Disease Du Jour: West Nile Virus Refresher
In this episode of the Disease Du Jour podcast, Dr. Erika Schwarz discusses West Nile virus (WNV) in horses.
Mosquito, which spreads West Nile virus (WNV)
West Nile virus (WNV) is considered endemic in the continental United States, and horses are one of the most commonly affected mammals. | Getty Images

West Nile virus (WNV) is considered the most common mosquito-borne virus in the continental United States, and horses are one of the susceptible species. In this episode of the Disease Du Jour podcast, Erika Schwarz, DVM, MPH, PhD, DACVM, provides a refresher on this common virus. 

History of West Nile Virus (WNV)

Schwarz started the podcast by discussing the history of WNV, which was first isolated from a febrile human patient in Uganda in 1937. The virus was first identified in the US in 1999, and today it is considered endemic to the lower 48 States. 

Affected Species

“Aside from humans, horses are the most common mammal that can be infected with West Nile virus,” said Schwarz. She noted that bats, rodents, skunks and alligators can also become infected, but this is incredibly rare. “Horses make up about 97% of non-human mammalian West Nile virus cases,” she said. 

Birds are also susceptible to the virus; there are a number of birds that are not considered reservoir species for the virus but can be infected and show clinical signs, including corvids like crows, magpies and ravens. 

Schwarz said that both horses and humans are considered dead-end hosts, which means that once they get the virus, it can’t replicate to high enough titers to be passed to a non-infected feeding mosquito. “The transmission cycle stops with us,” she said.

Clinical Signs of WNV in Horses

According to Schwarz, about 80% horses that are infected with WNV only experience mild disease, while about 20% of horses develop more severe clinical signs. “Horses are more likely than humans to develop severe neuroinvasive disease once infected,” she said.  

Schwarz said that approximately 20% of horses have flu-like symptoms, while around 1% will develop West Nile neuroinvasive disease. “Depending on where the virus localizes in the brain after crossing the blood-brain barrier, it can cause disease in the brain or the spinal cord, and that can manifest as changes in mentation,” she said. 

Some clinical signs to watch out for in horses with neuroinvasive disease include depression, hyperexcitability, chewing, pacing, locomotive issues, recumbency, cranial nerve aberrations, reduced reflexes, facial tremors, muscle fasciculations, lip drooping, ataxia, kneeling, dog sitting, limb weakness and paresis. 

“Mild cases generally have a decent shot at recovery, but as you probably guessed, the more neurologic the horse, the more guarded the prognosis,” Schwarz said. “Some of these animals will actually go down and won’t get back up.”

Diagnosing WNV

Schwarz said that for less severe cases, practitioners don’t always jump to diagnostic testing. Rather, they see how the horse does with supportive care; those cases typically recover. “I think a lot of cases of mild West Nile virus probably go undiagnosed,” she said. 

In the case of more severe neurologic disease, practitioners might pull blood to test for WNV. For anti-mortem testing, WNV is primarily diagnosed with serology using the IgM capture ELISA, which Schwarz said is rapid and highly accurate. Horses can be diagnosed with WNV from a week post-infection to about two months post-infection. She noted that diagnosis can be less clear in vaccinated horses, which typically won’t mount as significant of an IgM response as naïve horses. In those cases, diagnosis might require a plaque reduction neutralization test. 

Treatment for WNV in Horses

Treatment for WNV depends on the severity of the case. Mild cases might only require supportive care, such as anti-inflammatories or fluid therapy. Schwarz said that there are limited treatment options for horses showing significant neurologic disease. 

“Most horses that show severe neurologic disease have a guarded prognosis, especially if they become recumbent,” she said. “In the cases of these animals that are severely neurologic and do recover, many of them will also have long-term complications that may change their ability to perform their job. They might not be rideable anymore. There might be permanent behavior changes. There might be permanent locomotive changes.”

WNV has a 30-40% mortality rate in horses, and there are no approved antiviral medications for the disease. 

Prevention Strategies

According to Schwarz, the best options for preventing WNV are mosquito control and vaccination. She recommends using fly spray and fly sheets, removing manure when possible, and controlling standing water, which can become a breeding ground for mosquitoes. 

Schwarz emphasized the importance of vaccination for horses. They typically require an initial dose and a booster, and then they require revaccination on a yearly basis. “It’s important to get horses vaccinated in the spring so they have enough time to mount an antibody response that’s going to be protective during the most significant part of the summer when those mosquitos are most active,” she said. 

About Dr. Erika Schwarz

Schwarz, DVM, MPH, PhD, DACVM, is a Clinical Veterinary Microbiologist at the Montana Veterinary Diagnostic Laboratory. As Microbiology Section Head, she provides diagnostic technical expertise, veterinary consultation, and oversees daily operations and clinical testing in the Bacteriology, Virology, Molecular Diagnostics, and Serology sections. She joined the Montana team in late 2019. Schwarz received her Bachelor of Science, Master of Public Health, Doctor of Veterinary Medicine, and Doctor of Philosophy degrees from the University of Florida. She is also board certified by the American College of Veterinary Microbiologists. As a classically trained virologist, she has an extensive background working with a wide variety of human and animal pathogens. Her professional interests include infectious diseases at the human-domestic animal-wildlife interface, diagnostic techniques for high-consequence pathogens, translational medicine, and One Health. 

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