Louping ill was a condition that most sheep farmers had not ever experienced in Ireland until recent years. Interestingly in 2012, a publication found a sheep level prevalence of 8.5%, and a flock level prevalence of 9.8% in Ireland, based on serology.

Positive animals, based on antibody testing, were more common in upland flocks. Louping ill is a tick-borne viral infection which predominantly affects sheep and red grouse, causing nervous signs (and chick mortality in grouse). The condition has been reported in humans (zoonosis) and has been documented occasionally in other species including goats, llamas, alpacas, swine, horses, deer, cattle, dogs, European elk and a number of small mammals (rabbits, hares, rats etc).

The name louping ill is derived from an old Scottish word whereby sheep loup or spring into the air. The disease in humans is characterised by flu-like symptoms or neurological signs after exposure, but is rarely fatal. Humans are most commonly infected by tick bites rather than directly from sheep contact, though they can be infected via exposure to tissues or instruments contaminated with the virus.

The virus has been shown to transfer to pigs via ingestion of raw meat from infected lambs. Four subtypes of louping ill virus exist – the British, Irish, Spanish and Turkish subtypes – though the Turkish subtype may be reclassified as a type of tick-borne encephalitis virus. Its main maintenance host in a region is sheep, as they are best at amplifying the virus. Red grouse are also capable of multiplying the virus but they die quickly so they are not as effective at maintaining disease as sheep. Mountain hares may play a minor role in sustaining tick infection levels.

Tick infestation of pasture is also a requirement, however, and as a result the disease is most commonly reported throughout upland areas wherever the tick vector, Ixodes ricinus, is found. There is a seasonal occurrence in the spring and autumn, coinciding with the activity of the tick vector. In endemic areas, most losses occur among unvaccinated younger sheep that are no longer protected by maternal antibodies (>2-3 months old), while animals older than two years are commonly immune.

Exceptionally, the virus can wane in a region, meaning that adult sheep fail to be exposed and as a result become naïve. This means that clinical signs can occur in all age groups of sheep (including older sheep) when the virus is re-introduced into the area. Louping ill virus can also be transmitted by other mechanisms. This virus is shed in the milk of goats, and to a lesser extent sheep, potentially infecting nursing lambs. The time from infection to the development of clinical signs (incubation period) for louping ill is six to 18 days in sheep.

Clinical signs

Louping ill manifests clinically as fever, inability to stand, salivation, nervous nibbling, star-gazing and hindlimb paralysis which progresses to coma and death in a few days. Fifty per cent of sheep with nervous signs die, though sudden deaths can also occur without other signs.

Concurrent tick-borne fever infection (Anaplasma phagocytophilum) is thought to amplify the effect of the louping ill virus and may increase the susceptibility to disease. Severe clinical signs can be seen in naïve sheep flocks moved into endemic areas (including purchase of replacements) though signs in animals that were always resident in the endemic area may be mild or subclinical.

Disease control

Many animals may develop neurological disease, and up to 60% of the flock can die. Control of disease depends on the control of ticks and immunisation against the virus. A variety of products is available for controlling ticks on sheep, though the persistence of cover can be quite short, so it is important to check the label before purchasing. A vaccine has been intermittently available under emergency (AR16) license against louping ill. This vaccine is administered as a single dose at least a month before the risk season (springtime) and has a duration of immunity of at least 18 months, which means booster doses are given every two years.

It is advisable for you to contact your vet in this regard as the application process for a special license for this product can take some time.