Is West Nile Virus a Forgotten Disease in North America?

West Nile virus is still a threat to horses, but veterinarians say vaccination rates are down.
Share
Favorite
Close

No account yet? Register

ADVERTISEMENT

mosquito
Lack of vaccination could explain the spike in West Nile virus cases. | Photos.com
After its arrival to New York 23 years ago, West Nile virus (WNV) spread rapidly across the country, becoming endemic within a few years. Scientists promptly produced a vaccine, and equine veterinarians recommended it for all adult horses. Even still, veterinarians say there are continued spikes in disease incidence, suggesting not all owners are keeping their horses current on WNV vaccination. This begs the question, is WNV already a forgotten disease?

Antoine Levasseur, DVM, a master’s candidate in veterinary epidemiology at the University of Montreal’s Faculty of Veterinary Medicine, said the number of equine West Nile virus cases reported in Canada varies from year to year. He and colleagues from the Canadian Food Inspection Agency (CFIA, the country’s USDA equivalent) generated data that showed WNV cases spiked in  Western Canada in 2007, 2016, and 2018. Lower incidence rates were reported in intermittent years.

Similarly, data from the USDA Animal and Plant Health Inspection Service show a somewhat cyclical pattern of WNV, with peaks in 2012 and again from 2016 to 2018. In 2019 and 2020, fewer than 100 cases of WNV were reported each year.

“And the reported number of cases is likely underestimated,” Levasseur said. “Most infections are either subclinical or mild, which means those horses are probably not tested for West Nile virus, especially if the horse recovered quickly. Without a positive diagnostic test, the cases are not notified to the CFIA.”

Vaccination Rates Down?

Levasseur said one reason for the continued spikes in WNV cases, of course, could be lack of vaccination.

“Vaccination (in horses) doesn’t seem widespread in Canada based on the data we collected and doesn’t vary a lot from year to year,” said Levasseur. “We know that vaccination coverage is less than 40%, which is pretty low for an endemic disease.

“We think that owners may be less likely to vaccinate horses in areas where West Nile virus is not frequent and where people do not hear about West Nile virus,” he added. “Hence, awareness may be key to controlling the disease.”

In addition, practitioners must booster the WNV vaccine the first year of its administration, requiring two veterinary visits.

“This could be a financial barrier, particularly in areas where WNV is not common,” said Levasseur.

Lack of vaccination could explain the WNV spikes in U.S horses, as well.

That said, other factors can contribute to the periodic surge in WNV cases, such as weather patterns and expansion of the reservoir hosts and vectors, says Nicola Pusterla, DVM, Dipl. ACVIM, professor of medicine and epidemiology at the University of California, Davis, School of Veterinary Medicine.

In 2021 researchers in Texas published a review article on WNV in humans, suggesting the deadly virus is already often overlooked. Levasseur said this attitude could explain why owners no longer feel the pressure to vaccinate their horses against WNV, despite the vaccine being very effective.

The American Association of Equine Practitioners (AAEP) considers the WNV vaccine to be a core vaccine alongside rabies, tetanus, Eastern equine encephalomyelitis (EEE), and Western equine encephalomyelitis (WEE). This means all horses should be vaccinated against WNV according to the AAEP vaccination guidelines.

“The vaccine industry has made it easy to vaccinate against WNV by incorporating this antigen in the already existing multivalent vaccines (EEE/WEE/tetanus plus WNV),” Pusterla noted, adding, “I would say that the majority of horses that are receiving their spring vaccines will also be vaccinated with WNV.”

Ronca et al. (2021) highlighted the importance of vaccinating horses against WNV annually, stating, “WNV may no longer be considered a novel or emerging pathogen in the U.S., but its clinical and public health significance has not diminished.”

Indeed, veterinarians report mortality rates between 22 and 44% in horses with clinical signs of disease, which include incoordination/ataxia, weakness, and paralysis. With no specific therapy for WNV, affected horses receive general supportive care, and recovered horses often have permanent neurologic deficits that potentially limit their future use, said Levasseur.

Finally, this vector-borne disease can “spill over” into human populations, and no vaccine against WNV exists for people.

Share

Written by:

Stacey Oke, MSc, DVM, is a practicing veterinarian and freelance medical writer and editor. She is interested in both large and small animals, as well as complementary and alternative medicine. Since 2005, she’s worked as a research consultant for nutritional supplement companies, assisted physicians and veterinarians in publishing research articles and textbooks, and written for a number of educational magazines and websites.

Related Articles

Stay on top of the most recent Horse Health news with

FREE weekly newsletters from TheHorse.com

Sponsored Content

Weekly Poll

sponsored by:

Where do you primarily feed your horse?
327 votes · 327 answers

Readers’ Most Popular

Sign In

Don’t have an account? Register for a FREE account here.

Need to update your account?

You need to be logged in to fill out this form

Create a free account with TheHorse.com!