Vaccination Programs for Foals and Weanlings

Absorption of maternal antibodies from colostrum is important for protection of foals against specific infectious diseases but also substantially interferes with the response of foals to many vaccinal antigens. It is now well documented that the response of foals from vaccinated mares to inactivated influenza vaccines is blocked until at least 6 months of age, and administration of influenza vaccines at an earlier age may induce tolerance, resulting in a negligible response to influenza vaccines administered later in life.1–6 Most foals from vaccinated mares fail to respond serologically to common vaccinal antigens when primary immunization is started at 3 months of age, even when three doses of vaccine are administered in the primary series.2 In the case of EHV-1, EHV-4, WEE, EEE, and tetanus, preliminary results suggest that administration of vaccines to 3-month-old foals with detectable levels of maternal antibody does not negatively impact their response to subsequent doses of vaccine administered after maternal antibody levels have declined.2,7 However, more consistent responses to these antigens are achieved if primary immunization is delayed until 6 months of age or older. Since foals are frequently exposed to EHV-4, EHV-1 and, in the southeastern states, EEE during the first 6 months of life, it is rational to commence vaccination against these diseases at less than 6 months of age only if the risk of infection is high. Administration of inactivated influenza vaccines should be delayed until foals from vaccinated mares are at least 6 months of age and, preferably, 9 months of age or older. Regardless of the age at which foals are first vaccinated and the inactivated vaccine to be used, at least three doses of vaccine, rather than the two recommended by most vaccine manufacturers, appear to be necessary for primary immunization.2 Author’s address: Dept. of Medicine and Epidemiology, School of Veterinary Medicine, University of California, Davis, CA 95616. r 1999 AAEP.