Vaccines are important for the health of the mare and her foal. The goal of vaccination is to stimulate production of antibodies that will prevent or reduce the severity of infectious diseases. We recommend vaccinating non-pregnant mares in the late winter or spring, a week or more before breeding and not during early pregnancy. We would like to avoid any adverse complications in early pregnancy if a mare experiences a reaction to a vaccination.

The first series of vaccines recommended for pregnant mares is an inactivated virus vaccine to prevent equine herpesvirus (EHV-1) or rhinopneumonitis abortion. The vaccine series should be administered at 5, 7, and 9 months of pregnancy using a commercially available vaccine approved for use in pregnant mares to prevent EHV-1 abortion. In mares housed on farms with a recent history of EHV-1 abortion, veterinarians may suggest starting the vaccination series against EHV-1 at 3 months of gestation (i.e., vaccine to be administered at 3, 5, 7, and 9 months of gestation).  

Equine rotavirus is a highly contagious pathogen that may be transmitted from foal to foal. The virus causes significant damage to the lining of the small intestine and results in malabsorption, maldigestion, and profuse diarrhea. A vaccine is available to be administered to pregnant mares at 8, 9, and 10 months of gestation. Antibodies developed by the pregnant mare will be passed onto the foal via “passive transfer” when the foal ingests antibody-rich colostrum. Anti-rotavirus antibodies will reduce the incidence and/or severity of diarrhea in foals at risk of exposure to this pathogen.

Clostridium perfringens and Clostridium difficile are bacterial organisms that have potential to cause severe enterocolitis or inflammation of the small and large intestine in neonatal foals. Neonatal foals may ingest the Clostridium organisms from the local environment (foaling stall, pasture, etc.). Toxins produced by the organisms cause severe damage to the mucosa and villi lining the small and large intestines. Clinical signs of Clostridium enterotoxemia include severe abdominal pain or colic, foul smelling and sometimes bloody diarrhea, dehydration, depression, abdominal distention, and/or rapid death. Unfortunately, there are no vaccines against Clostridium perfringens organisms or their toxins labeled and approved for use in the horse. There is anecdotal evidence that immune protection for a newborn foal may be gained by vaccinating the pregnant mare 6 weeks and again at 3 weeks before the due date with a Clostridium C&D toxoid vaccine approved for other livestock.

Finally, pregnant mares should be vaccinated against tetanus, eastern and western equine encephalomyelitis (EEE and WEE; also called “sleeping sickness”), influenza, rabies, and West Nile virus 4 to 6 weeks prior to foaling. This springtime vaccine series will allow the mare to produce antibodies, which will be sequestered by the mammary gland and passed to the fetus in the colostrum. 

Additional vaccines may be administered to late-term pregnant mares depending on geographic location, potential for exposure, and medical risk.