Vaccinating Horses With EPM

Q:What is your opinion on giving half-doses of routine vaccinations to horses with EPM (equine protozoal myeloencephalitis)? My veterinarian says that they are more sensitive to vaccines because their immune systems are compromised. However, I've also read that a half-dose of vaccine in an adult horse is worthless. My horse has already received half-doses of everything. What, if anything, can I do at this stage to ensure that he is protected?


A:I would not give half doses of vaccines, as that may not elicit a full and protective immune response. I would suggest spreading out the vaccinations rather than giving 5 or 6 antigens all on the same day. I would give individual vaccinations separated by at least 3 weeks.

Regarding how much protection a horse will get from half-doses, unfortunately there is no blood test to determine if your horse is fully protected after those half-doses. I would consider re-vaccinating only for those diseases that are a significant threat in your area.

Learn more about rabies and read more Q&A with Dr. Vaala in our free on-demand webinar: The Many Faces of EPM!

About the Author

Wendy E. Vaala, VMD, Dipl. ACVIM

Wendy E. Vaala, VMD, Dipl. ACVIM, is a senior equine technical services veterinarian at Merck Animal Health and specializes in internal medicine and equine neonatology and perinatology. Prior to joining Merck Animal Health in 2004, Vaala was an assistant professor in large animal medicine at New Bolton Center, where she helped develop the neonatal intensive care unit (NICU) and high-risk pregnancy program. After leaving academia, Vaala served on the staff of Mid-Atlantic Equine Medical Center and BW Furlong and Associates in New Jersey, where she started NICUs, foaling programs for mares with high-risk pregnancies, and medicine referral services at both practices. She is board certified in large animal internal medicine and an active member of the American Association of Equine Practitioners. She has been an invited lecturer at seminars and symposiums for horse owners and veterinarians in North and South America, Australia, and Europe. She also has authored articles for veterinary journals and is an assistant editor for the textbook Equine Reproduction.

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