WNV: Treatment and Prevention

West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational seminar for”P>West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational seminar “>West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational semina”West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational semin”West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational semin”est Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it, said William Saville, DVM, PhD, Dipl”st Nile virus (WNV) is the number one

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“West Nile virus (WNV) is the number one diagnosed neurologic disease in horses, or close to it,” said William Saville, DVM, PhD, Dipl. ACVIM, of The Ohio State University, at a March 10 Fort Dodge Animal Health educational seminar for veterinarians held in Lexington, Ky. “Because of the decreased number of cases in 2003 compared to 2002, people are starting to think we’ve got WNV under control, but we haven’t,” he said. “We need to convince horse owners that prevention is the only way to go.”


The veterinary and horse-owning communities have watched WNV radiate across the United States since 1999, spreading via infected mosquitoes and birds. Forty-four of the lower 48 states had confirmed WNV cases in some species in 2003. “Why has WNV moved from East to Westa so fast?” Saville asked. He said that crows (which are highly susceptible to the virus) can move up to 200 miles in east-to-west migratory patterns, and other migratory birds and container traffic such as trucks and planes might have aided the western movement of birds and mosquitoes.

Saville used 2002 to illustrate the spread and behavior of the virus. That year, the middle of the country was hit with more than 14,000 confirmed equine cases. He thinks that many people never bothered to have affected horses tested, so the number of cases was probably higher.

“In Ohio in 2002, one-third of our equine cases were in marshy areas” that had low vaccination rates and a lot of mosquito activity, said Saville. Also, “Not much was likely done in the poor areas of the state (in the way of prevention or reporting disease).”

Pennsylvania, New Jersey, Maryland, and Virginia were hotbeds for equine cases last fall, more so than in 2002. Saville attributed these flare-ups of cases to weather patterns. “We have no way of predicting where (WNV infection) will happen and how bad,” he said. “It’s totally weather-dependent,” meaning that wet weather followed by dry conditions creates more stagnant water pooling for mosquito breeding, and thus results in more mosquitoes that can become infected and transmit the virus.


Mosquito Vectors

More than 162 bird species have been confirmed as infected with WNV in North America, he noted. At least 36 mosquito species can harbor the disease, of which more than 16 species are competent vectors (they can spread the virus by biting). The proliferation of these mosquitoes is notable–one female mosquito can lay a raft of 200-500 eggs, and she can do that 15-20 times in her lifetime.

Many specifics on the mosquitoes that are responsible for spreading WNV are poorly understood. Scientists know that in temperate zones, adults harbor virus as they hibernate, explaining why cases are seen earlier in the year in some areas than others. “In Africa, mosquitoes can pass the virus transovarially (from an infected egg-laying mosquito to the young),” said Saville, “and maybe here, too.”


Diagnosis and Treatment

According to Saville, OSU receives three or four hundred severe neurological cases per year, in which differential diagnoses must be made, since several diseases cause horses to exhibit neurologic signs similar to those seen in WNV cases. One of the most distinguishing clinical signs of a WNV horse is muscle fasciculations around the face and shoulder area. “Muzzle fasciculations are the most bizarre clinical sign I’ve seen,” said Saville.

The top three differential diagnoses when you’re looking at a neurologic case include:

Rabies–Characterized by ascending paralysis (first affecting the hindquarters, then the trunk, then the forelimbs, etc.). If the horse’s condition goes downhill rapidly, Saville reminded veterinarians to send the head to their state laboratory for rabies testing, particularly if the diagnosis is questionable. “You can easily get fooled (into thinking this is something else) and no one wants to be exposed to rabies virus,” he said. “Luckily, we don’t see much of it–maybe one or two a year in Ohio.”

Botulism–Severe weakness is the primary sign in botulism cases. Muscle fasciculations are evident in the head, trunk, and limbs, and can appear similar to WNV clinical signs. “For some reason we’re seeing more and more botulism in Ohio. At least we have a good test to differentiate it from WNV (in the living horse),” said Saville.

EPM–Neurologic signs might appear more asymmetrically than in a WNV case. “EPM cases do occur, just not as rampant as was thought before we had WNV to contend with,” said Saville. “In my experience, horses with EPM seldom exhibit muscle fasciculations.”

On post-mortem examination, lesions in the white and gray matter are apparent in WNV cases. The most severe lesions occur in the spinal cord in the thoracic/lumbar area. “There will be moderate to severe spinal cord hemorrhage in some cases,” he said

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