Rotavirus Vaccination in Foals

THE EQUINE ROTAVIRUS VACCINE (Fort Dodge Animal Health) was first commercially available in 1996 for use in pregnant mares. As one of the few vaccines labeled for use in pregnant mares, it has significantly reduced the incidence and severity of rotaviral diarrhea in foals, not only in Central Kentucky, but also in other areas of concentrated horse breeding, such as Newmarket, England.

Before use of the vaccine, serious outbreaks of rotavirus had high morbidity and moderate mortality in foals less than 14 days of age. With use of the vaccine, protection is conferred via ingestion of colostrum from vaccinated mares, and decay of maternal antibody in foal blood over the first 60 days of life is expected. Also, as foals age, their gastrointestinal tract develops and is better able to mitigate a rotavirus infection, with foals beyond 90 days of age having mild or asymptomatic rotavirus infections.

Because of concerns among Kentucky veterinarians and farm managers of some foals developing mild rotaviral diarrhea around 60 days of age, a study was designed to determine if foals from vaccinated mares would respond to a rotavirus vaccine to boost immunity by age 60 days.

Two well-managed Central Kentucky Thoroughbred farms with 105 foals were included in the study. All of the pregnant mares on the farms had received the Equine Rotavirus Vaccine at 8, 9, and 10 months gestation, per label instructions. All foals born had greater than 800 mg/dl of immunoglobulin G (IgG), and none received any supplemental colostrum, plasma, or serum. Blood samples were obtained from foals at 24 hours of age and periodically through 75 days of age. Half of the foals were vaccinated with the Equine Rotavirus Vaccine at 30 and 45 days of age; the other age-matched foals served as controls. Foals were critically observed for signs of diarrhea, with appropriate diagnostic testing and treatment determined by the farm veterinarians.

Only one foal had a fever (103.1 F) after rotavirus vaccination; no other adverse reactions occurred. Serum samples from the foals were tested via serum neutralization to determine rotavirus antibody titers. Foals from both groups (vaccinates and controls) had high initial rotavirus antibody titers at birth, which decreased over time at the same rate through 75 days of age. There was no significant difference in the antibody titers between vaccinated and unvaccinated foals or between farms. No foals developed rotaviral diarrhea during the study, but five foals recovered from bacterial diarrheas.

The significance of this study is the confirmation again that high concentrations of maternal antibody will interfere with vaccination of young animals. No similar studies have been performed on the antibody response in foals from mares not vaccinated with the Equine Rotavirus Vaccine.

Also important to recognize is that proper farm management, routine disinfection, isolation/quarantine protocols, prevention of overcrowding, and other techniques are just as important as a proper vaccination program in the prevention of equine infectious diseases.

Dr. Roberta M. Dwyer (859-257-4757),
Maxwell H. Gluck Equine Research Center, University of Kentucky, Lexington, Kentucky