The Best Defense: Maximize Your Foal's Immunity

Steps breeders can take to maximize foal immunity from gestation to weaning

Welcome to the world, baby! It’s full of people, other horses, animals, big buildings, tall trees, fences, rocks, holes … and trillions of microorganisms. You’re born programmed to confront this physical world almost immediately, with long sturdy legs that can carry you alongside your mother within hours. Your immune system, however, is not.

Newborn foals are easy targets for essentially every kind of bacterium, virus, and other pathogenic (disease-causing) microorganism. That’s because, unlike humans, they’re born with a “clean-slate” immune system, meaning their bodies aren’t equipped yet with disease-fighting antibodies (specialized proteins produced by immune system cells in response to the presence of foreign material, capable of binding to the material and alerting immune cells about its presence). It’s up to us—along with the foal and his dam—to make sure he gets off to the right start from the earliest hours and has the best shot at lifelong health.

Born Vulnerable

Unlike women, mares have six layers of tissue separating their circulation from that of the fetus (women have three). This makes it impossible for the mare’s antibodies to get through, says David W. Horohov, PhD, professor and director of the University of Kentucky Maxwell H. Gluck Research Center, in Lexington.

That doesn’t mean neonates don’t have an immune system, though. They do; it just hasn’t had the chance to build resistance against disease. “While foals are born with an intact and functional immune system, they are naive in terms of having prior exposure to the various pathogens and other microorganisms they will encounter,” Horohov says.

What they do have at birth are low amounts of antibodies that are not yet optimized for pathogens, says Bettina Wagner, DVM, PhD, professor and department chair at the College of Veterinary Medicine at Cornell University, in Ithaca, New York. 

“Foals are just ‘warming up,’ getting their immune system and their immune cells ready to respond without actually having antibodies for a specific pathogen,” she says. “So far nobody knows what these early antibodies are directed against.”

Colostrum: The Super-Immunity Drink

Mares produce several hundred milliliters of colostrum before they start to produce milk, says Wagner. Rich in nutrients, colostrum fills a newborn foal’s digestive system with antibodies that represent the dam’s “entire lifetime of exposure to pathogens,” she says. That includes antibodies against microorganisms specific to that mare’s environment—whatever might be circulating on her farm, as well as those against pathogens the mare has confronted or been vaccinated against in the past. Colostrum also contains cytokines—proteins that work like signals for the immune system. “The cytokines stimulate the foal’s immune system to get started,” Wagner says.

While a mare’s colostral antibodies reflect everything her immune system’s ever fought, some of these disease-fighters become less active over time, says Wagner. That’s why it’s essential to vaccinate the mare during her pregnancy to restimulate her immune response to pathogens such as equine herpesvirus (EHV), influenza virus, Clostridium tetani, and more. 

“Since the foal relies on the mare for its initial antibody response, proper vaccination of the mare is an important step in providing the foal with the necessary antibodies,” Horohov says.

A Closing Gut & A Ticking Clock

The tricky thing about swallowing an entire life’s repertoire of antibodies is getting them where they belong—in the bloodstream. If they stay in the digestive tract, they’re not much use.

Fortunately, the newborn equine gut is “open” to absorb these antibodies and get them to their destinations in the body. There’s a caveat, though: It’s only open for about the first 18 hours of life. Then, the gut closes gradually over the next six hours, says Ibrahim Elsohaby, PhD, postdoctoral researcher at the University of Prince Edward Island, in Charlottetown, Canada, and the Faculty of Veterinary Medicine at Zagazig University in Egypt.

At 24 hours after birth, the gut is completely closed, and no more antibodies can get through the intestinal walls into the bloodstream. “By then, they’re just good protein,” Wagner says. “But they won’t do anything for the immune system.”

Signs of Failure

Sometimes it’s obvious that failure of transfer of passive immunity (FTPI)—not getting enough antibodies through ­colostrum—has or will happen. The mare died; the mare and foal were separated; the foal didn’t nurse within the first hours.

But sometimes it’s less obvious, says Wagner. “It can depend on the quantity of the colostrum or the quality,” she says. “Maybe the mare didn’t make enough for some reason or the foal wasn’t efficient at suckling. Or, possibly, the quality was poor because of a low rate of antibodies present in the colostrum.”

A baby that hasn’t gotten enough high-quality colostrum will start to get sick, usually within two to five days, she says. “The foal won’t be as active, is not moving around much, won’t seem alert, and might stop suckling and often develops a fever.”

Testing for Transfer

If, for any reason, you doubt your foal got enough antibodies through colostrum, you can—and should—run diagnostic testing. “Confirming that adequate passive transfer has occurred and, if not, replacing it with another source, is critical,” says Horohov.

Most commonly, the veterinarian will take a sample of the foal’s blood and test it for sufficient antibody transfer. This is the most important test and is often performed routinely in healthy foals around 48 hours after birth.

But if earlier on you’re concerned about the quality of the mare’s colostrum, you can have it tested, as well. Collect it before the foal suckles or within the first two hours after birth while it’s still at its highest quality, says Wagner. “Usually, it has high antibody amounts in the range of several hundred milligrams per milliliter,” she says.

New digital and optical refractometry tests on blood serum can detect FTPI within seconds, however, says Elsohaby.

Of course, that still requires sending the sample to a laboratory to separate serum from the rest of the blood using a centrifuge. But breeders might be able to perform the testing themselves if they allow the whole blood sample to sit for a couple of hours and separate naturally.

“Refractometers are simple, rapid, and cost-effective methods for assessing FTPI in foals with moderate to good accuracy,” Elsohaby says. “However, a second confirmatory test with higher specificity (fewer false positives) should be used to confirm positive results.”

With any positive result, even without confirmation, you’ll want to intervene to save that foal’s life, he says.

The Best Defense: Maximizing Foal Immunity

Solutions: Plasma & Colostrum Banks

If a foal has low antibody levels, it’s best to get supplemental colostrum from the same barn, says Wagner. “Many experienced and large-scale breeders keep frozen colostrum (freezing the colostrum doesn’t destroy its qualities) from their own mares, just in case one of their foals ends up needing it,” she says.

These breeding operations maintain an emergency colostrum bank by milking and freezing small amounts from mares with sufficient and good-quality colostrum. Then, if a mare has no colostrum, dies after giving birth, or cannot nurse the foal for medical reasons, they can give the banked colostrum to the newborn within the first 24 hours. This supplement from another mare still provides the essential antibodies to the foal, says Elsohaby.

When foals don’t receive passive transfer of immunity from their dams within 18 hours, they urgently need those antibodies from elsewhere. A common source is mare plasma delivered through intravenous perfusion, which usually takes 60-90 minutes. In rare circumstances foals might have reactions such as muscle-twitching, increased heart or respiratory rates, fever, colic, or collapse within 20 minutes of administration.

Pharmaceutical companies harvest plasma from immunized mares and prepare it for FTPI foals, says Wagner. While these mares are exposed to the most common pathogens, they might lack antibodies against those in the foal’s environment.

Administering colostrum instead of plasma gives the foal immune-strengthening components that plasma doesn’t have, such as cytokines, says Wagner. “In general, colostrum gives better ‘priming’ for the foal’s immune system,” she says.

Of course, the trick is to diagnose and supplement before that 24-hour window closes, our sources say. If 24 hours have passed, plasma is your only choice.

The Best Defense: Maximizing Foal Immunity

From Birth to Four Months: The Environment

Once you’ve navigated the risky first 24 hours with successful passive transfer, there’s not much more you can do. Just let nature take its course, our sources say.

By nature, we mean letting the foal get exposed to endemic environmental microorganisms. “You can’t have a sterile environment for your foal,” Wagner says. “Not only is it more or less impossible in a barn, but it’s also not really good for your foal’s health. For example, foals need environmental microorganisms to establish their normal gut flora. This, however, does not mean that their environment should be ‘messy.’ Normal cleaning, good general horse care, and sufficient nutrients and a balanced diet are required to keep mare and foal healthy.”

Be reasonable, Wagner says. Let the foal live a normal life, with access to clean—but not bleach-washed—barns and pastures. Wash your hands before handling your newborn to get rid of any pathogens picked up from whatever you were just doing. But don’t scrub yourself obsessively and certainly not between each time you touch the foal during a single visit.

The presence of pathogens helps the young immune system develop, says Horohov, by stimulating it. “Initially, these responses are somewhat immature, but over time they become as efficient as those of adult horses,” he says.

Keeping this in mind, foal managers should make efforts to avoid subjecting their charges to disease outbreaks, he adds. “Knowing that the immune system of the foal is not as mature as an adult also means minimizing the exposure of the foal to infectious diseases,” he says.

The best thing a breeder can do to ensure good immunity for a foal is to provide foal and dam with a low-stress environment. “Keep herds stable with the same horses in a group, and give them plenty of turnout time within an established hierarchy,” Wagner says. “Make sure they have plenty of hay or grass and good shelter. This welfare-friendly management keeps stress levels low, contributing to good immune system development.”

Four to Six Months: First Vaccines

Continue with the same young foal management techniques after 4 months of age, say our sources. However, by this time, the foal’s own immune system is starting to take over, and the effects of passive immunity are disappearing.

That makes it time for the foal’s first vaccinations, which should occur any- where from 4 to 6 months of age, depend- ing on his immune history. “Vaccination of the foal should begin once the mare’s passive antibodies have declined, as these may interfere with the response to the vaccine,” says Horohov. “In most cases, this means waiting until the foal is 6

months of age. However, depending upon the mare’s vaccination history and the success of passive transfer, this schedule may need to be adjusted.”

Breeders should work directly with their veterinarians to establish a custom vaccine protocol for their foals, he adds.

Four to six months is also about the age that foals should begin their first deworming program, Horohov says. “Recommendations regarding the use of anthelmintics are based on both the exposure risk of the foal and the life history of the parasite (e.g. ascarids),” he says. “Immunity to parasites develops slowly and is typically incomplete, so foals are highly susceptible and require monitoring (with fecal egg counts) and treatment—as do older horses.”

Diseases of Concern: Rhodococcus equi and Clostridium difficile

While antibodies usually attack pathogens as they approach cells, some pathogens get inside the cells before the immune system can attack. This requires the body to provide cellular immunity, as opposed to humoral immunity involving antibodies. But cellular immunity is something horses must develop as they age; it can’t come from passive transfer.

Rhodococcus equi is one such pathogen. While adult horses’ immune systems can destroy R. equi easily and without showing signs of infection, the bacterium can ravage breeding farms, bringing 1-to 6-month-old foals down with severe pneumonia and leaving the survivors’ lungs literally scarred for life.

“The foal is highly susceptible to infectious agents that require cell-mediated immune responses,” Horohov says, including but not limited to R. equi, which infects and multiplies in macrophages (white blood cells that respond to infection).

“Fighting these bacteria off requires activation of the macrophage by the cellular immune system and, specifically, the production of interferon-gamma (a key protein in immune response),” he says. “But foals are unable to produce interferon-gamma early in life.”

Researchers still don’t understand exactly how foals acquire the ability to make interferon-gamma, Horohov adds.

The all-too-frequent “diarrhea disease,” caused by the Clostridium difficile bacterium, also requires cellular immunity. Sometimes fatal, C. difficile attacks foals’ digestive tracts, which lack established populations of beneficial bacteria that can fight off bad bacteria, says Angelika Schoster, DrMedVet, DVSc, PhD, Dipl. ACVIM, ECEIM, senior clinical lecturer at the University of Zurich’s Vetsuisse Faculty, in Switzerland.

Probiotics have been marketed to help prevent C. difficile infections in foals, but their effects are contradictory, she says.

What seems to work best for improv ing the immune system of the foal gut is “letting nature take its course,” Schoster says. “Foals usually consume mares’ feces during the first weeks of life, colonizing the gastrointestinal tract with good bacteria. Breeders shouldn’t prevent foals from practicing this routine.”

Take-Home Message

The equine immune system is well- designed to develop over time, starting with a warmup in the uterus and sup- ported by passive transfer of maternal antibodies in the first 24 hours after foaling. We can ensure optimum immune status by vaccinating the dam during pregnancy, then monitoring colostrum intake and looking for signs of failure of passive transfer in the first 72 hours. After those critical days, our job as breeders is to see that mares and foals have healthy, stable, normal environments with minimal stress. A veterinarian-designed vaccination program completes the developing immune system profile.