The Best Protection

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Most horse owners routinely vaccinate their horses every year, trying to protect their animals from the most prevalent or dangerous equine diseases. Vaccines come and go, however, as manufacturing companies come up with new products or old ones are discontinued. Plus, different companies may market different types of vaccines for the same diseases. And new disease threats emerge periodically, such as West Nile Virus that appeared on the East Coast in 1999 and then swept westward across the country, or Venezuelan Equine Encephalomyelitis, which occasionally moves north from Central America. As a result. it’s not always easy for the equine professional to know which diseases are most concerning in their own region, or which vaccines to use.

The American Association of Equine Practitioners (AAEP) publishes guidelines for horse owners, and updates these as needed (see www.aaep.org). The AAEP recommends certain vaccines be considered a “must” for every horse, regardless of regional location or use, due to the ever-present risk of these diseases. These “core” vaccines include tetanus, Eastern and Western Equine Encephalo­myelitis, West Nile Virus, and rabies. Other vaccines—for influenza, rhinopnuemonitis (equine herpes virus or EHV), strangles, anthrax, botulism, equine viral arteritis (EVA), Potomac Horse Fever and rotaviral diarrhea—may or may not be important for certain horses, depending on whether or not they might become exposed.

At the recent annual AAEP meeting in Baltimore, Md., (December 4-8, 2010), vaccines and vaccinations were among the topics discussed. Julia H. Wilson, DVM (University of Minnesota) was one of the moderators for the vaccinations table discussion at the AAEP meeting, together with Dr. Amanda House (University of Florida). Some of the topics discussed included changes in vaccines that are available today, especially since some of the vaccines that were in use earlier have been taken off the market.

“Spokespersons for Intervet, the company that makes Prevenile, believe they have found the problem with that vaccine and expect to have it back on the market in 2012. There was a brief discussion regarding what was causing the reactions to Prevenile, but there is little new information on this, other than the fact they think it is some form of anaphylaxis,” she says. No numbers were given on what percentage of horses had this reaction.

“What Intervet is doing in the interim for their clients is releasing that same viral product, but killed—in a vaccine form—so the company will still have a West Nile vaccine in their lineup. This will be the product offered to the horse rescues that are part of their Unwanted Horse Veterinary Relief campaign,” says Wilson.

“With all the company mergers, Intervet is going to become part of Merial sometime this year. We don’t yet know how this will impact the availability of the various vaccines,” she adds.

“The newest West Nile vaccine (Innovator DNA) that Fort Dodge came out with just before they merged with Pfizer is no longer on the market. So now horse owners have the older killed virus option of West Nile Innovator, which works quite well, but you have to give it more frequently than the recombinant (Recombitek) or modified live chimera vaccines for yearlong protection,” she explains.

“We also talked a little bit about vaccine failures for Eastern Equine Encephalomyelitis (EEE). It seems to be mainly the young horses that most frequently experience vaccine failure. In areas like Florida, most veterinarians are still recommending that horses be vaccinated for EEE three times a year if using a killed vaccine, because of the risk for that disease,” Wilson says.

Lyme disease and leptospirosis were also discussed, and there was no consensus regarding vaccination for these diseases, since at this point there’s still no approved equine vaccine for either disease. Horse owners remain concerned about both of these diseases, including lepto’s potential role in recurrent uveitis (moon blindness) and abortion. Using the canine vaccine for Lyme disease or the cattle vaccine for lepto is an extra-label use, however.

“One of the papers that Dr. Amanda House brought to our attention was a report in the Australian literature about the first horse seen with a fibrosarcoma (cancer) at a vaccine site,” says Wilson. This incident was one of the new things that came up in the discussion.

“This may be an indication that there might be some validity to the argument some people are putting forth that many of our horses are unnecessarily put at risk by over-vaccination. The Australian horse developed the same type of cancer seen in cats and dogs at vaccination sites. When given an influenza booster in its neck, the horse developed a swelling at the site two weeks later, which kept progressively enlarging. It had to be surgically removed after a biopsy confirmed the cancer diagnosis. With just one report, we don’t know how great a risk this is for horses,” House says.

“We also reminded veterinarians to make sure their clients recognize that if they are going to change their plans for a certain horse for the year (such as take the horse to shows or other events, or retire it from competition, or breed it), they need to check with their veterinarian to make sure that they tweak their vaccine program accordingly,” she says.

“We had a good discussion with many of the pharmaceutical companies represented at the meeting. Among other subjects, we talked about the role of different adjuvants causing potentially different reactions,” says Wilson. Adjuvants are chemical complexes added to killed vaccines to increase the body’s immune response.

“When a horse reacts to a vaccine, the veterinarian may suggest switching to a vaccine for that disease that has a different adjuvant. For horses that repeatedly react to vaccines, another alternative that was discussed was administration of a nonsteroidal anti-inflammatory drug such as flunixin (Banamine) before or at the time of vaccination,” explains Wilson.

Vaccine insurance programs were another topic of discussion. “The vaccine manufacturers are offering to pay a significant proportion of medical expenses for a horse that becomes ill with a disease it has been vaccinated against, but only if a veterinarian administered the vaccine. Pfizer has launched this program now.”

Boehringer Ingelheim claimed to have something similar, as did Intervet and Merial, but Dr. Wilson hadn’t seen the details yet. “I think this might be a useful way for clients to understand that the vaccines are helpful, but are not 100 percent effective.”

When to Vaccinate

Some diseases are more prevalent in certain areas than in others, and some horseman may be reluctant to vaccinate against a disease they don’t see very often. “One of the questions I posed to the group was to ask how well their clients are accepting the new designation for rabies vaccination. Rabies has now been added to the AAEP list of core vaccines, to be given to all horses, even though it is not a risk in all areas. It is a very low risk in the Northwest, for instance,” says Wilson.

“One of the veterinarians who spoke up, from the Northwest, said he has simply incorporated the rabies vaccine into his wellness program and didn’t give his clients a choice. Nowadays, you don’t always know where the horses are traveling to, and what risks they might encounter somewhere else,” Wilson notes.