Foals, just as infants, are vulnerable to disease and infection because their young bodies are naive to the world of germs and bacteria. The inclination, particularly for horse owners who choose to vaccinate their own horses, is to administer vaccines as early as possible or as recommended by over-the-counter products. New research, however, seems to indicate that doing so might be a moot point in the very young.

Currently, the majority of vaccines offers only limited protection for the highly susceptible equine neonate. Attention should be given to decreasing exposure to pathogens--those organisms that can cause disease--through isolation and sanitation, as well as by eliminating stressors that reduce the foal's resistance.

Factors that affect a foal's immunization program include the management situation, geographic location, and risk versus cost benefits for the owner. Although it is impossible to make universal recommendations for the vaccination of foals, a rational plan can be developed if an owner understands the key decision points in passive and active immunization regimens.

Nature's Vaccination

Passive immunity is best understood as the immunity transferred to the foal by its mother through the colostrum the foal ingests as it first suckles. It is critical that the foal receive colostrum--which is loaded with maternal antibodies--within the first few hours of life.

All mares should receive booster vaccinations four to six weeks before giving birth using only killed (inactivated) agents. Typically, this includes vaccinations for tetanus, encephalomyelitis viruses, influenza, and rhinopneumonitis, with additional vaccines for Streptococcus equi, Potomac horse fever, and in some circumstances, botulism. In previously unvaccinated mares, an initial vaccination course should be administered with the last booster given four to six weeks before foaling.

It also is important to remember that the value of colostral transfer of passive immunity can be considerably increased if the mare is housed on the farm where she is going to foal for six to eight weeks before foaling. One general rule is that modified live vaccines (MLVs) are not given during pregnancy. Remember that an MLV will induce some type of infection in order to achieve immunization.

Passive immunization also is achieved by the oral administration of immunoglobin-containing products to foals in the first hours of life or by injection at any time. However, this policy is unnecessary if the mare is appropriately vaccinated during pregnancy.

For those foals which do not receive immunity through passive transfer, the administration of plasma transfusions is a common procedure and offers an opportunity to influence resistance to specific pathogens through the choice of product. Commercial equine plasma products that have been prepared from donors extensively vaccinated against common equine pathogens should be chosen.


A contentious issue in foal vaccination is the timing of the initial series of vaccinations. The problem largely results from the effects of maternal antibodies received through the colostrum and their variable half life. It is difficult to time these initial vaccinations in the foal so that they are effective and administered early enough so the young animal is protected after the waning of maternal antibodies. An additional consideration is the increasing perception that foals might be relatively immunologically unresponsive to many of the currently available vaccines.

The first step in determining when to start a foal vaccination regimen is knowing the duration of maternally derived antibodies. The rate of decline of maternal antibodies varies for individual foals. The protection given the foals from the maternal antibodies also can vary because of the infectious agent that caused the antibodies to be formed.

For many important pathogens, the concentration of maternal antibodies falls to non-protective levels in foals by two to three months of age. However, the remaining antibodies, which one could view as residual, can render the foal unresponsive to vaccination for weeks or even months to come.

In the case of equine influenza, maternal antibodies can persist until the foal is six months of age and prevent immune responses in foals younger than six months. Because of this, AAEP recommends that you begin foal vaccinations at three to four months of age, followed by boosters at four-week intervals. This is adequate for many foals, but a significant number are in a high-risk situation and might remain vulnerable to infection. A more intensive vaccination schedule would include an initial vaccination at two months of age and monthly boosters until six months of age, with further boosters at nine and 12 months of age. In the case of tetanus and rabies, an initial vaccination at three to four months and a booster four weeks later should be adequate.

Even when intense vaccination regimens are used in young foals, poor responses can occur. This could be a result of a relative lack of immune responsiveness to currently available vaccines in young foals. An alternative proposal is that the frequent use of vaccines in the face of persistent maternal immunity might induce a state of tolerance, which could prevent a satisfactory response to vaccines past one year of age.

What To Use

In making choices among products, one must consider the antigen contained in the vaccine and the means of delivery. For example, in the case of equine influenza vaccines, it is important to look for an equine influenza type 2 strain with a date from the late 1980s or ideally the 1990s. This will increase the likelihood that the vaccine will protect against currently circulating strains of influenza virus. Similarly, in the case of equine herpes virus vaccines, it is important to include both EHV-1 and EHV-4 antigens for protection against abortion and respiratory disease.

Most importantly, however, horse owners need to be aware that immunities develop based on exposure and protection. With the help of your veterinarian, you should be able to determine the appropriate vaccination schedule best for your horses, both young and old.

About the Author

Paul Lunn, BVSc, PhD, MRCVS, Dipl. ACVIM

D. Paul Lunn, BVSc, PhD, MRCVS, Dipl. ACVIM, is dean of the College of Veterinary Medicine at North Carolina State University, taking up that position in February 2012 after serving as a professor and head of the Department of Clinical Sciences at Colorado State University's College of Veterinary Medicine and Biomedical Sciences. Before moving to Colorado in 2003, Lunn worked as an equine medicine faculty member, and teaching hospital director at the School of Veterinary Medicine, University of Wisconsin-Madison. Lunn hails from Wales, where he grew up in a farming community. More recently his interests have been in equine immunology and infectious disease, and he continues to run a research program focusing on influenza virus and EHV-1 infection in horses. Outside of work, he and his wife Kathy enjoy hiking and raising terriers. Lunn also fly fishes and skis inexpertly but enthusiastically.

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