Louping Ill is a viral disease primarily affecting sheep and other livestock, causing severe neurological impact. It is caused by a virus transmitted through ticks, leading to illness and sometimes death in affected animals. The name “louping ill” originates from an old Scottish term describing the characteristic leaping or spring-like gait observed in infected sheep. While mainly an animal health concern, Louping Ill can also affect humans.
The Causative Virus and Its Carriers
Louping Ill is caused by the Louping Ill virus (LIV), a member of the Flaviviridae family, which includes other tick-borne encephalitis viruses. This virus is an enveloped, positive-sense single-stranded RNA virus. Four subtypes of LIV have been identified: British, Irish, Spanish, and Turkish, with their distribution clustering geographically.
The primary vector for transmitting the Louping Ill virus is the sheep tick, Ixodes ricinus (also known as the castor bean tick). These ticks acquire the virus when feeding on an infected host, such as sheep or red grouse, which maintain the virus. The virus then persists within the tick through its various life stages, allowing transmission to new hosts during subsequent blood meals.
The life cycle of Ixodes ricinus spans three years, with ticks feeding for only a few days annually as larvae, nymphs, and adults. This transstadial transmission means the virus is carried from one developmental stage of the tick to the next. While sheep and red grouse are the most ecologically significant transmission hosts, other mammals like deer can also carry the tick, contributing to the virus’s presence.
Recognizing Symptoms in Affected Animals
Clinical signs of Louping Ill in sheep appear between 6 to 18 days after infection. The disease presents with a biphasic fever: an initial febrile stage with non-specific signs like depression and loss of appetite, followed by a second rise in temperature associated with neurological symptoms. During this second phase, the virus invades the central nervous system, causing distinct neurological manifestations.
Affected sheep may display muscle tremors, incoordination, and an unusual “louping” gait (both front legs move forward together, followed by the hind legs). Other signs include hypersensitivity to sound or touch, jaw champing, and drooling or tongue protrusion. As the disease progresses, animals may exhibit posterior paralysis, become recumbent, and experience convulsions or fall into a coma. Mortality rates range from 1% to 4% in adult sheep and up to 60% in lambs, particularly in newly exposed flocks.
Cattle and horses can also be infected, though they often show less severe clinical signs than sheep. When symptoms do occur in cattle, they may include lateral recumbency, convulsions, and hyperaesthesia. Horses can present with ataxia, muscle tremors, and hyperexcitability, but usually recover.
Although rare, Louping Ill is a zoonotic disease, spreading to humans, often through occupational exposure from tick bites or contact with infected animal tissues. Human infections can initially cause flu-like symptoms such as fever, headache, and muscle stiffness, lasting several days. In some cases, the illness can progress to more severe neurological signs, including meningoencephalitis with symptoms like severe headache, neck stiffness, tremors, or a poliomyelitis-like illness causing paralysis.
Where Louping Ill Occurs
Louping Ill is predominantly found in the British Isles, with prevalence in Scotland, northern England, Wales, and Ireland. Its presence extends to parts of northern Europe, and similar viruses have been reported in Norway, Spain, Turkey, and Bulgaria. The distribution of the disease closely correlates with the habitats of its primary vector, the Ixodes ricinus tick.
The ecological factors contributing to the prevalence of Louping Ill in these regions include moorlands and upland pastures. These environments provide ideal conditions for sheep ticks to thrive, characterized by suitable vegetation and humidity. The presence of susceptible hosts, such as sheep and red grouse, in tick-infested areas further facilitates the maintenance and transmission of the virus.
The seasonal activity of Ixodes ricinus ticks, highest in spring, summer, and early autumn, dictates peak disease occurrence. This geographical and ecological alignment highlights the close relationship between the vector, host animals, and the virus in maintaining the disease cycle.
Identifying, Managing, and Preventing Louping Ill
Diagnosis involves observing clinical signs, conducting post-mortem examinations, and performing laboratory tests. Neurological dysfunction in tick-exposed animals is highly suggestive. Post-mortem examination of brain and spinal cord tissue can reveal characteristic lesions, providing a presumptive diagnosis confirmed through immunohistochemistry.
Laboratory tests confirm the infection. Polymerase Chain Reaction (PCR) can detect the virus directly in central nervous system tissue, while serological tests, such as hemagglutination inhibition or ELISA, identify antibodies against the virus, helping to differentiate between recent and past infections. There is no specific treatment for infected animals, underscoring the importance of prevention.
Prevention and control measures focus on reducing exposure to the tick vector and enhancing host immunity. Vaccination of susceptible livestock, particularly sheep, is a primary strategy, though commercial sheep vaccine availability has varied. Effective tick control strategies include applying acaricides to animals and careful pasture management to reduce tick habitats, such as burning heather or managing grazing patterns.
Biosecurity measures, such as limiting naive sheep movement into endemic areas, also help restrict disease spread. For humans working in affected areas or with potentially infected animals, precautions are advised due to Louping Ill’s zoonotic nature. Although rare, exposure through tick bites or contact with contaminated materials can lead to infection, making awareness and protective measures important.