By D. Craig Barnett, DVM

As with adult horses, vaccination of foals is an important component of an overall infectious disease control program.  Over the past few years, foal vaccination has been a contentious issue, with a considerable amount of confusion as to when to start vaccinating foals and how many doses they should receive.  Recent research in this area has lead to the development of new foal vaccination recommendations from the American Association of Equine Practitioners.

Nature’s Vaccination (Passive Immunity)

Passive immunity is the immunity transferred to the foal by its mother through the colostrum (mare’s first milk).  Because the mare has a thick, multi-layered placenta, there is no transfer of blood or antibodies from the mare to the foal when the foal is in the womb.  Consequently, the foal is born without the benefit of protective antibodies to help protect the foal against disease.  Therefore, the foal depends upon the ingestion of antibody-rich colostrum to protect it from harmful organisms the first few months of its life.  It is critical that the foal nurse and receive this antibody-rich colostrum within the first few hours of life.  It is common practice to vaccinate mares 4 to 6 weeks prior to the anticipated foaling date.  This prefoaling vaccination stimulates the mare’s immune system to produce more antibodies in her bloodstream that are then concentrated in the colostrums and thus enhances protection to the foal.  Typically, the prefoaling vaccinations given to the mare include vaccinations for influenza, rhinopneumonitis (equine herpesvirus), eastern and western encephalomyelitis viruses (EEE & WEE) and tetanus. Other vaccinations such as rabies, strangles, potomac horse fever, botulism, rotavirus or West Nile virus may be included as endemic conditions may dictate.  For practical purposes it is important to remember that the first and most important vaccination that a foal receives is the passive immunization delivered from the colostrum.  Therefore, it is important to booster vaccinate mares 4 to 6 weeks prior to foaling.

Vaccination (Active Immunity)

Active immunity is the immunity the foal develops following vaccination where the foal’s immune system actively develops its own antibodies. For years it has been common practice to start vaccinating foals at around 3 to 4 months of age.  However, recent research indicates that vaccines given this early may elicit minimal or no response in most foals.  Why?  As indicated earlier, ingestion and absorption of maternal antibodies from colostrum is important for protection of foals against specific infectious diseases early in life. Unfortunately, these maternal antibodies may also inactivate many of the vaccines given to foals during the first few months of life.  This effect is termed “maternal antibody interference” or “maternal antibody block”.  In foals from vaccinated mares, it is now well documented that maternal antibodies will block the foal from adequately responding to vaccination until the foal is around 6 months of age.  By 6 months of age, the levels of maternal antibodies have usually declined to a level that will not interfere with the foal responding to vaccination.  Therefore, foals born to vaccinated mares should not be vaccinated against most diseases until around 6 months of age.  Although information is limited, it is suggested that foals from unvaccinated mares should be vaccinated earlier, generally around 3 to 4 months of age.

Another important finding in recent research is that foals receiving the initial two-dose vaccination series recommended by most manufacturers generally do not develop an adequate response to vaccination and should instead receive a primary series of three doses of vaccine at 4 to 6 week intervals.

A sample vaccination schedule for foals born to previously vaccinated and unvaccinated mares is presented in the table below.  However, you should consult with your veterinarian regarding the specific needs of your horses.  A strong emphasis should be placed on the fact that disease control programs need to be tailored to your individual needs with consideration given to ages, types, activities and number of horses in your program as well as your geographic location.  Decisions regarding the optimal age to start the primary vaccination series in the foal should not be recommended without an understanding of the risk, incidence, severity or consequence of disease in the foal.  For example, a vaccination schedule for foals on a large breeding farm with incoming and outgoing horses and significant risk of disease may be totally different that one developed for the horse owner with a single mare and foal on an isolated farm.

Vaccinations for other diseases such as such as rabies, strangles, potomac horse fever, botulism, rotavirus or West Nile virus may be included as endemic conditions may dictate.  Consult with your veterinarian for use in your area.

Other Considerations

It is important to remember that vaccination is only one component of an infectious disease control program. A good program should address; reducing the potential for exposure to infectious agents (viruses and bacteria), minimizing factors that reduce resistance (i.e., reduce stress factors) and finally, enhanced disease resistance through vaccination.

As a horse owner, it is important to recognize that there is a critical time in the foals life when the maternal antibodies are declining and may be below levels required to protect the foal against disease, yet there are still enough maternal antibodies present to block the response to vaccination.  Consequently, the foal may be susceptible to infection before the primary vaccination series can be completed.  For most foals and most infectious diseases this critical time is generally between three and six months of age.  During this time it is very important to reduce your foals potential for exposure to infectious agents by avoiding the foals exposure to sick horses, outside horses or new horses on the premises.  During this time is it is also important to minimize stress factors (hauling, weaning, etc) that may reduce the foal’s resistance to disease.

We should also recognize that due to individual animal variation, not all foals that are vaccinated will respond adequately or equally.  In fact, in a general population of horses it is estimated that approximately 10% of the group will not respond adequately to vaccination.  Due to this variability in response to vaccination, it would seem logical to utilize a whole herd vaccination schedule in foals, as well as in the adult horse population, to minimize replication of infectious agents and to help protect those foals that do not respond to vaccination.  If all animals on the premises are current on vaccinations, you are less likely to have an individual animal that may be shedding virus or bacteria.  It would also seem logical and worthwhile to implement managerial practices to reduce infectious disease risk factors (isolate all incoming horses, pasture or house mares and foals away from other horses, especially show horses, etc.)  These managerial practices will help minimize risk of infectious disease to all foals and especially those foals that do not respond to vaccination.  Consult your veterinarian regarding any questions you may have regarding risk of disease and vaccination recommendations for you situation.