WNV: Still More Questions Than Answers

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Most horses live long, healthy lives — longer than ever before — thanks to smart owners who utilize good nutrition and available drugs and vaccines. However, too many horses still lose their lives each year to avoidable infections either because they weren’t inoculated or because their owners neglected to minimize the risk of exposure. The most recent example of this is the West Nile virus.

When the first human case of West Nile was detected in New York in 1999, we told you to keep an eye on your horses, even before the first equine cases were reported. The disease continued its spread in 2000, exploding in 2001 into one of the worst possible places: Florida, where abundant swamps and flourishing mosquito populations create an ideal environment for its spread.

The first confirmed equine case of WNV in 2001 was in July, at least a month earlier than in previous years. The numbers rocketed from there, and by Nov. 20, the USDA reported 417 horses confirmed infected nationwide in 18 states, 303 of them in Florida. The total for the entire country in 2000 was 59.

However, there’s a flip side to these sobering numbers: The death/euthanasia rate has dropped from 40-50% in 1999 to about 24%. Why is unclear.

Early recognition is an unlikely factor, since we don’t have a proven therapy to kill the virus. It’s more likely a commitment to intensive nursing and supportive treatment to wait the disease out — it takes eight to nine days to turn the corner if the horse is going to.

It’s also encouraging that the infection rate in last year’s hot spots of New Jersey and New York is down, despite that the disease remains active in mosquitoes and wild birds. Again, the reasons for the drop aren’t clear. It’s possible many horses acquired a natural immunity from the prior year’s exposure, and intensive mosquito-control efforts are almost certainly involved, as many horse owners took measures to decrease exposure on their premises.

Vaccine Success
Fort Dodge received a conditional license from the USDA for its WNV vaccine on Aug. 1, 2001. It’s too soon to know for sure how effective this vaccine is, but it’s certainly getting one heck of a field test.

William C. Jeter, DVM, of the Department of Animal Industry in Florida, said that as of Nov. 13, 254,870 vaccine doses were distributed to Florida veterinarians — enough for over 125,000 horses for the recommended double-dose series.

However, large-scale vaccinations didn’t get underway until mid-August, with the second dose given three to six weeks after the first. With this time frame, evaluation of the response couldn’t begin until well into the fall and won’t be complete before spring.

The number of new cases appearing from late October (94) to mid December (452) didn’t drop, but that figure could be misleading. With diseases spread via horse-to-horse contact, vaccinations slow the spread by creating a “buffer zone” of protected horses. However, since mosquitoes spread WNV, you must look at the rate of infection in unvaccinated horses and compare it to the rate in vaccinated ones.

The state of Florida is tracking the statistics. Dr. Jeter told us there were 294 new cases of WNV between early September and Nov. 13. Of those cases, 49 were in horses that received one dose of vaccine (16.6%); seven in horses that received two doses (2.4%); vaccination status was unknown in 128; 110 were not vaccinated. From Nov. 13 to Dec. 15, 149 new cases were reported, of which 23 (15.4%) were in horses with one vaccination and five (3.6%) in horses that received two.

To evaluate these numbers more critically, you’d have to look at the total number of horses in Florida and the percentage of the total population that was vaccinated, as well as when positive horses started showing symptoms rather than when their test results were confirmed.

We did learn that in horses that received two doses, all cases occurred within two to three weeks of the second dose. This could mean the vaccine isn’t effective until two to three weeks after the second dose or that the vaccine might precipitate symptoms in horses that are already infected and incubate the virus.

Dr. Silvio do Valle, a private practitioner in Ocala, Fla., said of the five confirmed positives in his practice (a five-county area that accounted for 42 of the cases in the state confirmed as of Nov. 20), two horses became symptomatic after their second dose, one the next day and another in less than a week.

Dr. do Valle estimated 97% of the horses in the practice had been vaccinated, with over 700 doses administered. Dr. do Valle didn’t see West Nile problems later than three weeks after the second dose. If the vaccine precipitates symptoms, it wouldn’t be that unusual. The same thing happens with injectable strangles vaccines and even with some viruses.

It’s too soon to draw any conclusions about the vaccine’s effectiveness. However, it appears that if you’re going to use it, you should begin the two-shot series well in advance of the anticipated time of exposure, so that it can be completed two to three weeks ahead.

The Nutrition Connection
Your best defense against an infectious disease like West Nile remains a healthy horse with a strong immune system. While some viruses, like a “hot” flu strain, may be invasive enough to get past just about any animal’s immune defenses, disease will always be far less severe in a strong, healthy horse. Some diseases, like EPM and West Nile, may not be able to take hold at all unless the animal is in a compromised state.

Stress of any type, from heavy exercise programs and shipping long distances, to loss of a companion, pregnancy, foaling, and weaning, even stall confinement, always sets the stage for infections.

Stressful life events are difficult to avoid, but you can minimize other stresses and work to avoid exposing your horse to possible sources of infection when he’s stressed. Performance horses should be well-conditioned for the job they are asked to do. Frequent stops for feeding, watering and unloading, if possible, can minimize shipping stress.

Nutrition is extremely important to immunity. A horse that is losing weight or receiving poor-quality protein sources is more susceptible to infections. The status of vitamins and trace minerals in the diet is also critical to immune function. Many of the nutrients commonly deficient in domestic diets — like vitamin C, vitamin E, selenium, zinc and copper — are also among the most important for immune function. More is not better, however. The trick is to provide optimal levels of these key nutrients. (See March 2000 for information on building a strong immune system and mosquito-control practices.)

Bottom Line
If you’re in an active WNV area — or an area expected to experience it for the first time this year — weigh the benefits with your veterinarian on the two-shot WNV vaccine. In addition, practice mosquito control and feed antioxidants to build immunity. Keep up-to-date on your EEE, WEE, rabies and tetanus vaccinations. While we’re not underplaying WNV, these vaccines are proven, and the diseases carry a high death rate.

Also With This Article
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Click here to view ”Old News But Still Big Worries.”
Click here to view ”2001 WNV Cases In Horses.”