Vaccinating foals is important, but when should you do it and what should you give?

Foals, just as infants, are vulnerable to disease and infection because their young bodies are naive to the world of germs and bacteria which can cause those problems. So 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 may be a moot point in the very young.

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

Factors that effect a foal 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 the key decision points in passive or active immunization regimes are understood.

Nature's vaccination

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

The value of passive immunity can be considerably influenced by vaccination of the mare in order to maximize the concentration of the antibodies present in the colostrum. 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 is also important to remember that the value of colostral transfer of passive immunity can also be considerably increased if the mare is housed on the farm where she is going to foal for six to eight weeks before foaling. This allows adequate time for the generation of immune responses to pathogens present on the farm and subsequent transfer of these antibodies into the colostrum. One general rule is that modified live vaccines (MLVs) are not given during pregnancy. Remember that an MLV will induce some type of infection itself in order to achieve immunization.

Passive immunization is also achieved by the oral administration of immunoglobin-containing products to foals in the first hours of life or by injection at any time. It is relatively common practice to administer tetanus anti-toxin to neonatal foals. However, this provides relatively short-lived protection and carries the risk of inducing serum sickness in the foal. This policy is unnecessary if the mare is appropriately vaccinated during pregnancy.

For those foals who 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 should be chosen that have been prepared from donors extensively vaccinated against common euqine pathogens.

Timing

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 it is effective and administered early enough so as not to leave the young animal unprotected after the waning of maternal antibodies. An additional consideration is the increasing perception that foals may be relatively immunologically unresponsive to many of the currently available vaccines.

The first step in determining when to start a foal vaccination regime is knowing the duration of maternally derived antibodies. The rate of decline of maternal antibodies varies for both individuals and different infectious agents. For many important pathogens, the concentration of maternal antibodies in foals falls to nonprotective levels by two to three months of age. However, the remaining antibody, which one could view as residual, can still render the foal unresponsive to vaccination for weeks or even months to come.

Because, in the case of equine influenza, maternal antibodies can persist until six months of age and prevent immune responses in foals younger than six months, AAEP recommends beginning 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 may 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. In the case of tetanus and rabies, an initial vaccination at three to four months and a booster for weeks later should be adequate.

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

What to use

Currently available vaccines include many highly effective products that are safe, provide long-term immunity and are practical to use. In making choices between products, one must consider both 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 both 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.

Permission has been granted by AAEP for this one-time reprint.