West Nile`s “Sneak” Attacks

Horsemen have been dealing with the nightmare of West Nile virus for five years. Between 1999 — when it first appeared in the Long Island area — and 2003, a total of 20,716 horses with officially confirmed cases of WNV have been recorded by the U.S. Department of Agriculture. However, the true number, including horses that were never tested, is likely much higher.

West Nile earns the title of the worst encephalitis epidemic ever recorded in North America. The virus has been documented in at least 138 different species of birds and 43 species of mosquitoes. Most of the mosquito species prefer to feed on birds, keeping the virus well circulated through this reservoir. However, there are also different types of “bridging” mosquitoes, which feed on infected birds and then pass it on to a horse or human.

Some birds are easily killed by West Nile, particularly crows and blue jays. However, the birds that develop high levels of the virus but don’t die are a bigger worry, as they’re the ones that keep it in circulation. Mosquitoes can also pass the virus on to their next generation through their eggs, and the virus survives the winter with mosquitoes that make it through. Passage of virus via the placenta, milk or blood transfusion has been documented in people.

Attempts to predict WNV cases and the outcomes have failed dismally so far. The problem is there are simply too many different factors to take into consideration, including weather patterns, number of susceptible birds/people/horses exposed, a variation in numbers of mosquitoes overall and species of mosquitoes that might predominate from year to year, which is also linked to weather.

Nationwide totals for 2003 were down considerably from 2002, which some claim is due to vaccination, but we don’t think it’s that simple. Natural exposure, which results in far more symptom-free infections than actual disease, is a major factor. Yearly temperatures, rainfall figures and natural water collections all can play a role in the level of natural exposure.

The appearance of the disease in an area motivates owners to vaccinate. Despite this, however, an examination of the county-by-county equine-case numbers for many states shows a similar pattern of the disease continuing to be the most active from year to year in areas that were hardest hit, such as Marion County, Fla., and Weld County, Colo.

Even states that don’t show a sharp county-to-county difference in equine cases, such as Kentucky and Texas, have areas showing up as “hot spots” from year to year. The number of horses living in a particular county certainly also strongly influences the number of cases noted. But no matter how we look at the data, we find no clear evidence that vaccination alone is lowering case numbers.

Estimating Your Horse’s Risk
In a way, it’s pretty simple: If your horse lives in the United States, he’s at risk. Surveillance systems, such as dead-bird/mosquito testing and sentinel bird flocks, may not give you any advance warning that the virus is active in your area. However, some red flags of high risk include:

• Residence in an area rich in natural collections of water.

• High rainfall, especially following a period of drought.

• Residence in an area with a high number of cases the previous year, although risk will decrease over time as the percentage of immune horses rises.

• Confirmed cases within your county or in closely neighboring counties and farms.

Natural Immunity
Natural immunity is a sticky question because we just don’t have equine-specific information. To date, no instances of suspected reinfection with WNV in a horse confirmed to have already had the infection once have been reported to the USDA. Virologists and West Nile experts agree, however, that immunity to this virus following a natural infection is likely longstanding, if not lifelong.

Incubation Period And Symptoms
Experimental infection of horses with West Nile virus suggests the incubation period is between five and 15 days after exposure. However, many owners reported noting that their horses appeared depressed and isolative several weeks before showing clear symptoms of West Nile virus. Why this occurs is not clear. If you note your horse acting ill in any way during West Nile season, consider the possibility of a WNV infection. Antibody tests for acute infection become positive about seven days after exposure.

Symptoms of WNV vary from none to a nonspecific depression to full-blown neurological signs. Some horses continue to eat well even if down, while others have a poor appetite. Some show dramatic weight loss during the acute, early infection, while others hold their weight well or don’t show the effects of weight loss and generally decreased condition until several months later. Horses that do show loss of body condition generally do so despite a good appetite and high-quality diets.

On the neurological end, signs are highly variable both in type and severity. Muscular twitching in the head or body are common. A variety of gait deficits may be seen, from mild toe dragging and stumbling to severe ataxia and inability to stand, or anything in between. Encephalitis and cranial nerve signs aren’t seen as often as ataxia and balance difficulties but may occur. Many horses show a withdrawn or “spacey” personality change.

Non-Apparent Cases And Death Rate
Non-apparent cases are horses whose blood antibody levels confirm they have been exposed/infected but never showed any neurological signs. We’ve known this happens since the first cases on Long Island in 1999.

A University of Florida Veterinary School study in 2001 looked at horses in counties with various levels of WNV activity and found antibody evidence of exposure in anywhere from 15 to 68% of sampled horses in WNV-positive areas. The USDA’s National Veterinary Services Laboratory in Ames, Iowa, reported that all of the lab’s own horses, which are housed outside, became positive over a two-year period, although none of them were ill.

Fatality rates vary but run 25 to 30%. Since some owners/veterinarians may decide not to treat an individual ill horse, this number is likely higher than the true death rate would be if all horses were intensively treated. However, if we assume a worst-case scenario with a 30% death rate there, for every 30 horses bitten by infected mosquitoes, three will actually show West Nile symptoms and one of these will die or be euthanized.

Relapses And Long-Term Effects
While West Nile infections in the early years suggested the horse would either die or get over it completely, as experience with the disease increased, owners and veterinarians noted that horses may seem to recover only to have return of milder symptoms several months later.

Relapses may be precipitated when a horse that has seemed to recover experiences some stress, such as a return to work. A variety of long-term problems have also been reported, including weight loss, weakness and ongoing neurological deficits or behavior changes.

Data on virus persistence specific to horses is not yet available, but a study conducted by the University of Minnesota and presented at the 2003 AAEP meeting reported that 40% of horses tha t survived West Nile were reported by the owners to have residual problems six months after the infection. Of those horses, 9% experienced acute relapses within two months of their original infection. Whether horses with persistent deficits will be permanently affected or will recover gradually over a longer period of time is unknown.

Vaccine Efficacy
We still don’t know exactly how well the WNV vaccines work. Because there’s still no 100% reliable way to cause West Nile symptoms experimentally, the challenge studies done with both currently available vaccines provide only limited information. Efficacy of the new Merial vaccine remains to be seen under real-life conditions.

The Ft. Dodge vaccine, which appeared in late 2001, does not appear to be 100% effective. That said, there continue to be more cases of WNV in unvaccinated than vaccinated horses. However, the number of unvaccinated vs. vaccinated horses in any given state or county is often unknown or unavailable.

Even when information is available on doses shipped to a given state or area, we often don’t know when those doses were administered or how many were initial two-vaccine vaccine series versus boosters requiring only one.

Both private and state veterinarians in many heavily hit states are recommending boosters every four to six months, further complicating trying to determine how many horses are receiving the vaccine.

The largest survey conducted to date that looked at the influence of vaccination and other factors on West Nile was from Colorado State involving cases from Nebraska and Colorado during the 2002 epidemic. That study did not draw any conclusions about how well the vaccine might actually prevent West Nile, but it did note a 36% fatality rate in unvaccinated horses versus 20% in those that had received at least one dose of the vaccine. Of 493 horses in that study, 88 received both doses of the vaccine. Of these, 74 became symptomatic within four weeks of receiving their second dose.

Side Effects
Both the Merial and Ft. Dodge vaccines showed a low incidence of the usual type of vaccine reactions on pre-market testing. We won’t know any more about the new Merial vaccine until it has been in more extensive use.

Owners of horses with a history of reactions to a certain manufacturer’s brand of vaccines should consider switching brands — again, for this or any other vaccine. In addition, it is OK to use the Merial vaccine as a booster if your horse previously received the Ft. Dodge vaccine. Both vaccines stimulate a rise in the same antibody class, serum neutralizing antibodies. Merial recently reported that boostering previously vaccinated horses with their vaccine produced a 6.2-fold increase in antibody titer, compared to a 4.8-fold increase when boostered with the Ft. Dodge vaccine. The booster vaccinations were given 4.5 to 6.25 months after the initial series.

Of more specific concern to many owners has been that their horse developed full-blown symptoms of West Nile within a short period of time (24 hours to three weeks) of receiving the West Nile vaccine. However, neither killed vaccines nor Merial’s new DNA-recombinant vaccine can actually cause West Nile because neither contains live virus.

The usual explanation for these cases is that the horse must have been infected before the vaccine had a chance to be fully effective. That’s likely, of course, but it’s also theoretically possible that a horse already infected with West Nile could be pushed over the edge by vaccination because of the vaccine putting more burden on the immune system, or tying up the antibody response the horse is attempting to mount against the virus.

Either way, the best advice we have is to plan ahead if you’re going to vaccinate, giving it before the West Nile/mosquito season begins. We’d avoid vaccinations, especially first time vaccinations, during the West Nile season. If you must vaccinate after the season starts, check first to determine if the horse is showing antibodies, indicating he has already been naturally exposed.

Bottom Line
The toll this disease takes on both owners and horses is tremendous. About the only good news is that once a horse has been exposed, the immunity is likely to be long-lasting and, as the number of immune horses eventually rises, we should begin to see a drop in the number of cases. Fortunately, current estimates are that only about 1 out of 10 infected horses will actually show symptoms of the disease. West Nile is here to stay, though, and maintaining vigilance is your best protection.

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