This focus article has been prepared by Amanda Carson, Sian Mitchell, Paul Phipps, Michele Macrelli and Elizabeth Dunnett of the APHA.
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Ixodes, Haemaphysalis and Dermacentor are the three hard tick genera indigenous to Great Britain. Within these genera, Ixodes ricinus (the sheep tick) is the most common and widely distributed tick species encountered on people, livestock and companion animals in Great Britain (Fig A).
Ticks feed for relatively short periods and spend most of their lives in the environment, occupying specific biotopes. The epidemiology of ticks and tickborne disease is therefore governed by climate and land use. Climate change, greening, wildlife conservation (particularly deer, which act as maintenance hosts) and creation of wildlife corridors contribute to expansion in geographical ranges.
Tick-transmitted diseases present in the UK are listed in Box 1.
Diseases present in the UK that can be transmitted by ticks
Protozoal: babesiosis, theileriosis
Bacterial: tickborne fever, lyme borreliosis
Viral: louping Ill
Zoonotic: lyme borreliosis (Borrelia burgdorferi, sensu lato), human babesiosis, human granulocytic anaplasmosis and louping Ill are the most common. Recently tickborne encephalitis virus, which is endemic in many European countries, has been found for the first time in a very small number of ticks in two locations in England
Tickborne fever (TBF) is caused by Anaplasma phagocytophilum and transmitted by I ricinus.
A phagocytophilum multiplies in neutrophils, which are destroyed, producing profound neutropenia lasting for two to three weeks. Clinical signs include high fever (>40°C), polyarthritis, illthrift or abortion. Pregnant sheep exposed to infected ticks for the first time are likely to abort. Naive rams may be infertile for up to one month after infection.
The immunosuppressive effect of A phagocytophilum can also leave an animal vulnerable to concurrent infections, including pneumonia, septicaemia due to Bibersteinia species, louping ill or tick pyaemia (Fig B).
The introduction of a PCR for TBF has made diagnosis easier. It has identified situations where movement of naive ewes and lambs onto tick-infested pastures has led to infection with A phagocytophilum and other concurrent infections.1
Louping ill is caused by a neurotrophic flavivirus which is transmitted by I ricinus. It affects predominantly sheep and red grouse, but can affect people, cattle and other animals. Following an infected tick bite, the virus multiplies in the drainage lymph node and is then carried via the blood to the central nervous system.
In areas where louping ill is endemic, sheep less than two years old are most commonly affected, as older animals have acquired immunity. Affected sheep may display a high-stepping gait (especially the hindlegs) and may progress to seizures, paralysis, coma and death. Some animals may only display milder signs when under exertion; for example, at gathering. Lambs are protected for two to three months by colostral antibodies.
Diagnosis is by histopathology of the brain or virus isolation following postmortem examination, or a serological haemagglutination inhibition test (HAIT). The Moredun Institute has a research PCR test.
Of Veterinary Investigation Diagnosis Analysis (VIDA) diagnoses of louping ill between 2002 and 2019, 477 were in sheep, 112 in cattle and 36 in birds. The diagnoses made in sheep and cattle were in sample submissions from the regions shown in Fig C, from areas that include grazed hill/upland habitats frequently associated with ticks.
Louping ill vaccine is not currently available. In the absence of a vaccine the Moredun Institute has produced a booklet, ‘Best practice guidelines for LIV control in sheep flocks and on grouse moors in the absence of a vaccine’, which is available at www.moredun.org.uk/sites/default/files/louping_ill_best_practice_booklet.pdf
Tickborne encephalitis (TBE) virus is a closely related flavivirus that, although known to be less virulent than louping ill virus for sheep, can cause TBE after transmission to people by infected I ricinus ticks.
A recent study has identified TBE virus in ticks from culled deer in Great Britain.2 Although no autochthonous cases of clinical human disease have been diagnosed in the UK, these early research findings indicate that TBE virus should be considered as a potential cause of encephalitis in patients.
Tick pyaemia is caused by Staphylococcus aureus and may present as either a septicaemia or localised abscess formation. Typically, two- to 10-week-old lambs are affected. Abscessation can be found in a wide range of locations and this determines the clinical disease seen. Presentations can vary from dull, illthrifty lambs to lameness or neurological signs and sudden death.
The bacteria are believed to gain entry into the bloodstream either by direct inoculation during tick feeding or from local superficial wounds. However, the immunosuppressive effect of concurrent A phagocytophilum infection can aid the development of tick pyaemia. Control is dependent on reducing exposure to ticks.
The seasonal distribution of louping ill, TBF and tick pyaemia are shown in Fig D.
Human infection with Babesia divergens, the cause of redwater in cattle, has been described in Scotland.5 It has been shown experimentally that B divergens can infect spleen-intact sheep, but does not induce clinical signs.6
Babesia venatorum has recently been identified for the first time in the UK sheep population.7 This is a zoonotic parasite, which normally infects deer, using I ricinus as its vector. It is found in Europe, but its route into the UK is not known– it may have been brought in by migratory birds. However, the possibility of introduction by infected imported animals from Europe has not been investigated.
Cases of B venatorum in people in Europe are rarely observed, but have been reported in men over 50 years old (hunters) with immune deficiency, including splenectomies.
In Great Britain, Theileria species have been reported in livestock associated with transmission by H punctata. However, these reports have not generally been linked with disease. During April 2005, mortality associated with very heavy infestations of H punctata was investigated in a group of 60 ewes and their lambs grazing north Kent marshland; more than 25 ewes died. In 2012, then novel PCR techniques detected infection with Theileria luwenshuni associated with disease and mortality in the same flock.8
H punctata has been reported in the UK for more than a century but little is known about its distribution. In recent years, reports of this tick species to the Public Health England (PHE) Tick Surveillance Scheme have increased. This rise may be attributable to increased tick surveillance activities or to the increased distribution of this species in parts of England.
A paper reviewing published reports of H punctata and all data held by the Biological Records Centre and PHE, and summarising a number of field studies conducted by PHE and the APHA over the past eight years, has been published.9 It would appear that H punctata may be expanding its range across the eastern part of the South Downs National Park.
Toxoplasma gondii with the potential to cause both human and animal toxoplasmosis has been found in ticks; however, studies on the possible transmission of T gondii from infected ticks to mammalian hosts are limited.10
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