It is important to vaccinate broodmares 4 to 6 weeks before foaling for their own protection, as well as to maximize concentrations of immunoglobulins in their colostrum to be passively transferred to their foals. The majority of vaccines used in late gestation broodmares  to optimize immunoglobulin transfer via the colostrum do not carry a “safe for use in pregnant mare” claim. However, this is an accepted practice and clinical experience indicates these products are safe for this purpose. If the practitioner has specific safety questions or concerns, he or she is encouraged to contact the manufacturer for additional information.

It is important to recognize that simply vaccinating the mare is not sufficient for protection of the foal; successful passive transfer must also occur. The foal must receive adequate amounts of high quality colostrum and absorb adequate amounts of specific colostral immunoglobulins before absorption of macromolecules ceases (generally 24 to 48 hours post-foaling). Specific colostral immunoglobulins provide protection against field infections for several months, but also may interfere with foal responses to vaccines, a phenomenon termed “maternal antibody interference.”

Although protective concentrations of maternal antibodies decline with time, vaccination of a foal while these colostral antibodies are present - even at concentrations less than those considered to be protective - is often of minimal value because of maternal antibody interference. Consequently, a foal may be susceptible to infection before the primary vaccine series is completed. Management directed at minimizing exposure to infectious agents is key during this interval.

Reviewed by the AAEP Infectious Disease Committee in 2021.