By Michael Mahaffey

With the announcement of the voluntary recall of the Prevenile West Nile Virus vaccine by its maker, Intervet/Schering-Plough, in May of this year, a new wave of concern regarding the safety of the vaccines we give our horses washed over owners across the country. Reports that severe reactions to the vaccine had resulted in the deaths of several horses elicited a flurry of responses, from happiness that a horse who had received the vaccine hadn’t reacted badly, to confessions that similar reports of reactions have caused owners to stop vaccinating their horses altogether.

But is refusing to vaccinate a wise course of action to take? A trio of leading veterinarians and researchers suggest that choosing not to vaccinate for fear of what might happen may be the very thing that puts your horse’s life at risk

Why Vaccinate?
In the simplest terms, vaccines protect horses from potentially debilitating or life-threatening diseases by stimulating their immune systems to develop antibodies that help fight off potential infecting agents.

Dr. Hoyt Cheramie, a veterinary surgeon certified by the American College of Veterinary Surgeons who currently works for Merial as a Veterinary Professional Services Manager, views vaccines for horses, just as in people, as a form of insurance, albeit one that comes with an element of risk.

giving a shot

Vaccinations are vital to maintaining a healthy horse, but they do not come without risk. (CPG File Photo)

“The vaccine’s job is to make a response in the body, and it’s an inflammatory response,” he says. “Unfortunately, horses tend to get a little over-inflamed as a species, and when that happens, the potential for very bad problems—for a species with very specific gut issues and feet issues—is probably multiplied compared to other species. We don’t see founder in dogs and cats and people. We don’t see colic the way we do in horses in those species, and I think that’s where people get afraid.”

The choice of whether or not to vaccinate is typically a simple one for Cheramie. To him, the dangers most diseases, like West Nile Virus, pose to a horse’s long-term health almost always outweigh the potential risks that vaccination poses. The more virulent and prevalent the disease, the more certain Cheramie is to recommend vaccinating against it.

“I would look at it as one of those cases where if my horse gets this disease, it’s going to die,” Cheramie says. “If my horse gets a vaccination, there’s a very, very small chance that it might get a lump or a bump. It might get a little fever, and then even a minute chance that it might get something that might be a fatal event. In those cases, I think it warrants protecting those horses. Particularly when I can’t protect them by keeping them away from other horses.

“Even if you have a single horse in a pasture, you can’t protect them from [a disease],” he adds, “because you can’t keep the skunks away. You can’t keep the mosquitoes away. You can’t keep the rusty nails and the dirt away.”

The decisions of what to vaccinate for and how frequently a horse should be vaccinated is based on the core vaccinations recommended by the American Association of Equine Practitioners and the conditions in which a horse lives and competes.

“Vaccination is something we do on a risk-based basis,” says Dr. Paul Lunn, head of the Department of Clinical Sciences at Colorado State University, who specializes in immunology and infectious diseases of the horse and serves as an AAEP on-call veterinarian for infectious diseases. “This is where you and your veterinarian need to engage in a risk-benefit analysis to determine what your horse’s risk status is at its age and whether its lifestyle is going to be at sufficient risk for catching this disease.

“Some vaccines are what we call core vaccines,” Lunn says. “These are vaccines for diseases that are common and are really serious—including West Nile Virus, Eastern Equine Encephalomyelitis, Western Equine Encephalomyelitis, Tetanus and Rabies. They threaten the lives of horses.

“We only designate core vaccines as such when there is a really good vaccine available. Not just that there is a vaccine that is partially efficacious, but one that is really powerful. The core vaccine should be the no-brainer decision that ‘my horse should get this.’”

Lunn says our tendency to treat our horses the way we treat our children leads to the excessive attention we pay to the potential effect of a vaccine. We will not tolerate any side effects.

But side effects are unavoidable, according to Cheramie, which many horse owners know—making it hard in some cases for horses to get the protection they need.

bump lump mp

Some vaccines are known to be strong provokers of local responses, such as lumps, bumps or soreness. (Megan Parks)

“I’ve seen horses die from what I thought were vaccination reactions,” Cheramie says, “but I would suspect that many more horses every year die from not being vaccinated than from being vaccinated. I think the way many horse owners feel about it is it’s something that they did to their horse. I’ve dealt with clients that had reactions to vaccines when I was in practice, and that’s what they feel bad about. They feel they caused their horse to suffer—whether it just got sick or something severe happened to it—because they chose to vaccinate it.”

In his practice, Cheramie would discuss the process of vaccinating a horse with a client and look at the risk/benefit ratio—what is the risk of not vaccinating, what is the risk of vaccinating and what is the benefit of vaccinating. In almost every case, if the exposure risk of not vaccinating was great and that potential outcome of that horse getting exposed was great, he recommended vaccinating.

However, the urge some horse owners feel to stop vaccinating for some conditions, like West Nile Virus, can come from reasons as varied as only a few cases having occurred in their area to the simple desire to save a few bucks. But this can be a dangerous game.

“People have really wanted to stop vaccinating for West Nile Virus because the incidents got so low,” Cheramie says. “Most people would not stop vaccinating for EEE or Tetanus, because they’ve always done it. West Nile Virus, they saw it come, and they think they saw it go because we have reduced the population of susceptible horses tremendously by vaccination.

“If you look at the [Centers for Disease Control] reports, the level of West Nile infected mosquitoes, birds and people seem to be fairly constant. We’ve decreased the prevalence in horses because we vaccinate for it. If a horse goes unprotected and is not naturally immune, it puts that horse at risk of developing the disease.”

Are Vaccines Safe?
While most vaccines sold in the U.S. are safe, their use does not come with a 100 percent risk-free guarantee.

“Great strides have been made in the last decade or so in terms of improving both the efficacy and safety of these products,” says Dr. David Horohov, the William Robert Mills professor of Equine Immunology at the University of Kentucky Gluck Research Center. “It’s fair to say that we now have vaccines that are as efficacious as human vaccines, and that wasn’t always the case. We’ve come a long way in the last 10-20 years in providing good, safe vaccines.

“That’s not to say that something is not going to happen [when they are administered]. Some of those animals, for one reason or another, are going to have a reaction to that vaccine, even under the best of circumstances. We’ll never have a 100 percent safe vaccine, only because we’re dealing with animals of different genetic backgrounds and susceptibilities and tendencies. We hate when it happens, but I’m afraid it’s something that’s just going to happen as long as we vaccinate.”

The companies that manufacture vaccines, such as Cheramie’s employer, Merial, know it is in their best interest to make sure the vaccines they produce and sell are as safe and reliable as possible.

“Vaccines that are put out and labeled by pharmaceutical companies go through efficacy trials and challenge trials to show that they work,” Cheramie says, “and they also go through fairly rigorous safety trials prior to marketing them. They have to show to the USDA [United States Department of Agriculture], which is the governing body for biologics in the U.S., that they are safe for use.

“We go out and give the vaccine to hundreds of horses. For example, when Merial came out with Recombitek West Nile, I believe there were over 700 horses that we tried it on before we proved to the USDA that it was safe. And that’s beyond all the horses that we used in our trials to determine that it was safe.”

If vaccine makers don’t address problems when they arise, they will not only lose their ability to sell the questioned vaccine, but they will seriously damage their credibility in the process. This accountability causes companies go to enormous effort to minimize potential incidents and detect problems as early as possible.

Events such as the Prevenile recall are exceedingly rare due to the amount of testing for both safety and efficacy that goes into vaccine development.

“When something like this happens, it’s quite surprising to everyone involved,” Horohov says. “I tell people there’s no reason to throw the baby out with the bathwater. A problem with a vaccine does not necessarily mean that vaccination is the problem. It just means that there may have been a quality control or other issue with that product that needs to be resolved.”

Lunn believes the Prevenile incident is a good illustration of how things should go when a problem with a vaccine is detected.

“In this instance, Intervet started to suspect there were reactions to vaccination,” he says, “including the deaths to a small number of horses, and they believed they noticed these problems in association with Prevenile. As soon as they thought there might be a problem, they pulled this product from the market. They were very, very aggressive to take as low-risk a course as possible to protect the horses and horse owners. There was a rapid response and the risk was reduced by removing that product from the market.”

Check back next time for Part 2 of “Should We Vaccinate?”


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