The most common tick found in Britain is the sheep tick (Ixodes ricinus), which, despite its accepted name, is not host-specific, getting its blood meals from many different hosts including man.
They are found mainly in woodland, moorland and heath, where the soil and sward is moist as they are prone to desiccation. To feed they climb vegetation and wait for animals (or humans) to pass, grabbing on to the host when they brush against the plants.
If they do not attach they will return to the ground to rehydrate and then try again. They have three stages from larvae to nymph and then adult with all being blood-sucking and with the full life cycle usually taking between two to three years.
There are many diseases that can be spread by ticks but those most evident in sheep in the United Kingdom are:
Tick-borne Fever (TBF) – caused by the rickettsia called Anaplasma phagocytophilum, which spreads into the blood from the bite of an infected tick.
The incubation period is five to 13 days and the main symptom is that of a high temperature (40.5 to 42°C) which can last for four to 22 days. Pregnant ewes will probably abort two to 8 days after infection; mortality is low but may be a problem in ewes that have aborted.
Bought-in sheep from areas which are not affected are at most risk as they are likely to be used for breeding and must be exposed before they become pregnant to avoid abortions.
Although the clinical signs are vague the effect on the blood is dramatic with a profound fall in white blood cells and a consequent reduction in immunity. In endemic areas lambs pick up the infection early and are relatively resistant to disease even though there is no transfer of protection via the colostrum, gaining immunity in response to the infection.
Tick Pyaemia – seen in lambs as a consequence of the suppression of the immune system brought on by TBF. When the lambs are bitten by a tick, staphylococci bacteria can gain entry via the wound leading to a bacteraemia. From this, multiple abscesses form throughout the body leading to symptoms that are often similar to Joint-ill.
Louping-ill – a viral disease (Arborvirus) which can affect the brain giving rise to incoordination (‘louping gait’),trembling, paralysis, convulsions and death. Infection at the same time as TBF gives rise to a mortality rate approaching 100%.
Affected animals that recover are immune for life. Lambs can receive temporary immunity lasting up to three months via the colostrum if their mothers are immune. A vaccine is available for the control of this disease.
Ticks can be controlled by the use of synthetic pyrethroid based pour-ons or plunge dipping (limited available products) which can also aid in the control of lice and fly strike.