The goal of vaccination is to develop and maintain both individual and herd immunity against infectious diseases. Commercial vaccines are available for rabies, encephalomyelitis (Eastern, Western, and Venezuelan), tetanus, influenza, equine herpesviruses 1 and 4, botulism, equine ehrlichiosis (Potomac horse fever), equine viral arteritis, rotavirus, West Nile virus, and Streptococcus equi (strangles). Vaccination programs are formulated based on the animal's age, use, and level of exposure. Broodmare vaccination is important to provide active immunity for the mare and passive immunity for the foal via transfer of colostral antibodies. Vaccination guidelines for foals have been modified because of the interference of maternal antibodies with the initial vaccination response. Sources such as the American Association of Equine Practitioners can provide the most current equine vaccination recommendations for horses in the USA.

The following vaccination recommendations assume that foals are born to vaccinated mares and have absorbed adequate colostral antibodies with IgG levels >800 mg/dL.

Foals with failure of passive antibody transfer (ie, IgG levels <200 mg/dL) and/or foals born to unvaccinated mares can receive their initial vaccination for equine herpesvirus 1 and 4, tetanus, and Eastern and Western equine encephalomyelitis beginning at 3–4 mo of age, followed by a second dose 4–6 wk later and a third dose at 10–12 mo of age. These foals can receive their first dose of rabies vaccine at 3–4 mo of age, followed by a booster at 12 mo. Influenza vaccination can be started at 6 mo of age. Foals born to mares that have never been exposed to or vaccinated against West Nile virus can receive their first vaccination at 3–5 mo of age.

Tetanus

Recommended for all foals and horses. Initial vaccination at ≥4−6 mo of age, with a second dose 4–6 wk after the first dose and a third dose at 10–12 mo of age followed by annual boosters. Broodmares should receive a booster 4–6 wk before foaling. A horse with an unknown vaccination status that sustains an injury should receive a dose of tetanus antitoxin along with a dose of tetanus toxoid. A second dose of toxoid should be given 4 wk later.

Equine Herpesvirus 1 and 4 (Rhinopneumonitis)

Recommended for all foals and horses. Initial vaccination begins at 4–6 mo of age followed by a second dose 4–6 wk after the first dose, and a third dose at 10–12 mo of age. Young horses are most susceptible and should be vaccinated at 6-mo intervals. Pregnant mares are vaccinated against equine herpesvirus 1 during months 3, 5, 7, and 9 of gestation and 4–6 wk before foaling.

Encephalomyelitis (Eastern and Western)

Recommended for all foals and horses. Initial vaccination at 4–6 mo of age (3–4 mo of age in highly endemic areas), with a second dose 4–6 wk later, followed by a third dose at 10–12 mo of age. Young horses and at-risk horses should receive semiannual boosters thereafter. Broodmares should receive a booster 4–6 wk before foaling. Other adult horses can be maintained on an annual revaccination schedule.

Influenza

Recommended for all foals, broodmares, and horses at risk of exposure, usually as a result of showing, racing, or shipping. Initial vaccination using the IM vaccine should begin at ≥6 mo of age followed by a second dose administered 4–6 wk later and a third dose at 10–12 mo of age. If the modified live, intranasal vaccine is used, a single dose can be administered at 6–7 mo of age followed by a second dose at 11–12 mo of age. Pregnant mares should receive an annual IM booster 4–6 wk before foaling. If the mare was not vaccinated during the last trimester of pregnancy, then the vaccination series for her foal can begin as early as 5 mo of age. Young performance horses should be vaccinated every 6 mo. Adult horses are usually vaccinated annually.

Rabies

Recommended for all foals and horses. Initial vaccination should begin at 6 mo of age, followed by a second dose 4–6 wk later, and then annual revaccination. Broodmares can receive a booster before breeding or 4–6 wk before foaling.

West Nile Virus

Vaccination of all foals and horses in the continental USA is recommended. Initial vaccination using the inactivated or recombinant canarypox vaccine can begin at 5–6 mo of age, followed by a second dose 4–6 wk later. A third dose should be administered at 10–12 mo of age, before the next vector season. If flavivirus chimera West Nile virus vaccine is used, the first dose should be administered at ≥5 mo of age and a second dose given at 10–12 mo of age, before the onset of the next vector season. Pregnant mares should receive a booster 4–6 wk before foaling. All adult horses should be revaccinated annually.

Potomac Horse Fever

Vaccination is suggested in areas where the disease is endemic. Initial vaccination can begin at 5–6 mo of age, followed by a second dose in 3–4 wk and a booster at 1 yr of age. Annual boosters in the spring are recommended. Pregnant mares should receive a booster before foaling.

Botulism

Vaccination is recommended for horses in the mid-Atlantic states and other regions of the USA where the disease is common. Initial vaccination involves a series of 3 doses administered at 4-wk intervals followed by annual boosters. Foals from vaccinated mares can begin their primary vaccination series at 2–3 mo of age. Broodmares that have never been vaccinated should receive an initial series of 3 doses administered at 4-wk intervals during the last trimester, followed by annual boosters administered 4–6 wk before foaling.

Strangles

Use of this vaccine is restricted to farms where strangles is endemic or risk of exposure is high. Initial immunization with the IM vaccine involves a 3-dose series administered 4 wk apart beginning at 4–6 mo of age. If the intranasal vaccine is used, vaccination can begin at 6–9 mo of age with a second dose given 3–4 wk later, and a third dose administered at 12 mo of age. Broodmares on endemic farms should receive an annual booster, using the IM vaccine, 4–6 wk before foaling. Due to the increased risk of inducing immune-mediated purpura hemorrhagica, horses with titers to the SeM surface protein of Streptococcus equi in excess of 1:1,600 may not need to be revaccinated.

Rotavirus

On farms where foal rotaviral diarrhea is a problem, pregnant mares should be given a 3-dose series at 3- to 4-wk intervals, during the last trimester of pregnancy. Foals obtain passive immunity through absorption of colostral antibodies.

Last full review/revision July 2011 by Wendy E. Vaala, VMD, DACVIM