs can also attach to small mammals and birds. Like larvae, nymphs can also suck the blood of large mammals. After sucking blood, they shed their skin and the nymphs become adult ticks that overwinter again.
Adult females climb back up the vegetation to find a new host. Males also try to find a host animal, not to suck blood but to find a female to mate with. Adults are found on larger mammals such as hares, deer, moose, cattle and dogs. The females suck blood for 7-10 days, until their size is multiplied. The transition from egg to adult tick which lays eggs takes about three years.
Ticks are vulnerable to drought. Large numbers will be found in damp places with grass, small shrubs, scrub and open woodland with good deer populations. In areas where deer rest, the tick population can be very high. In Norway, the sheep tick is generally active from April to November when the temperature is above 5-7 degrees.
Many people believe that only adult females suck the blood of humans because they are easy to spot and attach firmly to the skin. However, larvae and nymphs are more common on humans. Larvae are tiny and sit so loosely that they easily drop off without being noticed. Nymphs are slightly larger and easily fall off when scratched. Larvae and nymphs can also be difficult to detect because of their size. Larvae are generally free of pathogens. The nymph stage is considered to pose the greatest risk in terms of Lyme disease because nymphs are far more numerous than adults.