West Nile virus in horses — What do you need to know to diagnose the disease?

West Nile virus (WNV) is an emerging arthropod-borne pathogen that causes disease in horses and humans, as well as in a variety of other animals. The virus is maintained within the avian wildlife population that acts as a natural reservoir and amplifier for the virus, which is transmitted mostly by mosquitoes (Culex spp.) (1).

In humans, WNV infection is considered a serious zoonotic disease. The infection is asymptomatic in 80% of cases and appears as a flu-like syndrome in 20% of cases. Neurological disease develops in less than 1% of infections. In these, however, case fatality may be as high as 20%, and chronic disease develops in most survivors. In Canada, the number of human cases reported varied, from 21 to 190 in the past 5 years (1).

The first case of WNV in horses was diagnosed in Canada in 2002 (2). West Nile virus is an immediately notifiable disease under the Health of Animals regulations in Canada (3) and is also a notifiable disease in many provinces (British Columbia, Alberta, Saskatchewan, Manitoba, Ontario, and Quebec). From 2013 to 2017, the number of cases reported in horses in Canada via passive surveillance has been 57, 21, 19, 46, and 54 per year respectively, reflecting a continued presence of the virus, and need for horse owner awareness of the risks and preventative measures.

Infection in horses can be asymptomatic; however, approximately 20% of horses infected will develop clinical signs. These signs include ataxia (stumbling, staggering, wobbly gait, or incoordination) combined with circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle fasciculation, proprioceptive deficits, blindness, lip droop/paralysis, teeth grinding, fever, or acute death. Horses that develop neurological disease may have up to 30% case-fatality, and another 10% to 20% recover with residual neurologic deficits. The clinical manifestation of WNV infection may resemble other reportable/notifiable diseases [rabies, eastern equine encephalitis (EEE)], which should be ruled out to confirm disease. Correct identification of WNV cases in horses is important since surveillance indicates the presence of a pathogen and its vector in a geographic area, and the risk of infection to other equines and humans. Cases of WNV infection in horses (and other domestic animals) are reported to the animal health and public health federal-provincial-territorial entities. West Nile virus is also a notifiable disease to the World Organisation for Animal Health (OIE).

A horse is classified as a positive case based on clinical signs AND laboratory testing

The horse must have a clinical presentation of ataxia and at least 2 of the following: circling, hind limb weakness, inability to stand, multiple limb paralysis, muscle fasciculation, proprioceptive deficits, blindness, lip droop/paralysis, teeth grinding, fever, and acute death (4).

Clinical history, location of the horse, as well as vaccination status on the submission form will help confirm the diagnosis and determine the area at risk.

Laboratory testing involves identification of the virus or of a specific immune response to WNV (4). The identification of WNV may involve isolation from tissue [preferred diagnostic tissues are brain or spinal cord; blood or cerebrospinal fluid (CSF)] or by a positive polymerase chain reaction (PCR) to WNV genomic sequences in tissues and appropriate histological changes or, a positive immuno-histochemistry (IHC) for WNV antigen in tissue and appropriate histological changes.

Serological diagnosis is useful for an animal with clinical signs. Detection of IgM antibody to WNV by enzyme-linked immunosorbent assay (ELISA) testing in serum or CSF in a horse that is not known to have been recently vaccinated confirms the diagnosis if associated with compatible clinical signs. Immunoglobulin M (IgM) antibody detection by ELISA is the test of choice since IgM antibodies are short-lived and indicative of a recent infection. There is no need for paired samples to confirm the diagnosis. In some horses, vaccination with specific vaccines has been reported to cause increases in IgM concentration for up to several weeks after vaccination; therefore, the date of the most recent WNV vaccination, if known, should accompany the test request (5,6). Otherwise, confirmation of a case requires an associated 4-fold or greater change in IgG ELISA test or serum neutralization (SN) test antibody titer to WNV in paired sera, in which the first serum should be drawn as soon as possible after onset of clinical signs and the second drawn at least 7 d after the first. Unless paired serum samples are submitted, the elevated titer to WNV antibody by SN test in serum or positive IgG ELISA test on a single sample will only allow classification as a possible case (4).

Clinical history and appropriate test selection contribute to effective and efficient diagnosis of West Nile virus infection on an individual basis and with a One Health perspective.