Louping ill virus: an endemic tick-borne disease of Great Britain

Abstract

In Europe and Asia, Ixodid ticks transmit tick-borne encephalitis virus (TBEV), a flavivirus that causes severe encephalitis in humans but appears to show no virulence for livestock and wildlife. In the British Isles, where TBEV is absent, a closely related tick-borne flavivirus, named louping ill virus (LIV), is present. However, unlike TBEV, LIV causes a febrile illness in sheep, cattle, grouse and some other species, that can progress to fatal encephalitis. The disease is detected predominantly in animals from upland areas of the UK and Ireland. This distribution is closely associated with the presence of its arthropod vector, the hard tick Ixodes ricinus. The virus is a positive-strand RNA virus belonging to the genus Flavivirus, exhibiting a high degree of genetic homology to TBEV and other mammalian tick-borne viruses. In addition to causing acute encephalomyelitis in sheep, other mammals and some avian species, the virus is recognized as a zoonotic agent with occasional reports of seropositive individuals, particularly those whose occupation involves contact with sheep. Preventative vaccination in sheep is effective although there is no treatment for disease. Surveillance for LIV in Great Britain is limited despite an increased awareness of emerging arthropod-borne diseases and potential changes in distribution and epidemiology. This review provides an overview of LIV and highlights areas where further effort is needed to control this disease.

Fig. 1.

A schematic of the LIV genome, translation of the viral polyprotein and proteolytic cleavage to generate individual proteins. Closed arrowheads indicate cleavage by a virally encoded protease, whereas open arrowheads are cleavage sites mediated by host proteases such as furin (Grard et al., 2007).

Fig. 2.

Map of the British Isles showing the distribution and number of LIV diagnoses in animals in British counties from 1975–2013.

Fig. 3.

Pathology associated with infection with LIV. (a) Subacute polioencephalitis observed in the brain of a British sheep. Note the perivascular cuff (arrow), neuronophagia, vacuolation of the neuronal cytoplasm and gliosis. Image shows a formalin-fixed section of the red nucleus stained with haematoxylin and eosin (original magnification ×200). (b) Detection of LIV antigen in the cerebral cortex of a sheep with neurological disease. Note the distinct brown labelling of the cortical neurones associated with a subtle perivascular and menigeal response. The staining was obtained using monoclonal antibody LM3.3 kindly provided by Moredun Research Institute (original magnification ×20).