West Nile (WN) fever (WNF), also referred to as WN encephalitis, is an infectious, non-contagious disease in horses caused by the flavivirus West Nile virus (WNV). WNV is insect vector-borne by various species of mosquito and there are two strains of the virus, lineage 1 and lineage 2. WNV is naturally maintained in infection cycles between wild birds through mosquitoes but the infection may spill-over into other species, including humans (WNV is zoonotic) and horses when infected mosquitoes feed on them. However, infected horses do not act as a source of sufficient virus for feeding mosquitoes to infect other animals and, as with humans, are considered ‘incidental’ or ‘dead-end’ hosts of the virus.
Clinical signs of WNV infection can vary markedly from subclinical (no obvious signs seen) through to severe neurological disease, with a high fatality rate (see Clinical signs below). Although WNF describes the disease seen in humans, fever is not always a feature of the disease in horses. WNV occurs worldwide, with the only report of an equine clinical case in Great Britain being in 2013 in an imported horse that recovered. The pattern of cases in infected countries is seasonal, linked to when mosquito numbers rise in a region, such as in the late summer months (so called vector seasons).