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 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.

Notes on Treatment

West Nile virus treatment is mainly supportive, Saville stated. The use of slings in recumbent WNV cases is horse-dependent. "Some just will not accept the sling no matter what you do," he said.

"Some practitioners tell me some drug (sedative) intervention has been needed to even sample or treat some of their cases (because of heightened aggression due to infection)," said Saville.

Different practitioners have different preferences as to what ancillary treatments they will use, but he said there isn't enough efficacy data for him to know whether to recommend any of them. Even though these medications have been used to treat human patients and some have been used on horses, the results have been mixed and no controlled investigations have been reported at this time. He listed the options, which include:

  • Ribavirin, an antiviral medication has been used in humans;
  • Interferon alpha (oral or intravenous). This medication has been used in both humans and horses. Some veterinarians have used interferon alpha routinely to treat their cases and believe that it does make a difference;
  • Hyperimmune serum; and
  • Hyperimmune plasma (which has been tested with success in guinea pigs).

While WNV has become a household curse in the horse industry, Saville said there is one question that he consistently gets from horse owners: Can my horses infect me?

"No, it's not likely, as the level of viremia (virus in the bloodstream) in the horse is too low," he said. In one study, seven viremic horses were exposed to over 600 naïve (never before exposed to WNV) mosquitoes, and none of the mosquitoes picked up the virus from the viremic horses. Regardless, Saville recommends wearing gloves when treating WNV cases.

This highlights the many questions that remain about understanding WNV infection. "Does it take one virus particle or 100, or more?" said Saville. "We don't know, so none of our students or faculty work without gloves (on WNV cases).

"How many mosquitoes bite a horse at any one time? Are these horses getting an overwhelming challenge? I can't answer it, but it poses an interesting question," he said.


Clearly, prevention is the most effective way to avoid having to experience WNV at all. "I don't think we cure (the horses) very well," said Saville. "That's why we've got to build a case for prevention versus cure."

The two methods of prevention include vaccination and mosquito control. Saville briefly discussed the two available WNV vaccines (from Fort Dodge and Merial), noting that anywhere from 28-87% of the estimated U.S. horse population in a given state has been vaccinated against WNV in 2003 or total based on sales of West Nile-Innovator.

"Since there were less cases last year, some people are cutting back on vaccinating. But this disease isn't gone!" he warned.

He added that in some states in 2003, many horse owners used the WNV vaccine instead of the Eastern equine encephalitis (EEE) vaccine, which likely led to last year's EEE outbreak. The mortality rate due to EEE is higher than that of WNV (80-90% for EEE vs. 33% for WNV). "Now you don't have to swap out one vaccine for another--you can use one vaccine for both diseases," he said. "At this point, I recommend the combination vaccine (combining WEE, EEE, WNV, and tetanus protection) in the spring, then the monovalent (protecting against one disease, in this case WNV) booster if recommended by your veterinarian."

According to Saville, a study presented at Ames, Iowa, in September 2003 demonstrated that the combination vaccine resulted in a higher WNV antibody titer compared to the monovalent WNV vaccine. "So the combo doesn't reduce anything in terms of immune response," he explained. "The same increase in titer occurred with the tetanus fraction in the combination vaccine. Although the WNV antibody levels (achieved in vaccinated animals) weren't significantly different with the combo vaccines (compared with the monovalent WNV vaccine), it is encouraging that the responses were higher."

Depending on the geographic location, practitioners recommend different vaccine protocols. Ohio officials recommend two doses three to six weeks apart in horses that haven't previously been vaccinated. (A seminar attendee from one Central Kentucky equine hospital noted that their veterinarians give three initial doses.) The second dose should be given no later than April 15 if the mosquito season starts in mid-May. "If a horse is highly stressed such as with showing or racing, give an additional booster the last week of July or the first week of August," Saville suggested.

"It's totally a regional decision," he reminded. "In Florida, some booster every three months. In Texas, some vaccinate three times a year, starting in February."

Saville recommends that foals from WNV-vaccinated mares be given their first WNV combination vaccine after three months of age, a booster in three to six weeks, and an appropriate booster at around six months. Foals from unvaccinated mares should receive their first WNV vaccine dose at one month and a booster two months of age, their third shot at six months, and then boosters yearly or more often, depending on region and your veterinarian's recommendation.

No vaccine is 100% effective. Reasons for vaccine failure include:

  • An overwhelming virus challenge (being bitten by many infected mosquitoes at once?);
  • Failure of the animal to generate an appropriate immune response; and/or
  • Stress or a waning immune response due to age or other unknown factors.

Integrated Mosquito Control

"I'm not an entomologist and I don't know much about bugs, but as a practitioner, you need to provide clients with some information regarding mosquito control," he said. "Providing your clients with fact sheets about mosquito control around horse farms would provide the owners with information to not only protect their horses, but also protect themselves.

"Decrease the sources of mosquito breeding around horses and farm personnel, and decrease the risk of WNV," he instructed. "It takes less than one week of stagnant water for mosquito to mature into adults."

Here are some recommendations for cutting down mosquito populations in and around your barn:

  • Stagnant pools should be drained or non-toxic larvicides used (such as BTI, which is a bacterial mosquito larvicide available from any garden store and approved by the EPA, even for use in water troughs).
  • Remove old tires, cut them in half so they cannot hold water, or treat them with larvicides. "You can potentially get over 5,000 mosquitoes out of only one tire," said Saville.
  • Add shallow-feeding fish to ponds (if they aren't native to your state, make sure they are only placed in ornamental ponds and won't make it into natural water sources).
  • Use mosquito magnets (propane-powered mosquito traps).
  • Install fans in barns.
  • Remove or trim tall vegetation around ponds, old equipment, tubs, etc.
  • Keep all birds out of the barn, which can be a big challenge. Saville recommends visiting www.birdbgone.com.
  • Turn over any containers or equipment that can hold standing water.
  • Keep horses in the barn during the times of most mosquito activity.
  • Turn lights off in the barn at night.

"The virus is here to stay--it's not going anywhere--so it's something we're going to have to deal with," said Saville. "We've got to prevent as many cases as we can. Hopefully, in the next few years we will have the disease under control similar to EEE and WEE when horses are vaccinated."

--Stephanie L. Church and Christy West

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