Louping-ill

A tick-borne disease of sheep

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Sheep Diseases

Louping-ill

Louping-ill is an acute viral disease of the central nervous system transmitted by sheep ticks (Ixodes ricinus). The disease can also spread via droplet infection. The disease is caused by a member of the Flaviviridae family. It is antigenically closely related (i.e., stimulates a similar immune response) to other members of this family, which are transmitted by Ioxodid ticks. Most tick-borne encephalitides caused by these viruses primarily affect humans and it is only louping-ill virus that produces disease in domestic animals, especially sheep, as well as man. The disease also occurs in red grouse (Reid, 1978). Louping-ill used to be regarded exclusively as a problem of the British Isles and Ireland, but is now known to have occurred on mainland Europe in Norway, Denmark and Spain (Gilbert, 2015; Reid and Chianini, 2007).

Clinical Signs for Louping-ill

Following infection, between 5 and 60 per cent of animals develop clinical signs, which vary from slight ataxia to sudden death. There is usually biphasic fever with encephalomyelitis following the second rise. The animal is dull, the head is held low and it may head press. Muscle tremors and muscle rigidity are apparent, particularly on the head and neck. There is a louping gait with both front legs being moved forward together, followed by the back ones. Often there is hyperaesthesia to sound or touch. Jaw champing occurs and in some cases drooling of saliva and protrusion of the tongue. During the latter stages of the disease, lateral recumbency occurs with a normal corneal reflex. Some animals paddle whilst others become paralyzed. Death usually follows (Reid, 1991, 1990). It has been shown that tick-borne fever increases the susceptibility of sheep to louping-ill virus and invariably causes death from a hemorrhagic syndrome involving a systemic mycotic infection with Rhizomucor pusillus (Brodie et al., 1986; Reid et al., 1986).

The Disease Vector

The vector for transmitting Louping-ill virus is the common sheep tick, Ixodes ricinus, also known as the castor bean tick. Their lifecycle takes 3 threes, as the ticks only feed for a few days days each year, as larva in the first year, a nymph in the second and an adult in the third (Taylor et al 2007).

The vector for transmitting louping-ill virus is the common sheep tick, Ixodes ricinus, also known as the castor bean tick. Their lifecycle takes 3 years, as the ticks only feed for a few days each year, as larva in the first year, a nymph in the second and an adult in the third (Taylor et al 2007).

Ixodes ricinus the vector for the louping-ill virus, is a generalist parasite i.e., is not host-specific, it blood feeds on many mammals and is able to carry and transmit a wide range of tick-borne diseases. In European cattle it transmits Babesia divergens, Babesia bovis (cause of red-water), Anaplasma marginale, and in both sheep and cattle it transmits louping-ill virus, rickettsia and is associated with pyaemia caused by Staphylococcus aureus. Ixodid ticks also transmit Borellia burgdorferi, the spirochete responsible for Lymes disease in humans (Taylor et al., 2007). As transmission of the disease is mostly dependent on the sheep tick Ixodes ricinus, the incidence of louping-ill follows closely the periodicity of the tick (Beasley et al., 1978). Losses occur in two peaks: one in the spring followed in some areas by a second peak in the autumn.

Wildlife Reservoirs

Sheep, red grouse and mountain hares have been identified to be the most ecologically important transmission hosts for louping-ill virus, however clinical louping-ill virus has been reported in farmed red deer, roe deer, and the mountain goat Cantabrian chamois (Gilbert, 2015). Louping-ill has been reported to be pathogenic in experimentally infected laboratory mice (Sheahan et al., 2002), and mice are an important reservoir for the closely related tick-borne encephalitis viruses however there is no evidence that louping-ill virus is transmitted by the wild small rodent species most common in the UK (Gilbert, 2015).

Disease in Sheep

The disease may affect all ages of animals either where infection is present at a low incidence or where infection has only recently been introduced. Bought-in sheep may also be susceptible. In areas where the disease is endemic, losses primarily occur in lambs and replacement stock with older animals being immune. Lambs from immune ewes are protected by antibodies in colostrum and are unlikely to become infected in the first spring, unless the intake of colostrum is insufficient (Reid and Boyce, 1976). Heaviest mortalities are seen in the next spring in ewe lambs retained for breeding, as they become fully susceptible to the disease.

Control and Prevention of Louping-ill

A specific inactivated louping-ill vaccine is available, a single dose providing immunity for at least two years (Reid and Pow, 1995; Wells and Reid, 1978). In endemic areas it is common practice to vaccinate all replacement ewe lambs and bought-in sheep at least 28 days before exposure to tick-infested pasture. Another proposed control strategy is associated with the fact that ticks become ‘clean’ of louping-ill infection within two years if there are no reservoirs of the disease. The practice involves the removal of all susceptible sheep or systematic mass vaccination for two years. However, removal of all sheep from tick-infested pasture is often not practical and mass vaccination not justified as wild animals can reintroduce the disease (Reid and Chianini, 2007).

Ticks commonly inhabit areas of rough grazing, moorland and woodland.

Ticks commonly inhabit areas of rough grazing, moorland and woodland.

In areas where the prevalence of louping-ill, tick pyaemia and tick-borne fever is high, losses may be reduced by dipping (Reid and Chianini, 2007). However, sheep dip contains organophosphates which can be particularly damaging to the environment. In the UK farmers need a certificate of competence to be able to dip sheep and in the USA the use of products containing organophosphates are not permitted under AWA standards if there are other treatment options available, so topical ectoparasiticides, such as cypermethrin or deltamethrin containing products are now often used (Hardeng et al., 1992). Hill pastures may be improved to lower tick numbers and decrease the number of infections. However, this method may make the sheep susceptible to louping-ill and tick-borne fever infection when moved to tick infested unimproved ground.

A number of organizations do not permit the use of organophosphate products to treat livestock. Certified producers should consult with their certifying body. 

Treating Louping-ill

If there is evidence susceptible animals have been exposed to the disease, your vet may advise administration of the louping-ill antiserum within 48 hours of exposure to the infection. Recumbent animals must be humanely destroyed.

Good Practice Based on Current Knowledge

  • In louping-ill endemic areas a vaccination program should be discussed with your vet as part of the overall flock health plan. The extent of the vaccination program depends on the farm situation
  • In a closed flock eradication of the disease may be attempted through mass vaccination. However, the situation has to be right as the disease can be reintroduced through wildlife vectors
  • In the case of common grazing, the eradication of the disease is very difficult and just the replacement animals and bought-in stock should be vaccinated
  • Use ectoparasiticides to reduce the tick burden on lambs
  • Recumbent animals have to be humanely destroyed

 

Louping Ill References
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