On Dec. 18, 2000, a vaccine to prevent EPM was approved by the USDA. As of Jan. 25, a total of 43 states had approved the use of the EPM vaccine under USDA conditional licensure. The vaccine must be used under the supervision of a veterinarian. It requires a two-dose initial series at least two weeks apart, followed by an annual booster. The label says it should be used in healthy horses, with no age limit specified.

According to Tom Overbay, DVM, Director of Professional Services with Fort Dodge Animal Health, the pharmaceutical company that produces the vaccine, "We have every belief that this product will be effective. The safety already has been proven in field trials."

When asked about the small group of researchers and practitioners who were not in favor of releasing the vaccine, Overbay responded: "It would be a poor business decision to release a vaccine that would fail because every other vaccine we make and our reputation as a company would be threatened if we only released a vaccine for opportunistic reasons.

"Some of the assertions are wrong," he continued. "Some said we can't do a protozoal vaccine, that this is the first. Actually, this is our fourth protozoal vaccine. We have some for the cattle reproductive disease trichomoniasis and the diarrheal disease in dogs and cats from giardia."

Overbay said it is the adjuvant technology that helps those protozoal vaccines be effective. Adjuvants are substances to which the causative organism (or portion thereof) is adhered and that help dictate the immune response in the animal.

"Adjuvants help determine the type of immune response you get," he added.

Malaria is caused by a protozoal parasite, and there never has been a vaccine produced for malaria in humans. However, Overbay said that malaria affects the red blood cells, which don't express antigens. Therefore, they give no signal to the body to respond to the parasite infection.

"Besides the fact that these protozoal diseases express differently, we can use different adjuvants in veterinary medicine than can be used in human medicine. That makes a difference."

A nationwide field efficacy study on the EPM vaccine is underway (see sidebar page 37). There also are research projects underway to help differentiate a vaccinated horse which tests positive from a horse which tests positive because of natural exposure to the parasite.

About the Author

Kimberly S. Brown

Kimberly S. Brown was the Publisher/Editor of The Horse: Your Guide To Equine Health Care from June 2008 to March 2010, and she served in various positions at Blood-Horse Publications since 1980.

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