In this episode of Daily Vet Life, Melissa Fenn, DVM, MS, DACVIM, of Littleton Equine Medical Center, in Colorado, described a 4-year-old Quarter Horse gelding that became the first confirmed equine WNV case during the state’s unprecedented 2023 season. The horse initially presented for left forelimb lameness and generalized irritability before rapidly progressing to recumbency and severe neurologic deficits.
Although Fenn and her colleagues considered several neurologic causes, including equine herpesvirus, equine protozoal myeloencephalitis (EPM), rabies, and other viral encephalitides, examination findings quickly elevated WNV to the top of the differential list.
“The muzzle twitching and those fine fasciculations that you see over the muzzle and sometimes the torso and the neck, those are very classic of the disease,” she explained. She also noted that WNV tends to affect the thoracic limbs, unlike equine herpesvirus, which more commonly affects the hind limbs.
“The muzzle fasciculations, the predilection for the thoracic limbs, the behavior change, and lack of vaccination were overall what made his case so highly suspicious for West Nile,” said Fenn.
Blood testing confirmed a positive WNV IgM titer the following day, eliminating the need for cerebrospinal fluid collection.
Treatment remained largely supportive. Fenn and her team focused on fluid therapy, anti-inflammatory medications, antioxidants, neuroprotectants, nursing care, and physical support to safely manage repeated episodes of recumbency.
“In general, the treatment for West Nile virus is supportive care,” she said. “It’s making sure that they’re not going to hurt themselves when they’re falling down and not going to hurt other people.”
The gelding’s neurologic signs improved gradually over several days, but he developed septic thrombophlebitis followed by Clostridioides difficile colitis during hospitalization, extending his recovery. Despite these complications, he was discharged after 22 days. Six weeks later, his neurologic examination was normal, and today he has returned to a successful career in barrel racing.
Fenn cautioned that while many horses recover well, according to one study approximately 18% had incomplete recovery at six months, and about 9% experienced a relapse of neurologic signs within weeks to months after diagnosis.
Perhaps the most important takeaway for practitioners is prevention. “We can’t control the weather. We can’t control the mosquito populations. So our efforts really need to be focusing on prevention, which the two main aspects of are vaccination and mosquito control,” said Fenn. During Colorado’s 2023 outbreak, every vaccinated horse survived, while unvaccinated horses faced mortality rates of approximately 25% to 30%, she said.
Fenn also reminds veterinarians that WNV should remain on the differential list even in afebrile horses. “Only about one-quarter of these cases actually have a fever,” she says, adding that paresis and weakness can easily be mistaken for lameness, and early neurologic signs can resemble colic.
Listen to the full episode to hear Fenn discuss her diagnostic approach, treatment decisions, nursing considerations and practical recommendations for helping clients reduce WNV risk through vaccination and mosquito control.
About Dr. Melissa Fenn
Melissa Fenn, DVM, MS, DACVIM, is originally from Tucson, Arizona, but spent most of her childhood living in South America with her family. She received her bachelor’s degree from the University of Arizona and her Doctor of Veterinary Medicine degree from Cornell University. Following veterinary school, she completed a rotating internship at the Mid-Atlantic Equine Medical Center before returning to Cornell for a 3-year residency in Large Animal Internal Medicine. After the residency and obtaining her board certification with the American College of Veterinary Internal Medicine in 2019, she moved to Doha, Qatar, and spent a year working at the Equine Veterinary Medical Center, a busy referral hospital that provided specialized clinical services to the horses of Qatar, as well as teaching and research opportunities. After her time abroad, Dr. Fenn moved to Lexington, Kentucky, where she worked as a clinical veterinarian and served as the Head of the Internal Medicine Department at Park Equine Hospital. In 2021 she joined the team at Littleton Equine Medical Center. Her clinical areas of interest include emergency and critical care, neurology, and neonatology. These interests have translated over to her research focus on transfusion medicine. In her spare time, she prefers to be up in the mountains or traveling.
Related Reading
- Arbovirus Season: Remind Clients to Vaccinate Their Horses
- A Refresher on Protecting Horses Against Arboviruses
- Researchers Identify Potential New Transmission Method for West Nile Virus
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