by Kimberly S. Herbert and Stephanie L. Church
West Nile virus is a dangerous and scary disease, and it is spreading across North America very quickly. However, this is one of the few times when horses have it better than humans when it comes to facing disease. There is an effective equine vaccine against West Nile virus (WNV); there isn't one for humans--yet. But the management that you use to protect your horses is beneficial to you and your neighbors. And without proper property and mosquito management, no vaccine can be as effective.
This is a frightening disease for horse owners because the equine mortality (death) rate is high--about 30-40%, says William Saville, DVM, Dipl. ACVIM, of The Ohio State University. He says the fatality rate in humans is about 10-15%. Generally, the horses most susceptible to this disease are the young and old and those with compromised immune systems. (It's the same in humans.) It's also hard to protect horses from mosquitoes.
Veterinarians and human health officials in this country were mostly unconcerned about this disease prior to 1999--it had never before been seen in the Western Hemisphere. Therefore, information is being developed continuously on this disease in horses and humans, including hosts, carriers, and treatments, as well as management and prevention measures.
The complexion and geographic location of this disease changes daily, sometimes hourly. Thus we urge you to continue monitoring our web site at www.TheHorse.com/wnv for updates. Also on our web site you will find contact information for each of the 48 contiguous states' health departments and their information on WNV, as well as federal and university contacts and news releases.
The Horse magazine and TheHorse.com have continuously covered WNV, its spread, and things you can do to help protect yourself and your horses since the first cases in 1999. In light of the tremendous number of cases and the rapid spread of the virus across the United States, we have sifted through what is known about WNV in horses, the conditionally licensed vaccine from Fort Dodge Animal Health, and mosquito control, and we've brought it all together for you in the following pages.
What is WNV?
In August 1999, WNV was identified as the cause of encephalitis (brain and spinal cord inflammation) in birds, horses, and humans in New York. Prior to its introduction in New York, WNV had caused outbreaks in other parts of the world, and it is considered endemic in some areas. The virus isolated in the United States is suspected to have come from Israel, but how it was introduced into America is unknown, says Genevieve L. Fontaine, DVM, MS, Dipl. ACVIM, of the University of Florida. There was some thought that it might have been a bioterrorism attack, but there has been nothing official said on that since the first year it was speculated in the press.
West Nile virus is known to cycle between birds (reservoir hosts; a living creature needed for the virus' survival) and mosquitoes. However, it was discovered in October 2000 by Robert McLean, PhD, director of the United States Geological Survey's (USGS) National Wildlife Health Center in Madison, Wis., that birds can infect other birds without the presence of mosquitoes in the laboratory. It's not known how the transmission occurs, but theories include preening, contact with infected feces, or contamination of food sources. There's no evidence that mammals such as horses or humans can be a source of infection either by contaminating mosquitoes that feed on them or through direct contact with another animal.
Birds might get sick and die of encephalitis (crows and blue jays are particularly susceptible), or be unaffected and develop a lifelong immunity. A large number of dead crows is generally the first sign of viral activity in a given area, notes Fontaine.
"Horses and people contract WNV encephalitis through the bite of infected mosquitoes and are considered 'dead-end hosts' because there's no evidence that they develop sufficient viral amplification (viral reproduction in their bodies in significant levels) to complete the cycle," she says. "Horses and people are therefore not contagious. Other species such as cattle, camels, dogs, and cats develop an immune response without becoming ill, except for rare cases reported in cats and dogs."
It is theorized that migrating birds are the major means of spread of WNV from its original epicenter in New York to other areas of the country since 1999. It also was proven in 2000 that the virus can overwinter in mosquito populations. In areas where there are mosquitoes year-round, there is the potential for continuous cases. In areas where there are killing frosts, the winters offer protection from the virus since there are no mosquito vectors.
At press time, WNV had been documented in 41 states and the District of Columbia in either birds, mosquitoes, horses, or humans (see map at left). It has also spread to four Canadian provinces (see page 16).
Eileen Ostlund, DVM, MS, PhD, is head of the Equine and Ovine Viruses Section of the Diagnostic Virology Laboratory at the United States Department of Agriculture's (USDA) National Veterinary Services Laboratories (NVSL) in Ames, Iowa. She says, "The last count (of 2001 equine cases) was 738 confirmed cases. That is larger than the sum of all equine cases (of WNV) in recorded history." There were 1,424 cases reported at press time for 2002.
Most of those cases were in Florida, which was hard-hit by the virus in 2001. (For a closer look at the Florida outbreak and subsequent management, see "Florida Under Siege" on page 44.)
West Nile Virus Clinical Signs
Florida had more than 400 equine cases of WNV during an outbreak in 2001. That number is about the same this year, although the cases are farther south in naïve populations. Veterinarians there think quick vaccination and aggressive mosquito control has curtailed much of the problem this year in Central Florida, but practitioners are recommending additional boosters during peak mosquito season.
Maureen T. Long, DVM, PhD, assistant professor of large animal veterinary medicine at the University of Florida's College of Veterinary Medicine, describes the remarkable array of clinical signs exhibited by WNV-infected horses seen at the university.
- Flu-like signs, where the horse seemed mildly anorexic and depressed.
- Fine and coarse muscle and skin fasciculations (twitching).
- Hyperesthesia or hypersensitivity to touch and sound. Affected horses startle easily and were often described as "jumpy." About 10-20% of cases were unwilling to be handled or touched, some responding to stimulation with aggression.
- Changes in mentation (mentality), when horses looked like they were daydreaming or "just not with it."
- Occasional somnolence (drowsiness), but horses rarely went into convulsions or comas. Horses varied from being over-responsive to showing no response to stimulation in a matter of minutes.
- Propulsive walking (driving or pushing forward, often without control), but not as dramatic as a horse with Eastern equine encephalitis, another arboviral (transmitted by arthropods or insects) neurological encephalitis.
- "Spinal" symptoms, including asymmetrical weakness (the most important). Some horses showed asymmetrical or symmetrical ataxia (incoordination on one or both sides, respectively).
- Other nerve signs included paralysis of the lips, facial muscles, or tongue. Some also had a head tilt and difficulty swallowing.
"Horses that have paralysis of the lips usually leave a little grain behind in their bucket and will go and eat their hay," explains Long. "They also will drool a lot. A lot of the owners were uncomfortable managing a neurologic horse even for a short period because the course of the disease was extremely unpredictable and referred their horses (to the University of Florida)."
The average time that WNV-positive horses spent in the hospital was nine days, although some stayed as long as 40 days.
"There appeared to be a recrudescence (a reactivity of clinical signs after a period of abatement or inactivity) of mild to moderate clinical signs two to three days after the first signs abated," explains Long. So, just because the horse appeared to get better did not mean that he was cured.
Saville says the thing that has been most consistent in clinical signs is asymmetric incoordination (ataxia). The other frequently seen clinical sign is severe muscle fasculations or tremors. The NVSL has been the main reference laboratory for testing WNV veterinary cases since the fall of 1999. Ostlund published information stating that during the 1999/2000 outbreak, the following signs were seen in horses confirmed to have WNV.
- 85% incoordinated (ataxia)
- 50% depressed/apprehensive
- 40% weak
- 45% recumbent
- 40% muscle tremors in the neck and muzzle
- 25% fever
- 18% paralyzed or drooping lip
- 13% twitching muzzle
- 5% blindness
She adds that in 2001, the highest risk of disease in horses was in those over 25 years of age.
Saville reminds horse owners that another disease can cause muscle tremors--botulism. However, he points out that horses with botulism are not incoordinated or ataxic, only weak. Horses with WNV are ataxic and have muscle tremors. He says if a horse has both of those clinical signs, the chances of him having WNV are very high. (For a WNV report on one horse, see "Kentucky Case Study" on page 38.)
Serologic surveys (evaluating antibodies) show that most horses exposed to the virus don't become ill, but the ones which do become ill suffer from neurological disease, notes Fontaine. "The severity and nature of clinical signs depend on the area of the nervous system affected by the virus and the extent of the lesions (damage to nerves)," she says. "The most common sign is ataxia--with rear limb involvement more frequently observed--and trembling, particularly around the nose and lips. Some horses become hypersensitive to touch or sound, or conversely might be lethargic and unresponsive. A transient (temporary) fever might precede these signs.
"The onset of disease is generally acute (sudden), and in most cases, improvement is noticed within days," continues Fontaine. "Euthanasia is warranted in the most severe cases due to paralysis, or severe signs such as seizures or coma."
Testing, Treatment, and Outcome
Currently, the diagnosis of WNV is based on a blood test and on clinical presentation in horses located in an area with viral activity, notes Fontaine. She explains that after exposure to the virus, horses develop an immune response characterized by a rapid increase in IgM (a type of antibody) that lasts less than two months, and a delayed increase in IgG (another antibody type) that might persist for several months. Vaccination does not interfere with the ability to diagnose acute cases, because no IgM levels are detectable in vaccinated horses.
"West Nile virus is not a mortal disease (it doesn't always kill)," says Long. "The majority of horses are going to get better and deserve a chance. The ones with the flaccid (not stiff) paralysis are the ones that have a higher probability of not making it."
While there is no specific treatment protocol for WNV, Long gives guidelines based on the University of Florida's cases.
Supportive care includes anti-inflammatory medications, including flunixin meglumine (Banamine). Only in very severe cases should cortico-steroids and mannitol be used, and only at moderate anti-inflammatory doses. If the horse becomes recumbent (stays down), treatment gets tougher; Long recommends aggressive therapy with the anti-inflammatories dexamethasone and mannitol, and detomidine for prolonged tranquilization. Low doses of acepromazine can provide relief from anxiety in both standing and recumbent horses.
Around 70% of the Florida cases appeared to recover fully, although some had prolonged weakness, taking a month or two to return to normal. The effects in most cases were not permanent. Long encourages owners to begin light exercise with WNV patients after 30 days.
The good news from Long is that horses which have recovered from WNV haven't gotten the disease again; there seems to be a natural immunity after experiencing infection once.
Saville says that in the 1940s, the Japanese developed a killed vaccine to protect horses and humans against the deadly Japanese encephalitis. "With that killed vaccine, they have controlled this encephalitis virus in humans and horses," explains Saville. "The same vaccine has been used in China and Singapore with good success."
Saville says that Fort Dodge Animal Health extrapolated from the experience of that Japanese encephalitis vaccine development to create the WNV vaccine. "It should work the same," says Saville. "That's why I have confidence that it will work."
He adds that there is work ongoing at the Centers for Disease Control (CDC) to develop a DNA WNV vaccine for horses, and eventually for humans.
The vaccine developed by Fort Dodge Animal Health received conditional licensure from the USDA-APHIS Center for Veterinary Biologics in the summer of 2001. Because of that conditional licensure, each state veterinarian's office has to approve use of the vaccine. A conditional license means consumers can be assured of the vaccine's purity, safety, and have a reasonable expectation of efficacy (protection).
At the 2002 meeting of the American College of Veterinary Internal Medicine, Yu-Wei Chiang, MS, PhD, of Fort Dodge Animal Health, announced that the company had completed challenge model and efficacy studies. Results will help the vaccine gain full approval from the USDA.
"Significant protection against the viremia (presence of WNV in the bloodstream) was demonstrated (using the challenge model designed for this purpose)," says Chiang about the efficacy studies. "We are pretty close (to full licensure), and I hope that we get the full licensure."
Saville says that each horse owner needs to feel comfortable in asking his or her veterinarian about using the WNV vaccine. "I don't think any veterinarian is going to say not to vaccinate," says Saville.
The recommended vaccination schedule is to give the initial two doses three to six weeks apart. "It looks like they need about four weeks after the second dose to be well-protected based on what's happened over the last year," says Saville.
Some veterianrians recommend only two weeks between initial shots during outbreaks.
Last year, there were approximately 132 horses which had been vaccinated against WNV that developed clinical signs, reports Saville. Of those, 113 only had received one dose. Of the 19 remaining horses, 14 had onset of clinical signs within 15 days of the second dose. The other five had onset of clinical signs within 20 days of receiving the second dose.
"They (Fort Dodge) considered only three vaccine failures," says Saville. "That's a pretty low number since they had sold 1.1 million doses at that time."
Saville says this year's results might be a better indication of the efficacy of the vaccine because more horses have been vaccinated.
"We should have an incident report on every horse that was vaccinated that had clinical signs that were confirmed," says Saville. "Those have to be sent to the company. Veterinarians should report that."
As with any vaccination, Saville says, if a horse has a fever or is showing neurologic signs, definitely don't vaccinate him. "He could already be developing WNV," he says. "If you see horse that's obviously not right, you don't want to vaccinate him until you find out what the problem is."
Mosquito Management is Critical
"It doesn't matter what vaccine you use if you don't use other preventive measures," says Saville. "You need integrated pest management."
One of the reasons Saville and other university and public health officials in Ohio have been in the forefront on public education about WNV is because of the problems they faced with an outbreak of St. Louis encephalitis in 1975. He says the surveillance of mosquitoes was set up in Ohio for that purpose, and has been expanded to include WNV. There also is a very good working relationship between the various state agencies, university researchers and extension agents, and federal officials.
Joe Conlon, technical advisor for the American Mosquito Control Association, says that there are about 176 species of mosquitoes. How many of these species can transmit WNV is a matter of debate. Of the 79 species that have been tested, 26 have tested positive for WNV and six are considered primary vectors. Some of these species feed only on birds--they keep the disease circulating, but do not affect horses and humans. But some of the species that have tested positive breed near humans and are known to bite viciously.
The likelihood of any one mosquito carrying WNV is very low, explained Conlon. Speaking epidemiologically, he explains, an infection rate of 1% among mosquitoes in a particular population is considered a serious problem. This is because the low rate is countered by the vast number of mosquitoes. That is why, Conlon says, that the most important step is to lower the bite rate. The fewer bites, the more the odds are in your favor.
An obvious part of pest control is directly protecting your horse from mosquito bites. However, fly repellent isn't necessarily mosquito repellent, says Randall L. Crom, DVM, senior staff veterinarian for emergency programs at the USDA's Animal and Plant Health Inspection Service (APHIS), Veterinary Services. "Generally, compounds like synthetic pyrethroids applied topically will have decent effectiveness against mosquitoes," he says. "Nothing is 100%, however, especially after perspiration."
Some people feel that over-reliance on chemicals can damage the environment, build up chemicals in insect-eating creatures, and lead to increased chemical tolerance in mosquitoes. Others argue strongly against this position. Natural alternatives include sprays made with oil of citronella and/or oil of eucalyptus.
Another way to keep mosquitoes and flies away from horses is with physical barriers--face masks, ear covers, sheets, and leg covers. Many of these articles can be impregnated with fly repellent.
On the down side, no clothing covers a horse completely. A sheet, for example, leaves the sensitive belly exposed. Furthermore, some horses object to having their faces covered.
Stable management practices can also help repel mosquitoes and reduce their numbers. If there is not an automated system in the barn, consider manually spraying stalls and aisle walls. Also consider spraying the lower limbs of shade trees and any place where large congregations of mosquitoes are seen. Adult mosquitoes rest in weeds, so trim weeds around barns and houses and keep lawns mowed.
Barn fans can also help. Steven Hal-stead, DVM, equine programs veterinarian for the state of Michigan, explains that mosquitoes are, "fair-weather flyers that don't do well in air currents." Screens might work on barns, as long as they do not trap flies and mosquitoes inside.
"Overall," says Crom, "screens are useful as long as there are no mosquito breed-ing sites inside the screened area."
As far as mosquito control is concerned, "There are three steps: Source reduction, larval control, and if that fails, adulticides," says Don Barnard, PhD, research leader of the Mosquito and Fly Research Unit at USDA's Center for Medical, Agricultural, and Veterinary Entomology.
Reduce mosquito sources by eliminating breeding sites. "Many mosquitoes develop in small bodies of water or containers," says Barnard. Drain or eliminate anything on your property that traps water, such as old buckets, tires, or birdbaths. Water troughs for horses that are used infrequently can be drained and cleaned weekly to discourage mosquito breeding."
Horse owners should also eliminate standing water in pastures when possible. However, Barnard says it is not always environmentally acceptable or feasible to fix these wet spots.
Some mosquito breeding sites are on adjacent property, such as tire dumps. According to Barnard, many states have statutes that control the disposal of tires. Your local government can tell you if cleanup of these areas is in order.
Mosquitoes begin life as larvae in standing water (see "Know Thy Enemy" on page 42). Microbial larvicides and insect growth regulators are acceptable, safe ways to eliminate larvae without endangering the horse. The products are more expensive than some adulticides.
Microbial Larvicides--Bacillus thuringiensis israelensis (BTI) is one type of microbial larvicide commonly sold under the trade name Vectobac. This product's one-time application is better for water not loaded with manure or organic matter. Bacillus sphericus, often sold under the name Vectolex, is better for more organically "enriched" wet areas. It has some residual activity, with a slight ability to continue growing in the environment. BTI is also sold in a solid "donut" form in hardware stores under names such as "mosquito dunks" for smaller areas of standing water. These products aren't harmful to horses or other animals.
Insect Growth Regulators--These are sold as briquettes, liquids, pellets, granules, and concentrates. They contain synthetic chemicals that prevent larvae from emerging as adults. Altosid is a common trade name for the insect growth regulator methoprene.
Mosquito Predators--Barnard warns that larvae-eating fish have trouble getting into grassy, shallow areas where larvae live. Additionally, many of these fish aren't native to the United States, and there is significant concern about the introduction of foreign species.
Natural mosquito predators such as dragonflies and bats are an option, but they don't feed only on mosquitoes, and Barnard says that introduced mosquito predators also destroy beneficial insects. Bug zappers kill good and bad insects indiscriminately, and Barnard discourages their use.
Adulticides--Adulticides kill adult mosquitoes and can be applied from the ground or by aircraft as mists, fogs, or granules. All have instructions on their use from the Environmental Protection Agency.
"Timing, knowledge, and skill are very critical to proper application," Barnard explains. "If not applied properly, they could do a lot of damage to other animals or beneficial insects."
Mosquitoes can develop a resistance to pesticides, Barnard says. Therefore, it is important to have a reputable pest control operator help you plan and administer the application of pesticides to your property.
Stable Management--There is debate over the best turnout schedule to minimize horses' exposure to mosquitoes, mainly
because the mosquitoes will go wherever the horses go.
"One problem with keeping horses inside is that mosquitoes rest on the walls during the day and are active at night," explained Barnard. "Look in the cracks and crevices of the barn--typically in the corners up near the ceiling--for resting mosquitoes. One cure is spraying an insecticide to kill adults in the barn. Large screens over doors and windows are effective, and of course, good barn-keeping goes a long way in keeping the mosquitoes at bay."
There are many things that still aren't known about WNV. However, researchers believe WNV is now endemic in the United States. The virus has proven it can spread year-round in areas that have active mosquito populations. Experts predict that WNV is going to continue spreading across the United States, Canada, and possibly into South America.
Informed sources predict that the Fort Dodge Animal Health vaccine will receive full approval from the federal government, and that the vaccine will play a key role in protecting horses from illness. It is possible that a DNA vaccine will be developed in the future for horses and humans.
Owners will be the first line of defense for their horses. Proper management and mosquito control are critical to reducing all species' exposure to the disease. Being informed of the spread and incidence of WNV, and any new developments in the fight against it, will be key for horse owners and veterinarians. --Kimberly Herbert and Stephanie Church
KENTUCKY CASE STUDY
Nathan Slovis, DVM, Dipl. ACVIM, an internal medicine specialist at the veterinary firm of Hagyard-Davidson-McGee in Lexington, Ky., diagnosed the first Kentucky equine case of West Nile virus (WNV) in the fall of 2001. He gave the following description of the case.
The 3-year-old Quarter Horse filly first began showing neurologic signs on Aug. 23, according to her owner. Slovis says the filly appeared "drunk" and sedated on Aug. 23, but by the time she was admitted, she no longer had that "drunk" look. However, her neurologic signs were constantly changing.
"I figured it was rabies, EEE (Eastern equine encephalitis), WEE (Western equine encephalitis), or West Nile virus," says Slovis. He ran tests for all of the encephalitis viruses, and even took a spinal tap to rule out EPM.
Slovis says the filly was in the clinic for three days, and each day her neurologic signs changed. First she was ataxic and had the "shakes" or muscle tremors in her neck, face, and thorax. Any movement caused the tremors to commence. Her depth perception was affected as well. "She'd go for her water bucket and miss by five inches," he says.
On the second day, she was hypersensitive to touch. "If you barely touched her, she acted like she'd been stung," Slovis says. "She also was depressed."
By the third day, 90% of the shaking had disappeared, but her muzzle deviated and her neck was bent to the right. She had an unusually high gait with her front legs when she tried to move.
Slovis began treating the filly with DMSO and fluids twice a day, and a course of treatment that included dexamethasone, vitamin E, and Banamine.
All of her blood work was normal, but she had twice the normal protein counts in her cerebrospinal fluid, says Slovis. "That is indicative of inflammation in the central nervous system."
After intensive supportive care, the filly returned home and recovered.--Stephanie L. Church
KNOW THY ENEMY
Population control of mosquitoes centers on interrupting the mosquito life cycle. Water is essential to this life cycle--mosquito eggs are laid in water or damp areas. A floating raft of 200 to 300 eggs looks like a speck of soot, about one-quarter inch by one-eighth inch. Eggs hatch into larvae, then molt into pupae, both of which live in water. Even adult mosquitoes rest on top of water to dry out after emerging from the pupae stage (an intermediate stage).
To stop mosquito breeding, one must eliminate or treat all sources of standing water. Removing these breeding sites will get rid of eggs, larvae, and pupae. Mosquito larvae feed on organic matter, so cleaning the water that has to remain (such as in water troughs) will leave the next generation of larvae with nothing to eat.
Potential mosquito breeding sites exist anywhere water stands for days--cans, barrels, tires, toys, buckets, potted plant trays, horse troughs, in areas under or near water troughs, wheelbarrows, clogged rain gutters, puddles on flat roofs or near faucets or air conditioners, seepage from cisterns/cesspools/septic tanks, ornamental ponds, tree stumps, in trash along fencelines, swimming pool covers, other plastic covers or tarps, puddles, creeks, ditches, and marshes.
These water collectors should be emptied, turned over, removed, or filled in when possible. Clean and unclog gutters. If an object can't be moved, drill a hole in the bottom to drain the water. When watering lawns and gardens, look for puddles that remain for days, and landscape to eliminate them.
Bigger areas like ditches or swampy areas might require a community effort. If a roadside ditch needs to be left open for drainage but doesn't always drain completely, Joe Conlon, technical advisor for the American Mosquito Control Association, recommends keeping vegetation to a minimum. Remove dead leaves, plants, and grasses on the banks and mow or weed-eat. This will give the larvae fewer places to hide and mosquito predators a clear shot. You should consider that there might be environmental concerns and regulations preventing the filling in of wetland areas.
A mosquito takes at least five days to grow from egg to adult. Therefore, water sources that cannot be removed need to be cleaned and/or treated at least once a week with a chemical or biological larvicide such as mosquito dunks. Keep water troughs clean and swimming pools treated.
Some species of mosquitoes can lay eggs in as little as a few tablespoons of water. Thus, you should dump all standing water and keep your eyes open for new puddles that collect after a rain.
Comprehensive mosquito control includes killing adult mosquitoes and keeping new ones from growing up. Space spraying is not as effective outdoors as it is indoors, since it only kills the adults that are present with no long-term effect against future mosquitoes. Various traps and bug zappers can attract adult mosquitoes, so they are not recommended. If you use them to kill other insects, keep them away from horses.
"I would encourage people to throw their bug lights away," says Steven Halstead, DVM, equine programs veterinarian for the state of Michigan. "They destroy more helpful insects than harmful." Dragonflies and other mosquito predators can be zapped. Halstead also finds that traps work for mosquito surveillance, but do not catch enough mosquitoes to effectively reduce the population. --Katherine Walcott
FLORIDA UNDER SIEGE: A CLOSER LOOK
Last year, the Central Florida equine industry (the Thoroughbred breeding center of the state and home to tremendous sport and pleasure horse populations) was hit hard by West Nile virus (WNV). One of the largest veterinary firms in that area is Peterson and Smith Equine Hospital, a full-service referral hospital with 16 ambulatory practitioners and eight hospital veterinarians including internal medicine specialist Carol Clark, DVM, Dipl. ACVIM.
The area that was ravaged by WNV last year has seen a huge decline in the number of cases of WNV this year. Clark estimates that the practice has had a 75% decrease in the number of cases as compared to this time last year. Clark attributes this huge decline in active cases to proper vaccination and client education about mosquito control and eradication, and she adds that a big part of the campaign against WNV has been trying to get rid of mosquitoes and standing water.
There have been some problems with controlling WNV in the face of an outbreak. Some of the horses which were not yet fully protected by the vaccine were becoming ill with WNV. That led to speculation that either the vaccine didn't work, or that the vaccine was causing disease. "But it was that the horses were not (yet) being fully protected," explained Clark. "There was a lot of vaccination education...we had a second wave of panic over the vaccine."
As far as treating horses, Clark said veterinarians "looked to our colleagues up north because there's not much in the literature. It was simple supportive care, with variable success." Clinical signs extremely unique to disease included muscle twitching around the face and muzzle, said Clark. While those clinical signs weren't present in every case, most horses had it at some part of the disease process.
"Many had no signs of depression or fever," recalled Clark. "They were wobbly and had these fasciculations (twitching) around the muzzle. Some started with unilateral lameness and would quickly progress from there to involving both front limbs or both hind limbs. Those horses usually didn't do well and usually ended up going down."
Diagnosis is much better and faster this year, she said. This year, the state diagnostic lab and the University of Florida (under Maureen Long, DVM, PhD,) is doing testing. "She's especially looking at vaccinated horses and non-vaccinated horses to see what levels of antibodies are in horses," said Clark. "So far, the test looks like it can pick out true clinical disease. She can diagnose a horse with disease even if it was vaccinated."
Clark said testing shows that many horses get sub-clinical disease. "A lot of people are exposed, but very few get sick. It's something like 50-100 exposed for every one that gets ill. Certainly some horses that had disease went unnoticed. They will have gained some natural protection, but for an unknown period of time. I think they still should be vaccinated."
Clark feels that delayed treatment has a bearing on outcome. She said that because the virus has a predilection for the lower motor neurons (nerve cells that supply movement and strength), affected animals left in their normal environments might become fatigued out running around with pasturemates. With this increased weakness, they are more likely to go down. Affected horses should be started on anti-inflammatory drugs and kept stalled.
The fatality rate for diagnosed WNV cases in the Peterson and Smith practice last year was around 30%. The hospitalized cases had a higher mortality because "we were seeing the worst of the worst," said Clark. "We lost no more than 50%, though. For something as serious as an encephalitis, it was good."--Kimberly S. Herbert
WNV VACCINE Q & A
The following information is from Fort Dodge Animal Health, manufacturers of the WNV vaccine. Company officials recommend that horse owners consult with their own veterinarians to get more specific details about the threat to their horses.
Q: Can you tell me when the "Conditional License" for the West Nile virus vaccine will be made permanent?
A: We have submitted the data for review to the USDA, the licensing agency. Experimental design, challenge model, and of course, the results of comparison between vaccinated and non-vaccinated horses will be evaluated prior to full license. We are optimistic that the full license will result. We do not know when that will occur.
Q: I have friends who are afraid to give the WNV shots until more data is available. Any clue when that will be?
A: There is absolutely no reason to be "afraid" to give the vaccine. There is no safety decrement in conditionally licensed products. The safety standards are the same for such products as they are for all other veterinary vaccines on the market such as flu/rhino, tetanus, sleeping sickness, etc., plus we have extensive field data in a very short period of time. The vaccine is safe.
Q: I've been told that the vaccine caused West Nile virus in a horse. Is there any truth to this?
A: No, this is a killed vaccine. The virus in the vaccine has been effectively inactivated by very precise methods. During a very intense outbreak in Florida when some horses were vaccinated while incubating the virus, there might have been some horses that got sick regardless of vaccination (since they were already infected). These horses might have appeared to get sick due to vaccination, but there is no plausible evidence that vaccine can cause the disease.
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