Crossing Boundaries

Vaccination of broodmares is just one part of a complete disease prevention program, but its importance should not be underestimated. Immunization is one of the least expensive, and most effective, strategies horse breeders can use to protect broodmares and their foals from disease, and even death.

"When you vaccinate broodmares, you're not just protecting the mares, you're protecting their foals as well," says Tulio Prado, DVM, MS, Dipl. ACT, professor of theriogenology at the University of Tennessee's College of Veterinary Medicine. Foal protection comes in the form of passive immunity foals acquire as they absorb antibodies from the mare's colostrum. "The antibodies produced by the mare don't cross the placenta during pregnancy," Prado explains. "They are transferred from mother to foal only after birth through the colostrum, in the first 12 to 24 hours of life. This is why it is essential to maintain current vaccination of broodmares."

"Have a veterinarian check the foal's blood at 12-24 hours of age to make sure he got the colostrum and its valuable antibodies," urges Robert Holland, DVM, PhD, senior technical services veterinarian at Pfizer

Animal Health. "It's some of the best money you can spend on your foal's health. If he didn't get the colostrum, none of the vaccines matter to the foal."

According to Laurel Gershwin, DVM, PhD, professor of immunology at the University of California, Davis, School of Veterinary Medicine, "One of the most important things to remember about vaccinating pregnant animals is that it is necessary to use the inactivated--or killed--form of the vaccine. If you give a live vaccine to pregnant mares, there's a risk of abortion or fetal abnormality."

(Editor's note: There is a modified live EHV vaccine labeled for use in pregnant mares.)

Since there isn't a one-size-fits-all vaccination protocol for every broodmare or breeding operation, owners should work closely with veterinarians to design a plan that fits the needs of individual mares, as well as the entire herd. Factors to consider are the age and health of each horse, the geographic location of the farm, and any unique circumstances that heighten horses' risks of getting various diseases, including exposure to outside horses or to wildlife.

"You have to think about all the diseases that you need to vaccinate against," says Gershwin, "and consider whether or not each vaccine is really necessary."

According to Judy Marteniuk, DVM, MS, equine medicine/extension veterinarian at Michigan State University's College of Veterinary Medicine, "What you vaccinate broodmares against depends on which diseases are prevalent in your area. On the other hand, some vaccines are essential for all horses, especially for diseases that are mosquito-borne, because they exist in the environment. Every horse in the U.S. will be exposed to these diseases, even if it's a solitary horse on an isolated farm."

Eastern Equine Encephalitis

One of the deadliest mosquito-borne diseases is Eastern equine encephalit-is (EEE), a viral illness that attacks the cen-tral nervous system of its host and is al-most always fatal. Although the virus is most prevalent in the southeastern United States, it is found near wetland habitats along the eastern seaboard from New England to Florida.

Closely related to EEE are Western equine encephalitis (WEE) and Venezuelan equine encephalitis (VEE). WEE occurs mainly in the plains regions of the western and central United States, while VEE is endemic in South and Central America. For protection against EEE and WEE, horses require an initial two-shot series with inactivated vaccine, administered two to four weeks apart, followed by an annual or semi-annual booster.

"In endemic areas, semi-annual boosters are recommended," says Marteniuk. "In non-endemic areas, mares may require only one booster, given four to six weeks prior to foaling, if their foaling dates coincide with the onset of mosquito season. For mares that will foal earlier in the year, two boosters may be necessary. One is given four to six weeks prior to foaling for the protection of the foal, while the other is given four to six weeks prior to the onset of mosquito season to protect the mare."

"Vaccination for EEE and WEE is recommended for all broodmares," says Gershwin, "but in the absence of an outbreak, the VEE vaccine isn't routinely given to horses in the U.S. Vaccination against VEE should be considered if there is a threat of exposure with international travel."

VEE vaccination is sometimes recommended by veterinarians in U.S. southern border states and Mexico.

West Nile Virus

Like EEE and WEE, West Nile virus (WNV) is spread by infected mosquitoes, and it can cause encephalitis and death. Al-though most horses exposed to WNV don't get sick, the disease is fatal in a third of infected horses that develop neurologic signs. For most breeders, odds like these make vac-cination against WNV worthwhile. After the initial two shots given three to six weeks apart, semi-annual revaccination with the inactivated vaccine is recommended in warm, wet climates where mosquitoes live year-round. In non-endemic areas, mares may require only one annual booster, as long as it can be administered four to six weeks prior to the onset of both the mosquito season and the foaling date.

"The West Nile virus vaccine is usually given twice yearly," says Gershwin. "Administration of the vaccine should be avoided during the first 60 days of gestation, if possible. The first vaccine is given to mares before they're bred, and the booster is given four to six weeks before foaling. This way, the mares are protected, and the foals are protected through the colostrum."


Tetanus is a non-contagious disease caused by Clostridium tetani. Its spores, which are present in soil worldwide, can contaminate wounds and surgical incisions, and the umbilici of foals. In the body, Clostridium tetani produces a powerful neurotoxin that blocks transmission of nerve impulses, resulting in a type of muscle spasm known as tetany. If untreated, infection is usually fatal.

"Every broodmare should receive an inactivated tetanus toxoid booster annually four to six weeks prior to foaling, to ensure colostral protection for her foal," recommends Gershwin.

Rhinopneumonitis (Equine Herpesvirus)

Equine herpesvirus (EHV) is an organism transmitted from horse to horse via body fluids. Infection with EHV causes rhi-nopneumonitis, which is a highly contagious disease that can lead to respiratory infection, abortion, paralysis, and even death. Although there are several strains of EHV, EHV-1 is the strain responsible for causing equine abortion, generally in the last three months of gestation.

Because the EHV ("rhino") vaccines offer short-term immunity, boosters every two to three months are recommended. "As the old saying goes, herpes is forever, so giving the rhinopneumonitis vaccine is very important for all broodmares," says Marteniuk. "At the very least, you should give it according to the manufacturer's rec-ommendation, at five, seven, and nine months of gestation. On farms that have had several abortions, I recommend giving the vaccine at three, five, seven, and nine months."

Up to 90% of horses carry equine herpesvirus by the time they are two years old--some without showing clinical signs (latent carriers), writes George Allen, PhD, of the Gluck Equine Research Center in Equine Respiratory Diseases. Stress can reactivate infection, causing clinical signs. Vaccination of carriers helps boost the immune response to keep the organism under control, since it currently can't be eradicated.

According to Gershwin, "The vaccine is available as a combination of EHV-1 and EHV-4. When you give the mare her annual booster before breeding, use a combination of EHV-1 and EHV-4. During gestation, you should give the inactivated EHV-1 vaccine alone."

Atwood Asbury, DVM, Dipl. ACT, professor emeritus at the University of Florida and a retired dean of that veterinary school, says, "Many vet-erinarians give a combination vaccine with both EHV-1 and -4 to mares four to six weeks before foaling to make sure the foal is protected against both forms via colostrum. Discuss options with your veterinarian."

The AAEP guidelines also state: "Vaccines containing EHV-1 appear to offer some protection against EHV-4."

Vaccines for At-Risk Mares

For some mares, vaccination against certain diseases might be added to the core requirements, depending on the environment of that horse and the herd. Discuss these options with your veterinarian.

Rabies--"Whether or not to give your mares the rabies vaccine depends on where you live," says Gershwin. "If you live in an area where rabies is endemic, and there is a significant risk of exposure to wildlife vectors, you should definitely give the vaccine. If you live in an area where rabies is rare, you can probably skip this one."

Rabies is caused by a rhabdovirus that affects the nervous system, and since there is no effective treatment, the disease is considered to be 100% fatal. Unvaccinated horses can contract the disease from the bites of infected animals. Although skunks, foxes, raccoons, and bats are most likely to transmit the virus, the bite of an infected dog or cat can also cause infection. According to Gershwin, "If you're going to vaccinate for rabies, you should do it annually, before you breed the mare, because you're going to be giving her enough other vaccines during the pregnancy."

Vaccination guidelines endorsed by the American Association of Equine Practitioners state: "None of the licensed (rabies) vaccines are labeled for administration to pregnant mares...Some veterinarians administer the killed (rabies) vaccine to pregnant mares without reports of adversity." (More information: See page 18.)

Equine influenza--This viral respiratory disease most often affects young horses between one and five years old. "Influenza immunization is always a good idea if broodmares will be exposed to other horses," says Marteniuk. "It should be a core vaccine for mares having contact with other horses at shows, events, and breeding farms."

While vaccination doesn't guarantee com-plete protection against influenza, the disease in vaccinated horses is less severe than in non-immunized horses. Broodmares should get an annual vaccine be-fore breeding, as well as a pre-parturition booster.

"You can give the intranasal modified live virus annually before breeding," says Marteniuk, "but the booster should be an inactivated, injectable influenza vaccine, given four to six weeks prior to the birth of the foal."

Strangles--This highly contagious disease is caused by the bacterium Streptococcus equi. Infection most commonly

occurs in groups of hors-es younger than five years of age, especially weanlings and yearlings. It is characterized by severe inflammation of the mucosa of the head and throat that can lead to rupture of  lymph nodes under the jaw and death by asphyxiation. Intramuscular vaccines contain a protein from the strangles-causing organism and are considered safe for gestating mares. A modified live intranasal vaccine is also available.

"If you have a breeding farm that has had a lot of strangles, it is important to vac--cinate the brood string to ensure that foals get colostral protection," says Gershwin. "If broodmares will come in contact with outside horses, they should be vaccinated prior to breeding, with a booster vaccine administered four to six weeks before foaling."

According to Marteniuk, "The strangles vaccine is quite controversial. If the protection is intended for the foal, an inactivated intramuscular vaccine should be given to the broodmare four to six weeks prior to foaling. If the protection is for the mare only, you can give her the intranasal live vaccine before she's bred."

Special Circumstances Vaccines

Some vaccines are given to broodmares only when dictated by special circumstances. Specific management practices, environmental risks, and your veterinarian's recommendations will help you decide if these vaccines are necessary.

Equine viral arteritis (EVA)--This disease causes inflammation of blood vessels that can lead to hive-like reactions, limb swelling, and abortion. It can become established as a carrier state, with shedding of virus via semen from some in-fected stallions. According to Marteniuk, "If you're getting a mare ready to be bred to an EVA-infected stallion, give her the EVA vaccine about a month or so beforehand. If you don't, she won't settle, because the infection typically causes early embryonic death."

Botulism--This neuroparalytic disease is caused by Clostridium botulinum, an organism found in decaying plant material. Hay and hay silage can be contaminated with the bacteria during raking and baling. Infection of adult horses typically occurs when pre-formed toxins are eaten, leading to a paralysis that affects the head, limbs, and tail. The inactivated vaccine should be administered to broodmares that are fed hay silage, and to those that live in endemic areas. Mares should receive an initial three-dose series at 30-day intervals, with the last dose given four to six weeks before foaling. Thereafter, the vaccine is given annually, four to six weeks before parturition.

"Typically, you're giving the botulism vaccine to broodmares to protect the foals from shaker foal syndrome," says Gershwin. This syndrome, which usually strikes foals between the ages of two weeks and eight months, is caused by a gastrointestinal infection with Clostridium botulinum.

Potomac horse fever (PHF)--Caused by the bacterium Neorickettsia risticii, horses get PHF through accidental in-gestion of infected aquatic insects, including mayflies and caddis flies. The disease causes diarrhea, colic, and occasionally abortion. The development of severe dehydration, electrolyte imbalances, or laminitis can lead to death or might necessitate euthanasia.

"Vaccination may be considered in areas where the disease is endemic, but I don't recommend this vaccine, since it doesn't offer complete protection," says Marteniuk. While at least six strains of PHF are thought to exist, the vaccine is made from a single strain. Vaccination in at-risk areas consists of an initial two-shot series, followed by an annual booster.

Rotavirus--"Vaccination of mares for rotavirus should be considered on farms where risk of infection is high," says Marteniuk. "Passive transfer of colostral antibod-ies helps prevent rotaviral diarrhea in foals.

"Rotavirus is a vaccine you'll want to give your mares if they're going to be exposed to other horses, or if they live on endemic farms," she adds. "To protect the foals, the rotavirus vaccine is given to broodmares at eight, nine, and 10 months of gestation."

Your Veterinarian: The Key to Disease Management

Although a thorough understanding of immunization principles and practices is important for all horse owners, a veterinarian's input is critical to the success of the vaccination program.

"Your local veterinarian knows which diseases are prevalent in your area, and which vaccines are necessary for your broodmares," says Prado.

About the Author

Rallie McAllister, MD

Rallie McAllister, MD, grew up on a horse farm in Tennessee, and has raised and trained horses all of her life. She now lives in Lexington, Ky., on a horse farm with her husband and three sons. In addition to her practice of emergency and corporate medicine, she is a syndicated columnist (Your Health by Dr. Rallie McAllister), and the author of four health-realted books, including Riding For Life, published by Eclipse Press and available at or by calling 800/582-5604.

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