To vaccinate or not? DVMs sort out issues

As West Nile Virus (WNV) dominates headlines worldwide, the only vaccine on the market - available to horses - is quietly making noise in equine veterinary circles.

While 4 million doses have been distributed, the vaccine, West NileVirus Vaccine, produced by Fort Dodge Animal Health and conditionally licensedby the USDA, appears to segregate veterinarians over its efficacy, conditionallicense and the location of veterinarians in relation to the virus' spread.West Nile Virus had reportedly infected 3,453 horses in 28 states at presstime.

Veterinarians in the hotbed of viral activity, such as Florida, are recommendingtwice-yearly vaccinations, while others located where the disease has barelyregistered, are obstinately reluctant to vaccinate at all.

A staunch vaccine proponent, Dr. Michael Carinda, of Delray Beach, Fla.,says he reported the first and perhaps only case of West Nile in southernFlorida last year, a state, which overall saw almost 500 equine cases lastyear, and another 100 this year.

"Everybody is vaccinating without question, because of our mosquitopopulations," says Carinda. Since the state wages war on mosquitoesyear round, veterinarians here have agreed on twice-yearly vaccinations,though Fort Dodge only recommends an initial vaccine, 30-day booster, thenyearly vaccination.

Equally supportive of vaccination are Hagyard-Davidson-McGee and Rood& Riddle, two internationally renowned equine referral hospitals, inLexington, Kentucky. The state has reportedly confirmed 185 equine casesof West Nile in 2002.

The data speaks for itself, says Dr. Nathan Slovis of Hagyard-Davidson-McGee.

"By looking at the data in several states, it seems the vast majorityof the horses - upward of 90-plus percent that get the disease - do nothave a history of vaccinating. The ones that have been vaccinated appearto be protected.

"It seems the odds are in your favor that you won't get the diseaseif you vaccinate on time and correctly," says Slovis.

Yet, in states where the disease has yet to dominate, some veterinarianshesitate to vaccinate.

Conditional approval

For instance, in New Mexico where the virus appeared for the first timethis year in two equine cases, State Veterinarian Steven England says, althoughthe state recommends it, he brazenly questions the merits of a conditionallylicensed vaccine.

The vaccine has limitations such as, "The following statement shouldappear on all labels: this product is conditional" and "the licenseeshall demonstrate acceptable progress toward completion of host animal efficacy"

"That license had more restrictions on it than any conditional licenseI've seen in 15 years," says England.

Company officials from Fort Dodge Animal Health say there is nothingunusual about the license's restrictions.

"By definition, a conditionally licensed product must prove safety,purity and a reasonable expectation of efficacy while efficacy and potencystudies are ongoing," Fort Dodge claims in a position statement. "Aconditional license may be issued based on an urgent need for the productin the marketplace."

That said, the vaccine may be perfectly fine to use, concedes England."But to me, it's wrong when they've got a conditional license withthis many restrictions on it, to be just willing to throw it out there onthe open market."

Research not lacking

Not denying the product's current restrictions, Fort Dodge officialssay the company is "sufficiently funding" research necessary toobtain a full license and plans to continue until the vaccine is fully approved.

Challenge data has now been completed and Fort Dodge says it supportsefficacy claims. In addition, an abstract paper, "Equine Vaccine forWest Nile Virus," released by Fort Dodge at presstime, shows "significantprotection (94 percent of preventable fraction) against viremia in horsesvaccinated with the killed WNV vaccine and the long duration of the protectiveimmunity."

West Coast divided

Well-researched or not, on the West Coast, sentiment is mixed. Dr. DavidRamey, a Southern California equine veterinarian, says in light of datademonstrating the vaccine's efficacy, he still has reservations.

"For instance, people say, 'for horses that were vaccinated lastyear, we're not seeing as much of (the virus).' That's true, but it couldbe all the horses are developing a natural immunity," he argues.

Nevertheless, adds Dr. Kim Sprayberry, recently-relocated private practitionerin central California's Paso Robles, "Nobody wants their horse to bethe first horse."

A number of veterinarians on the West Coast were said to be vaccinatingfor West Nile before she arrived in January. Yet, the vaccine may not befoolproof, she notes, citing instances of horses that became infected thoughthey were vaccinated prior to infection.

"It's not a suit of armor, but when the virus arrives here withthis population of horses, if there's a large population of horses thatare already vaccinated and have at least some kind of protective antibodytiters, its ability to become rampant, virulent or damage a lot of animalswill be curtailed," she says.

Location matters?

Ramey still argues the disease may never truly penetrate Southern California,where he says mosquito concentrations are minimal.

He remains steadfast in spite of recent news reports, confirmed by theCenters for Disease Control and Prevention, of the first fatal case of WestNile Virus in an otherwise healthy woman living in Southern California.This case emerged without any reports of infected mosquitoes, birds or horsesin California.

Aside from the human case, Ramey says, "I seriously doubt if itwill ever be a disease that establishes itself in Southern California."

In southern Oregon, where the disease has not yet arrived, Dr. Cary Hills,of Rogue Valley Equine Hospital, says he's not taking chances and recentlybegan suggesting the vaccine to clients.

"I tell people it's an unproven, conditional license. But we arerecommending for the most part, because when it comes, it will come witha vengeance. In terms of the cost to the client, to get their horse vaccinated,it costs $45. That's often money well spent. Just like if you have car insurance,you don't expect to go crash your car the next day, but it's nice to havethat insurance."

That sentiment rings familiar in Texas, where 134 equine cases of WestNile were reported this year, from Houston to the Panhandle.

Dr. Terry Conger, state epidemiologist in Texas, notes, "Under theseconditions and considering the cost of the vaccination, I don't have anymisgivings about veterinarians selling it to horse owners. The mortalityrate is around 20 percent in non-vaccinated horses. That's severe, but itcould be worse."

What about efficacy?

The concerns about efficacy impact veterinarians across the country onvarious levels.

Fort Dodge answers, "No vaccine is 100 percent effective. Whileit would not be realistic to assume 100 percent of vaccinated horses willbe protected, our feedback from practitioners, which have had vaccine breaksis that the challenge was overwhelming, the incidence of potential lackof efficacy cases is very low and most horses responded more favorably thannon-vaccinated horses regarding treatment."

Given word of Fort Dodge's latest efficacy studies, and acknowledgingthe vaccine hasn't been reported to kill a horse, nor cause adverse effects,Ramey says he is still unconvinced.

"Under the best case circumstance, (veterinarians) are using wiseforesight to prevent a disease that could hurt their horse," says Ramey."But the worst case is that they're wasting their money to use an ineffectiveproduct."

At Rood & Riddle in Lexington, Ky., where veterinarians advocatethe vaccine, Dr. Bonnie Barr says she understands how questions of efficacycould raise eyebrows.

"What I tell people is my understanding is the vaccine was producedvery similarly to the vaccines for eastern equine encephalitis and westernequine encephalitis, which had been out for awhile. The WNV vaccine wasbased on those other vaccines. They're fairly comfortable that it is prettyeffective, but nobody knows for sure," she says.

"You have to talk to your veterinarian and see what you think isthe best thing," says Barr.

Editor's note: Several company representatives contributed to thestatements from Fort Dodge Animal Health.

(To view the complete Fort Dodge Animal Health abstract on the WNVequine vaccine, visit the equine section of the DVM Newsmagazine Web siteat www.dvmnewsmagazine.com/dvm/article/articleList.jsp?categoryId=361)