This article is based on the presentation made by Dr. W. David Wilson at the recent 14th annual Horseman’s Day seminar at UC Davis in April. Dr. Wilson is a professor in the UC Davis School of Veterinary Medicine’s Department of Medicine and Epidemiology and an international authority on diseases and complications unique to foals.

In this era of modern veterinary medicine, vaccination provides a means to protect our horses from potential infection by serious diseases. Most experienced horsemen realize that it is much easier and less expensive to protect their animals through vaccination than it is to treat them once they become infected. For vaccination to be most effective, it must be administered correctly and at the proper time. Recent knowledge regarding the proper timing of vaccinations for foals and weanlings has changed our thinking regarding the best use of some of the most common vaccines.

When designing a vaccination schedule for these young animals, it is important to identify and understand your goals. Are you attempting to provide immediate protection from diseases which pose a serious threat to the foal early in its life, or are you trying to provide long-term protection against diseases which are apt to occur later in the animal’s life? If your goal is to protect the foal early on, there is not enough time to build a protective immunity by vaccinating the foal itself (active immunization). To provide protection to this newborn, you must vaccinate the mother who will then pass protective antibodies to the foal through her colostrum (first milk). This process is known as passive immunization.

In order to maximize passive protection of the foal, broodmares should be maintained on a regular vaccination program and should be given booster doses of vaccine four to six weeks before foaling. These booster doses increase the level of specific antibodies in the mare’s blood and allow the antibodies to be concentrated at an even higher level in the colostrum. During the first 24 hours of life, the intestine of the foal is lined by special cells that absorb the antibodies directly into the circulation without being digested. This process provides the foal with a level of protective antibodies similar to its dam. Therefore, it is extremely important that foals receive colostrum within the first few hours of birth, preferably by nursing or, in weak foals, from a bottle or stomach tube passed by a veterinarian.

The antibodies the foal absorbs from the colostrum are made of protein and are gradually consumed by the foal’s body. The concentration of these passively acquired antibodies in the blood diminishes by 50 percent every month. Thus, the higher the concentration of antibodies in the mare’s blood, the more the foal absorbs through nursing and the longer the antibodies persist at a protective level. These antibodies may provide protection from specific infectious diseases anywhere from a few weeks to several months. During this time, the foal constantly encounters infectious agents in the environment and starts to develop its own immune system. By the time the protection from passive antibodies has waned, the foal has developed sufficient immunity of its own to ward off specific disease agents.

When trying to develop active immunization through vaccination of the foal itself, there are several things one needs to understand. When a vaccine is developed, the agent which causes the disease is either killed or slightly modified so that it can not cause disease, but it retains the ability to stimulate the production of antibodies against the disease. In order for these modified agents in the vaccine (antigens) to stimulate that response, they must closely resemble the original infectious agents and that is where the trouble begins when vaccinating foals.

If the antibodies that were given to the foal by its dam are still present in the foal’s body, they may react to the antigens in the vaccine you administer as if they are the real disease agents and neutralize them. This effect is called maternal antibody interference. It is a well recognized complication in puppies and kittens, and has recently been identified as a significant problem in effectively protecting foals through vaccination.

This issue becomes even more complicated within a herd situation because the level of antibody acquired by each foal varies. Each mare provides a different amount of antibodies to her foal and each foal maintains these passive antibodies for different lengths of time. Therefore, the duration of passive protection and maternal antibody interference with vaccination varies. For instance, if all foals received their first vaccinations at three months, those foals that no longer have maternal antibodies would be protected whereas those foals with persistent maternal antibodies would not. The horse owner would be left with the false sense of security that all foals were protected when some were not. Booster vaccinating mares with the same vaccines at a consistent time (four to six weeks) before foaling can maximize the uniformity of passive transfer, but even this approach will not ensure that all foals respond to the same extent because of the variations in absorption and duration of maternal antibody protection.

Recent research has shed considerable light on the subject of maternal antibody interference and has necessitated a significant revision of the recommendations for vaccinating foals. It is now recognized that it is necessary to administer at least three doses of vaccine in the primary series for foals rather than the two recommended by vaccine manufacturers, regardless of the age at which foals are first vaccinated and regardless of the type of vaccine used. Also, with the exception of tetanus toxoid, most foals from vaccinated mares fail to produce appropriate antibodies to the common disease agents in vaccines when the vaccination program is started at three months of age, even when three doses of vaccine are administered in the primary series. Therefore, it is now recommended that no vaccine should be administered to the foal before four months of age and in the case of influenza, not before six months of age.

In the case of equine herpesvirus (EHV-1 and EHV-4), and with equine encephalomyelitis (WEE and EEE), it does not appear that the maternal antibodies in the foal’s blood negatively impact the immune response to subsequent doses of vaccine administered later in life. Because foals are frequently exposed to the herpesvirus during the first six to 12 months of life, it is advisable to begin vaccination at less than six months of age, but three or more doses should be included in the primary series. In the case of equine encephalomyelitis, foals are less likely to encounter this disease except in the Southeastern United States. If there is a high risk of infection, it is advisable to begin vaccination at four months of age but again, three or more doses should be included in the primary series. If the risk of infection with WEE and EEE is low, then begin vaccinating (three doses) at six months of age.

In contrast, the influenza vaccine is definitely affected by the maternal antibodies that remain in the blood until the foals are at least six months of age. The administration of influenza vaccines at an earlier age may adversely affect the production of appropriate antibodies, both immediately and in response to vaccination later in life. This can result in the animal being unable to obtain a vaccinated immunity as an adult. Therefore, no influenza vaccine should be administered to a foal before it is six months old.

As outlined earlier, decisions regarding the optimal age to start the primary vaccination series for different diseases can be simplified by considering whether the primary concern is the immediate prevention of disease in the foal, or immunizing the foal to prevent disease later in life. For further information on specific diseases, see our vaccination recommendations.

 Chart I

Rotavirus — (for horses on farms with a history of rotavirus infection)

a.) Vaccinate dam during  8th, 9th, and 10th month of gestation.

b.) Repeat three doses each pregnancy.

c.) Foal vaccination is not indicated.

 Botulism type B (shaker foal syndrome) — (on individual farms or in geographic areas with a history of this disease)

a.) Vaccinate dam during 8th, 9th, and 10th month of gestation.

b.) Vaccinate dam four to six weeks before foaling in successive pregnancies.

c.) Vaccinate weanlings at four months of age or older if risk of infection continues.

 

Equine herpesvirus type 1 (EHV-1) —

Vaccinate mares during 5th, 7th, and 9th month of every pregnancy with approved inactivated vaccine.

 

EHV-4/EHV-1, tetanus (and strangles on problem farms) —

a.) Vaccinate dam four to six weeks before foaling.

b.) Start foal vaccination at four to six months of age.

c.) Administer at least three doses in the primary series.

d.) Booster vaccinate at three to four month intervals (EHV-1 and EHV-4), six month intervals (strangles), or at 12-month intervals (tetanus) thereafter.

 

Influenza —

a.) Vaccinate dam four to six weeks before foaling.

b.) Start foal vaccination at six to nine months of age with three or more doses.

c.) Booster vaccinate young horses at three to four month intervals until two years old if risk of infection remains high.

d.) Maintain all horses on the farm on a program of boosters at four to six month intervals.

e.) Mature performance, show and pleasure horses constantly exposed to potential infection should be booster vaccinated at four month intervals.

Western and Eastern equine encephalomyelitis (WEE and EEE) and Venezuelan equine encephalomyelitis (VEE) in problem areas

a.) Vaccinate dam four to six weeks before foaling.

b.) Start foal vaccination at six months of age or in the spring of the yearling year (three doses) and booster vaccinate annually thereafter.

c.) In Southeastern US, where the risk of EEE infection is high, start foals at four months (three or more doses).

 Rabies —

a.) Vaccinate dam before breeding.

b.) Start vaccinating foals at six months of age or older (two doses).

c.) Booster vaccinate as yearlings and then annually thereafter.

 

Equine Viral Arteritis (EVA) —

Before vaccinating for EVA, it is advisable to evaluate serum titers from blood samples for EVA. It is important to establish negative titers before vaccinating for EVA in regards to the shipment of horses abroad. Vaccination is only recommended for horses on breeding farms with a high risk of infection and for mares being bred naturally or by artificial insemination (A.I.) to carrier stallions. Vaccination of pre-pubertal weanling colts with modified live vaccine is effective in preventing the establishment of a carrier state later in life.

a.)   Vaccinate dam before breeding using modified live vaccine.

b.) Start foal vaccination at six to 10 months (one dose) and give booster doses annually thereafter.

 

Chart II

 Sample vaccination schedule for breeding farms in the Western US**

a.) Booster mares in endemic areas against rabies (and EVA is risk exists) before breeding.

b.) Booster mares against tetanus, WEE, EEE, influenza, EHV-1/4, *strangles, *botulism, and *rotavirus four to six weeks before foaling.

c.) Vaccinate foals as follows:

                 4 months - EHV-1/4, *strangles

                 5 months - EHV-1/4, *strangles

                 6 months - tetanus, WEE, EEE, *rabies

                 7 months - tetanus, WEE, EEE, EHV-1/4, *strangles

                 8 months - *rabies, *EVA

                 9 months - influenza, tetanus, WEE, EEE

                10 months - influenza, EHV-1/4

                1 year - influenza, *strangles

Thereafter:

                                - tetanus, WEE, EEE and *rabies boosters annually

                                - influenza and EHV-1/4 boosters at four month intervals

                                - *strangles boosters at six month intervals

* Diseases listed with an asterisk are included in the program only when conditions of significant risk prevail or when indicated for other reasons. Consult your attending veterinarian for specific recommendations in your area.

 ** Additional vaccines such as botulism and rotavirus can be added to the program in high risk areas.

The heat wave that swept across the U.S. this week broke temperature records and strained power grids as people cranked up the air conditioning in an attempt to keep cool. Horses also suffered in the extreme temperatures, prompting a shut-down at several tracks and stopping carriage rides in New York's Central Park.