Fighting Back

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As part of any good equine health program, vaccinations are a must. Lately, however, choosing what to vaccinate for has become a confusing issue for many barn owners and managers because of some new drugs on the market.

An established rule of thumb says a vaccination program should be based upon the type of disease and a horse’s likelihood of exposure to it—and most experts stand by this logic. Dr. James Corley of Lafayette, La., recommends that you “vaccinate first for diseases that are highly fatal and to which your horse may most likely be exposed.” These include diseases like tetanus and encephalomyelitis (sleeping sickness). After those, he says, “follow with vaccination for diseases that are highly contagious even if not highly fatal, especially in horses involved in group activities.”?

While the commonly held rule works well with regard to tried-and-true pharmaceuticals and their target diseases, what about the newly-released vaccines?

In 2001, the USDA conditionally released vaccines, both manufactured by Fort Dodge Animal Health, for equine protozoal myeloencephalitis (EPM) and West Nile virus (WNV), which are both currently undergoing studies as to their efficacy and potency. There is also a relatively new intranasal vaccine formulated to prevent strangles.

With regard to the injectable vaccines for EPM?and WNV, Roberta Dwyer, a veterinarian and researcher with the Maxwell H. Gluck Equine Research Center in Lexington, Ky., says, “these vaccines are usually only being recommended where there is a problem with the diseases, or horses are being shipped to an area where there is a problem.”?The ultimate decision, she points out, rests with the individual veterinarian and the client.

While owners might request that their horses be vaccinated for EPM and WNV, some veterinarians are hesitant to administer the drugs.

“I don’t use either,” says Frank Reilly, a practicing veterinarian in Pennsylvania. He feels that the drugs are too new and unproven to be used with confidence.

Rob Daily, the director of equine business for Fort Dodge, explained that the company was in the process of conducting efficacy trials for both. Results of the trials of the West Nile vaccine are expected this year, but the full study for the EPM vaccine will take longer to complete. Daily also notes that both vaccines were 99 percent reaction free in the early going and they have been maintaining that level. “Should anyone report a problem,” he says, “it is addressed immediately.”

In the United States, horses along the West Coast and from central states eastward are most vulnerable to EPM. The vaccine is a killed vaccine that stimulates antibodies to Sarcocystis neurona, a parasite that is considered the main cause of EPM, and is given in two doses at least three to six weeks apart followed by an annual booster. With regard to the Fort Dodge drug, there has been some concern as to whether horses vaccinated would test positive for EPM when the Western blot test was used.

According to Dr. Daniel K. Howe, a researcher with the Gluck Center, “at this point, the Western blot test cannot distinguish between natural exposure to Sarcocystis neurona . . . and immunization with the new EPM vaccine. Consequently, horses given the vaccine will test positive, and this appears to include testing of the cerebrospinal fluid.”?According to Howe, this has been a major concern to both researchers and veterinarians because the test won’t be useful for ruling out EPM in horses that have been vaccinated. “However,” he says, “if we assume that the new vaccine provides at least some protection against EPM, the benefits of vaccinating probably far outweigh the value of getting an accurate test result.”

To tackle West Nile virus, which is spreading down the East Coast to Florida after first appearing in New York in 1999, Fort Dodge released a vaccine this past year and distributed it more widely than originally planned. While many people used the vaccine in late summer, it is recommended that it be administered in the spring. It is also given in an initial two-dose series three to six weeks apart followed by an annual booster. Again, its efficacy is not known, and it may be late 2002 before solid results are released.

The third relatively new vaccine is another Ford Dodge formula and is an intranasal spray called Pinnacle IN, which is aimed at preventing strangles. This vaccine is an alternative to the intramuscular injection, which can cause painful injection site reactions and abscesses. Again, some veterinarians are hesitant to use the new vaccine until its safety is better determined because there have been reports of adverse reactions like swollen lymph nodes and nasal discharges. However, the majority of horses so far respond better to the intranasal form than the injection.

Naturally, you should talk to your veterinarian about diseases specific to your area and make educated and informed decisions on the new vaccinations.

When EPM Takes Hold

For a horse that is infected, a new antiprotozoal oral paste called Marquis was released by Bayer in the summer of 2001. It is the first FDA-approved treatment for EPM and consists of a daily dose of 5 mg/kg for 28 days. The active ingredient is ponazuril, a compound that crosses into the central nervous system and works to fight the Sarcocystis neurona parasite. While it has proven effective in clinical trials, it may not have any effect on damage already done to the central nervous system, and early diagnosis and prompt treatment is important. Marquis has a wide safety margin, but has not been evaluated in broodmares or foals. For more information on this new treatment, call (913) 268-2517 or visit their Website at www.EPMnews.com.

The Essential Defenses

The American Association of Equine Practitioners recommends the following vaccinations. This list is only meant as a guideline and for final evaluation, you should discuss the needs of your horses with your veterinarian.

For primary immunization, an initial vaccination is required, followed by a repeat dose in three to four weeks.

  • Tetanus—Should be given annually to all horses. Foals initially at two to four months of age. Broodmares at four to six weeks before foaling.
  • Encephalomyelitis—Should be given annually in the spring to all horses. Foals initially at two to four months. Broodmares at four to six weeks before foaling.
  • Influenza—Should be given to most horses every three months. Foals initially at three to six months. Broodmares biannually, plus booster four to six weeks pre-foaling.
  • Rhinopneumonitis—Should be given to most horses every three months. Foals initially at two to four months. All broodmares at least during fifth, seventh and ninth months of gestation.
  • Rabies—Should be given annually to all horses. Foals initially at two to four months.
  • Strangles—Should be given biannually for high-risk horses. Foals at 8 to 12 weeks. Broodmares biannually with one dose four to six weeks prior to foaling.
  • Potomac Horse Fever—Should be given biannually for high-risk horses. Foals at two to four months. Broodmares biannually with one dose four to six weeks prior to foaling.

—SC