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Experts encourage vigilance regarding West Nile Virus

by Wyoming Livestock Roundup

This year’s wetter-than-usual summer has led to an increase in the mosquito population across several states in the West, and many experts are advising individuals and animal owners to be mindful of West Nile Virus (WNV) and its effects.

“WNV is a viral disease in the Flavivirus family which can result in fever and neurological disease,” reads a U.S. Department of Agriculture Animal and Plant Health Inspection Service (APHIS) Fact Sheet, published on Dec. 14, 2022. 

According to APHIS, WNV was first detected in the U.S. in 1999 and is now considered an endemic disease. 

The virus is maintained in nature through a transmission cycle between mosquitos and wild birds, in which birds act as the host and mosquitos act as the vector.

While there is little documentation of the virus infecting livestock, mosquitos can transmit WNV to pets, humans and horses, posing a serious threat to the two latter of the three.

WNV in animals 

An article published by Texas A&M University’s School of Veterinary Medicine and Biomedical Sciences on Aug. 19, 2021 explains most infected animals are asymptomatic, although WNV can lead to encephalitis – inflammation of the brain – and/or meningitis – inflammation of the spinal cord lining. 

Other symptoms may include fever, weakness, trembling, head tremors, inability to walk and inattention.

“Dogs and cats can become infected but are unlikely to show signs of disease, and infected dogs and cats are unlikely to infect mosquitoes, other animals or people. Fortunately, WNV is not a major concern for companion animal health,” states Dr. Sarah Hamer, an epidemiologist and associate professor at Texas A&M and director of the Texas A&M Schubot Center for Avian Health, in the article.

On the other hand, WNV does pose a deadly threat to horses. According to the American Association of Equine Practitioners (AAEP), horses represent 96.9 percent of all reported non-human mammalian cases of WNV. 

“Equine WNV complications range from debilitating to lethal. According to AAEP, horses showing clinical signs of WNV infection have an approximate 33 percent mortality rate, and research indicates 40 percent of horses that survive the acute illness still exhibit residual effects, such as gait and behavioral abnormalities, six months post-diagnosis,” explains Texas A&M University’s School of Veterinary Medicine and Biomedical Sciences. 

J.D. Ragland, PhD and Texas A&M University AgriLife Extension agriculture and natural resources agent, urges horse owners to keep a watchful eye on their horses. 

In a Texas A&M AgriLife Newsletter, written by Blair Fannin and dated July 6, Ragland notes symptoms in equids include high fever; incoordination, including stumbling, staggering and/or appearing sluggish; inability to stand; going off feed and acute death.

Physical protections

In an effort to reduce risk of transmission, Ragland encourages individuals to be vigilant of their surroundings.

He says, “Standing water in nearby lakes, trenches or even household items like flowerpots and wheelbarrows should be removed.” 

He also advises backyard pools and water tanks or buckets, if not in use, should be drained.

“People with household pets should monitor their movements and try to limit them to indoor places,” Ragland adds.

Additionally, Ragland suggests people wear long-sleeve shirts and long pants to cover their skin; apply and reapply insect repellant with ingredients like DEET, lemon eucalyptus oil and picaridin and avoid outdoor areas during dusk and dawn, which is when mosquitoes are most active.  

Medical preventions

In addition to physical protections, there are some medical prevention options individuals should keep in mind in an instance when WNV is suspected.

According to Texas A&M AgriLife, there are annual vaccinations for animals, and most veterinarians offer core vaccines for tetanus, rabies, encephalomyelitis and WNV.

Vaccinations remain the primary method of reducing risk in horses. Protocol involves an initial administration of two doses at an interval of three to four weeks, followed by an annual or semi-annual booster shot. 

Many experts recommend administering annual vaccines in the spring or during a suitable time before the mosquito season. 

“If a horse has not been previously vaccinated, the initial dose should be administered as soon as possible,” Ragland states.

Diagnosis and treatment

In a June 27 article by Mallory Pfeifer, the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) notes several serological tests may be used to diagnose WNV, and the most reliable test for horses is the Immunoglobulin M (IgM) antibody-capture enzyme-linked immunosorbent assay, which can confirm recent exposure to the virus. 

“The IgM antibody rises quickly after exposure but is relatively short lived,” TVMDL explains. “A positive result indicates infection likely occurred within the previous six weeks. Data indicates little IgM is detected as a result of recent WNV vaccination.” 

APHIS shares treatment consists of supportive care and is based on the severity of clinical signs, but prognosis may be poor for horses with severe neurologic signs.

Hannah Bugas is the managing editor of the Wyoming Livestock Roundup. Send comments on this article to

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