The Disease

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 to other members of this family, which are transmitted by Ioxodid ticks. The tick-borne encephalitides caused by these viruses are primarily a problem of infections in man and it is only louping-ill virus that produces disease in domestic animals, especially sheep, as well as man. The disease also occurs in grouse (Reid, 1978). Louping-ill used to be regarded exclusively as a problem of the British Isles, but is now known to have occurred on mainland Europe (Reid, 1991a).

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 pyrexia 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 paralysed. Death usually follows (Reid, 1990; Reid, 1991b). It has been shown that tick-borne fever increases the susceptibility of sheep to louping-ill virus and invariably causes death from a haemorrhagic syndrome involving a systemic mycotic infection with Rhizomucor pusillus (Brodie et al., 1986; Reid et al., 1986).

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.

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, older animals being immune. Lambs from immune ewes are protected by antibodies in the 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.