West Nile Virus in Horses: Still a Threat

Over the last two decades, West Nile virus (WNV), an orthoflavivirus transmitted by mosquitoes, has become endemic on all continents except Antarctic and continues to be a major threat to the health and well-being of both horses and humans. The virus had an explosive emergence in North America in 1999, followed by rapid expansion throughout the US and parts of Canada over the first five years, which resulted in often catastrophic disease in over 25,000 US horses. With the rapid deployment of vaccines, the US equine caseload decreased dramatically by 2006.

Once established in a new area, reported cases in humans and horses, depending on vaccine status, will occur yearly, but can vary in intensity from year to year. The virus can also cause explosive outbreaks in new locations or among those that have only experienced sporadic cases.

While most mosquito-borne viruses are typically associated with tropical or subtropical climates with high precipitation, WNV is endemic in a variety of climates, including temperate, continental and semi-arid areas. The continental US and Europe are two great illustrative examples of the adaptability of this wide-ranging virus. During the last 10 years, 2,484 cases of equine WNV were reported in 46 US states. Climate did not appear to be an issue since the states reporting the highest number of cases of WNV infection (Texas, Pennsylvania, California, Colorado, Utah and Montana) spanned all six US National Oceanic and Atmospheric Administration defined climate regions.

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In Europe, most WNV activity in equids was initially reported in regions around the Mediterranean basin; however, during the past two decades, WNV activity was reported in more central and northern countries and regions such as Western France (Landes), Eastern Austria (Vienna), Hungary, and Northeastern Germany (Berlin). In 2024, a spike in the number of cases occurred with 494 affected premises reported in 10 countries, representing a 50% rise in reported equine cases over the two previous years. Northern Germany had the highest number of reported cases, approximately 200, with disease occurring in two federal states where activity had not been previously reported. There were 55 affected premises, typically with one or two cases, reported in Austria, a country in which the previous highest case count was four in 2019.

(Ed. note: In Canada, 41 equine WNV cases were reported in 2024; in Saskatchewan, Manitoba, and Ontario, the only provinces currently providing mosquito pool testing, 343 mosquitos pools were reported.)

WNV outbreaks are not confined to North America and Europe. Outbreaks have been reported in Central and South America, the Middle East and Australia. Australia is home to WNV lineage denoted as Kunjin virus, or WNV subtype 1b, which underwent a change in virulence resulting in a severe WNV outbreak in 2011 affecting over 1,000 horses.

Nine lineages are now recognized with at least three (Lineage 1a, L1b and L2) associated with neurological disease in horses and cause similar clinical signs and pathology. When data is analyzed across multiple studies, mortality is about 30-40% for all lineages. Neurological disease, often with severe outcomes, remains the primary clinical presentation in the nonvaccinated horse.

While early studies suggested an increased risk of mortality for older horses associated with underlying health issues, in fact all unvaccinated horses less than one year old infected with WNV died.

Contrary to observations for other viral epidemics, the virulence of this virus has not declined over time. West Nile fever is not recognized in the horse and all clinical manifestations observed are neurological. Affected horses had one or more neurological abnormalities consisting of changes in mentation (30%), locomotion (70%) and/or cranial nerve deficits. A common feature (approximately 80%) of WNV in horses was the presence of muscle tremors or fasciculations. In an analysis of pooled data from 14 studies (2,182 horses), approximately 30% of horses became recumbent, necessitating euthanasia for humane reasons.

Many studies examined risk factors for disease, including intrinsic factors such as age, sex or breed predilection with pooled analysis demonstrating no inherent bias. While early studies suggested an increased risk of mortality for older horses associated with underlying health issues, in fact all unvaccinated horses less than one year old infected with WNV died. Multiple extrinsic factors such as hair coat, housing, use of fans, etc. have been analyzed and were found equivocal to disease. Vaccination overrides all risk factors in terms of effect on susceptibility to disease.

With no specific antiviral therapies, vaccination is the only intervention that successfully prevents severe neurological disease in the horse. Currently all marketed vaccines have comparable efficacy (over 90%) against known lineages that infect horses, and vaccines are marketed in North America, the United Kingdom and Europe. The American Association of Equine Practitioners identifies WNV vaccination a core component, which indicates that horses should be immunized annually to prevent disease. Vaccination against WNV is considered a ‘best practice’ under the guidelines provided by The Equine Infectious Disease Surveillance of the United Kingdom. While annual boosters are recommended, horses are not fully protected unless the initial immunization consists of two injections given four weeks apart. Foals must receive an additional booster 10 to 12 months prior to the next mosquito season or eight weeks after the second dose.

With continuous expansion of this virus within and beyond an established endemic area, coupled with (international) travel of horses in and out of these areas, veterinarians and owners should consider all horses irrespective of location and climate at risk for WNV disease if not vaccinated.