Louping-ill is caused by Louping ill virus (LIV) , the only flavivirus known to be endemic in the UK. The virus is closely related to the tick-borne encephalitis virus. Louping-ill is an infection of the central nervous system, and is acquired from the bite of an infected tick. It primarily affects sheep but can infect cattle, horses, pigs, dogs, deer, and wildlife species; it is an important disease of grouse. The name 'louping-ill' is derived from an old Scottish word describing the effect of the disease in sheep whereby they 'loup' or spring into the air.

Louping-ill also occasionally affects people.

Epidemiology

Louping-ill is endemic and occurs regularly in areas throughout the British Isles where pastures are infested with the sheep tick (Ixodes ricinus). Such areas are found mainly on hilly areas of rough upland grazing or moorland. LIV variant viruses occur elsewhere in Europe, in particular Spain, Turkey, and Greece, where they cause similar disease in sheep and goats.

Certain occupational groups are at increased risk of acquiring louping-ill virus infection; of the 45 published reports of human clinical louping-ill virus infections worldwide, most occurred in laboratory personnel working with the virus, or those whose likely exposure was to infected sheep (see table 1).

Table 1: details of exposure and disease type in 45 human cases of louping ill

Type of exposure and disease	Number of cases
Laboratory exposures
Bi-phasic illness	11
Influenza-like illness	14
Haemorrhagic disease, no encephalitis	1
Natural exposures
Shepherd/crofter/sheep farmer/vet	9
Butcher/abattoir	3
Unknown	5
Forestry worker (local area endemic)	1
GP (local area endemic)	1

(Adapted from Davidson 1991 )

Serological surveys have found further evidence of exposure to louping-ill virus in these groups, in particular in abattoir workers.

There are no recent reports of louping-ill virus infections in laboratory personnel or other potentially exposed persons.

The disease in animals

Louping-ill causes disease mainly in sheep and grouse in particular upland areas, and is uncommon elsewhere or in other species. Louping-ill in sheep is also known as infectious ovine encephalomyelitis. There are occasional large outbreaks in sheep on upland pastures of Great Britain, with significant losses when sheep previously unexposed to louping-ill virus are put onto pastures where the infection is endemic. Clinical louping-ill is likely to cause significant suffering in clinically affected sheep, however many animals have asymptomatic infections. A recent cluster of cases in horses occurred in Devon (see Hyde, 2007 ). 

Infection of red grouse may significantly reduce grouse population and therefore have an effect on local economies (i.e. grouse shoots).

• Further information available here (Moredun Institute)
• Defra disease profile (PDF)

The disease in humans

Human cases are very uncommon, and most of the recorded cases have occurred in laboratory or abattoir workers. The incubation period is thought to be 4 to 7 days. Clinical disease resembles the European form of tick-borne encephalitis, and is usually biphasic. There is an initial 'influenza-like' illness lasting a few days with a moderately raised temperature, headache, muscle pains and malaise, followed by a remission of about a week. Fever then recurs with signs of meningoencephalitis or paralytic poliomyelitis. Blood white cell counts are depressed in the initial phase and raised in the second. Convalescence in humans may be prolonged, but after recovery most patients have no long-term ill effects. In laboratory and slaughterhouse workers the disease may be limited to the first phase and mistaken for influenza.

Transmission

Besides tick bites, the virus can be transmitted by contact with infected animal tissues, particularly if these are handled in a manner which generates aerosols. Although not usually regarded as a foodborne disease, it has been demonstrated experimentally that acutely infected sheep and goats can shed the virus in their milk. Transmission by consumption of infected unpasteurised milk may therefore be theoretically possible but has not been documented. There is a slight possibility of hill walkers acquiring disease through being bitten by infected ticks in regions where the disease is enzootic. The fact that both clinical and sub-clinical infections are rare in rural areas but are more frequent among laboratory and slaughterhouse workers, may indicate that handling infected viscera and needle-stick injuries may be more important routes of transmission to man than tick bites.

Transmission between people has not been recorded. There may be a hypothetical risk of transfer by blood transfusion, however this has not been documented.

• Resources
  Added/updated: 6 August 2009