Equine West Nile virus 2022
by Jenny Alonge
As of September 13, the United States has confirmed 28 West Nile virus (WNV) cases in horses in 2022. The first case was identified at a private facility in Volusia County, Florida, on January 11, 2022, and states affected now include Florida, Tennessee, California, Minnesota, Colorado, Indiana, Utah, Illinois, and Idaho. In addition, 198 cases of WNV in people have been reported to the Centers for Disease Control and Prevention (CDC) this year.
WNV is the leading cause of arbovirus encephalitis in equids and humans in the United States, and is considered an endemic disease, potentially causing significant health issues for affected hosts. The mortality rate for U.S. horses exhibiting clinical WNV signs is approximately 30% to 40%, and about 40% of surviving horses experience residual neurological issues for six months or more. WNV significantly threatens horses, and veterinarians should be proactive in educating horse owners about this disease.
West Nile virus emergence
WNV is a flavivirus with similar characteristics to the viruses that cause St. Louis encephalitis, Japanese encephalitis, and yellow fever. The following are highlights of WNV’s emergence:
- 1937 — WNV was first isolated from a febrile patient, whose only presenting sign was fever, in the West Nile district of Northern Uganda.
- 1951 — The first recognized WNV epidemic occurred in Israel in a small town outside Haifa. Young children were primarily affected, and the main symptoms included fever, headache, myalgia, anorexia, abdominal pain, rash, and vomiting. Also in 1951, WNV studies were conducted in the upper Nile Delta region. Serosurveys demonstrated that WNV was endemic along the Nile, with seroprevalence rates about 60%. Mosquitoes were identified as vectors, and the epidemiology, ecology, and clinical characteristics were further elucidated.
- 1957 — A WNV outbreak that occurred in Israel affected elderly nursing home residents, who experienced severe neurological signs—the first report of this WNV manifestation.
- 1996 — A large outbreak around Bucharest, Romania, was the first WNV outbreak centered in a predominantly urban area, and where the majority of symptomatic cases involved central nervous system (CNS) signs.
- 1999 — In late August, a cluster of severe WNV encephalitis cases occurred around Queens, New York, affecting eight people, who all but one developed severe, acute flaccid paralysis. Veterinarians subsequently reported 25 equine cases by the year’s end, all in New York.
Since WNV arrived in the United States in 1999, the virus has spread westward, and now has been detected in all contiguous states.
West Nile virus basics
WNV poses the greatest threat in late summer, when mosquitoes are most active. Other WNV information includes:
- Transmission — Birds serve as a WNV reservoir, and mosquitoes are infected when they take a blood meal from an infected bird, and then act as vectors to spread the virus to other birds, animals, and humans. Some bird species, such as the crow family, are more susceptible to the virus, and dead crows in an area can indicate WNV is present. Other infected animals are incidental to the cycle in birds, and humans and horses are considered dead-end hosts (i.e., an infected horse or human can’t transmit the disease to other animals or humans).
- Clinical signs — The incubation period for humans and horses is about 3 to 14 days, and clinical signs vary, depending on infection severity. Potential signs in humans include headache, fever, neck stiffness, muscle weakness, disorientation, tremors, seizures, paralysis, and coma. In horses, signs include lethargy, ataxia, weakness, lameness, partial paralysis, muscle twitching, recumbency, fever and an altered mental state.
- Diagnosis — Several serological tests are available to diagnose WNV. For horses exhibiting clinical illness, the most reliable test is the IgM antibody-capture ELISA, which can confirm recent WNV exposure. A positive result indicates infection probably occurred during the previous six weeks. Vaccination doesn’t seem to interfere with this test.
- Treatment — Since WNV has no cure, treatment is aimed at supportive care and reducing CNS edema and inflammation. Protecting the horse from self-induced trauma is also important in WNV encephalitis management. Sling support, protective leg bandages, and a helmet typically benefit horses with moderate to severe WNV.
- Prognosis — Approximately 30% to 40% of horses with WNV encephalitis will die from the infection or encephalitis complications. However, the survival rate is about 80% to 90% for horses who remain standing without assistance throughout the infection.
West Nile virus prevention
The U.S. incidence of WNV varies widely from year to year, but climate change has affected geographic and seasonal patterns of mosquito activity, increasing the exposure risk across the country. Prevention is key for your patients’ protection, and you should relay the following tips to your clients to help decrease their horses’ risk.
- Vaccinate your horse — Four USDA-licensed WNV vaccines that are highly effective at preventing infection are currently available. Horses typically should be vaccinated in the spring, prior to mosquito season, but every six months in areas such as Florida, where mosquitoes are active all year long.
- Eliminate standing water — Eliminate containers, tires, and any other items that can hold water to prevent potential mosquito breeding sites.
- Clean water receptacles — Clean all watering receptacles at least once a month.
- Keep your horse inside — During peak mosquito activity (i.e., dusk and dawn), keep your horse inside.
- Install fans — Install fans over your horse’s stall to deter mosquitoes.
- Avoid lights — Leave the lights off in the evenings and overnight to avoid attracting mosquitoes.
- Use insect repellent — Apply insect repellent to your horse to help repel mosquitoes.
WNV appears to be here to stay, but we have resources at our disposal to help protect the horses in our care. Educate your clients to ensure their horse receives the necessary vaccinations to prevent this dangerous disease.
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