The First Immunity: How Foals Build a Healthy Immune System

First nursing of colostrum is not the end of immune development. It is only the beginning.

Within minutes of birth, a foal enters a world filled with bacteria, viruses, fungi, parasites, and countless environmental challenges. Yet unlike human babies, foals are born with almost no antibody protection from their dam. Their immune systems are immature, inexperienced, and still under construction. In many ways, the neonatal foal begins life immunologically vulnerable and dependent on the mare for survival.

For decades, breeder education surrounding neonatal immunity has focused primarily on one concept: making sure the foal nurses and receives adequate colostrum. While this remains critically important, modern research has expanded our understanding of how immunity develops in foals. Colostrum is not simply the foal’s “first milk.” It is the foundation of early immune development and the bridge between maternal protection and the gradual maturation of the foal’s own immune system.

Borrowed Immunity

Unlike humans, horses possess an epitheliochorial placenta, which prevents significant transfer of antibodies from the mare to the foal during pregnancy. As a result, foals are born with very limited circulating maternal antibodies and must acquire immune protection after birth through ingestion of colostrum, the antibody-rich first milk produced by the mare.

The antibodies contained within colostrum are called immunoglobulins. Immunoglobulins are specialized proteins produced by the immune system that recognize and help neutralize bacteria, viruses, and other pathogens before they can cause disease. The primary immunoglobulin measured in foals is IgG, which serves as one of the foal’s earliest and most important forms of immune protection during the first weeks of life.

Successful transfer of passive immunity requires several things to happen correctly in a very short period of time: the mare must produce high-quality colostrum, the foal must stand and nurse effectively, and the foal’s intestinal tract must absorb those antibodies before “gut closure” occurs. The foal’s intestine is uniquely capable of absorbing intact antibodies during the first several hours after birth, but this ability declines rapidly. By approximately 20–24 hours of age, antibody absorption becomes negligible.

This foal is standing and has latched on to nurse from his dam within two hours of birth.

Most healthy foals will stand and nurse successfully without intervention, but modern neonatal management increasingly emphasizes monitoring rather than assumption. Many veterinarians recommend confirming adequate passive transfer through IgG testing between 12–24 hours after birth, particularly in foals that experienced difficult deliveries, delayed nursing, weakness, illness, or questionable colostrum intake. In general, IgG concentrations above 800 mg/dL are considered adequate, values between 400–800 mg/dL indicate partial failure of passive transfer, and values below 400 mg/dL are considered failure of passive transfer requiring intervention.

Importantly, once the foal is beyond approximately 24 hours of age, oral colostrum is no longer an effective way to transfer antibodies because the intestine can no longer absorb them efficiently. At that point, intravenous plasma transfusion becomes the primary method used to raise IgG levels. Plasma can be lifesaving and is highly effective at correcting low IgG concentrations, but it is important to understand what plasma does and does not replace.

Colostrum is designed by nature to be delivered through the gastrointestinal tract during a very specific developmental window. In addition to antibodies, it contains immune cells, growth factors, antimicrobial proteins, hormones, and signaling molecules that interact directly with the intestinal lining and developing microbiome. Plasma primarily replaces circulating antibodies, particularly IgG. In other words, plasma can successfully correct failure of passive transfer, but it does not fully recreate the complete biologic experience of normal colostrum ingestion.

This distinction is especially important when considering why a foal required plasma in the first place. A healthy foal from a healthy mare with poor-quality colostrum may do extremely well after plasma correction. However, premature foals, critically ill foals, septic foals, weak foals, or foals with neonatal maladjustment syndrome (“dummy foal syndrome”) may face broader developmental and immune challenges beyond low IgG alone. Passing the IgG test is therefore not the end of the story. It is the beginning of the foal’s independent immune development.

Modern research has revealed that colostrum is far more complex than previously appreciated. While immunoglobulins are essential components, colostrum also contains immune cells, growth factors, antimicrobial proteins, and signaling molecules that help shape the newborn foal’s developing immune system.

Research suggests that broodmare health and management during late pregnancy may influence colostrum quality and passive transfer success. Factors such as premature lactation (“running milk”), placentitis, poor maternal health, stress, and inadequate late-gestation nutrition may all negatively impact colostrum quality.

Because colostrum production is closely tied to the mare’s overall physiologic state during late gestation, minimizing significant stressors may help support optimal colostrum production and immune transfer. Illness, excessive transport, overcrowding, heat stress, abrupt social changes, poor nutrition, and other management-related stressors can influence cortisol levels, inflammatory signaling, hydration status, appetite, and metabolic balance during a hormonally complex period immediately preceding foaling. Healthy mares generally produce healthier colostrum, healthier milk, and healthier foals.

This mare was hospitalized for an illness prior to foaling. Her foal received a plasma transfusion after a failed IgG check.

For most broodmares, supporting neonatal immune health does not require exotic supplements or aggressive feeding programs. However, mares do require balanced late-gestation nutrition with appropriate calorie intake, protein quality, vitamins, trace minerals, and body condition. Thin mares, mares on poor-quality forage alone, or mares with underlying health problems may be less likely to produce optimal colostrum and milk quality.

Vaccination of the mare during late gestation also plays an important role. Vaccinating broodmares approximately 4–6 weeks before foaling helps maximize antibody concentrations within colostrum, improving passive antibody transfer to the foal during the critical first hours after birth. These vaccines commonly include core vaccines such as tetanus, Eastern and Western encephalitis, West Nile virus, and rabies, along with regionally appropriate risk-based vaccines depending on geography and farm exposure. In some regions, this may include vaccines such as Potomac Horse Fever, rotavirus, influenza, EHV, or strangles. Vaccination programs should always be developed in consultation with the farm veterinarian.

Today, many breeders and veterinarians use Brix refractometers to estimate colostrum quality. Colostrum testing at or above approximately 23% Brix is generally considered good quality. This inexpensive tool allows breeders to quickly assess colostrum before the foal nurses and identify mares that may require supplementation with frozen colostrum or closer veterinary monitoring.

A Brix refractometer directly measures the total solids and sugar content in a liquid. When used with colostrum, this measurement indirectly estimates the concentration of immunoglobulins (IgG), which indicates colostrum quality.

Many breeding farms now maintain frozen colostrum banks collected from healthy mares with high-quality colostrum. However, not all frozen colostrum is equally useful or safe. Ideally, donor colostrum should come from healthy mares, be tested for quality before freezing, and be clearly labeled with the mare’s identity, collection date, and Brix value. Colostrum should be thawed gently in warm water rather than microwaved, as excessive heat can damage important immune proteins.

Breeders should also remember that unscreened donor colostrum may create concerns in situations involving neonatal isoerythrolysis (NI), where incompatible maternal antibodies can attack the foal’s red blood cells. For this reason, some owners may prefer coordinating with established veterinary colostrum banks or using plasma support under veterinary guidance when concerns about donor compatibility exist. Likewise, foals raised by nurse mares may still require careful passive transfer monitoring. While a nurse mare can provide milk, nutrition, maternal behavior, and social support, she cannot recreate the original colostrum transfer window if the foal failed to receive adequate colostrum shortly after birth.

An Immune System Under Construction

Even after successful passive transfer, the foal’s immune system remains immature for weeks to months after birth. This is one of the most important concepts emerging from modern neonatal immunology research.

The newborn foal’s infection-fighting cells do not yet recognize, coordinate, and eliminate microbes as efficiently as those of an adult horse. Likewise, the lining of the respiratory and gastrointestinal tract is still learning how to distinguish between normal environmental organisms and dangerous pathogens. This helps explain why neonatal foals remain particularly susceptible to infections despite having received adequate colostrum.

Importantly, the immune system does not develop in isolation. After birth, the foal begins interacting with the outside world through the skin, respiratory tract, gastrointestinal tract, pasture, soil, forage, water, and other horses. Research increasingly suggests that these normal environmental interactions help “educate” the developing immune system.

The gastrointestinal microbiome, the enormous community of microorganisms living within the digestive tract, appears to play a particularly important role in immune development. As foals begin nursing, nibbling forage, grazing pasture, investigating soil, and interacting with their environment, they are gradually establishing a diverse microbial population within the gastrointestinal tract. Foals also commonly consume small amounts of manure from their dam and other horses, a normal behavior known as coprophagy. While unpleasant to human sensibilities, this behavior is believed to help populate the developing gastrointestinal tract with beneficial microorganisms, particularly bacteria involved in the digestion of forage and fiber.

This microbial diversity appears to help educate the immune system to appropriately respond to normal environmental organisms while still recognizing harmful pathogens. In practical terms, this means that healthy immune development likely benefits from clean but natural environmental exposure rather than complete isolation or excessively sterile management.

Exposure to normal environmental organisms is a key part of a foal's immune development.

That does not mean foals should be raised in dirty or heavily contaminated environments. Overwhelming infectious exposure remains dangerous to immunologically immature foals. However, a foal raised exclusively in a stall or small paddock with minimal environmental diversity, little turnout, limited social interaction, and little exposure to natural forage or grazing environments may experience a very different pattern of microbial development than a foal raised in a more natural herd and pasture setting.

Modern management, therefore, becomes a balancing act between minimizing dangerous infectious exposure while still supporting normal environmental and microbial interaction. In practical terms, this means focusing on clean, dry, well-ventilated housing rather than attempting to create sterile conditions. Damp bedding, excessive manure accumulation, poor ventilation, overcrowding, and high ammonia levels can all increase infectious and respiratory stress on developing foals. Likewise, heavily trafficked barns with frequent horse movement may expose young foals to significantly greater infectious pressure.

Farms that both breed foals and actively travel horses to shows, clinics, or sales should pay particular attention to biosecurity practices. Horses returning from high-contact environments may carry respiratory pathogens despite appearing clinically normal. Strategic quarantine procedures, minimizing nose-to-nose contact between recently traveled horses and young foals, avoiding shared water sources during quarantine periods, and reducing direct exposure to recently traveled horses can help decrease infectious risk during vulnerable stages of immune development.

Season may also influence management considerations. Foals born during spring and early summer generally experience earlier access to pasture turnout, diverse forage exposure, fresh air, and broader environmental interaction. In contrast, winter-born foals in colder climates may spend more time indoors with reduced pasture exposure and greater barn confinement. While winter-born foals can absolutely develop healthy immune systems, management during these periods may require additional attention to ventilation, turnout opportunities, footing conditions, stocking density, and overall environmental quality.

Once the foal is healthy, nursing well, and the umbilicus is dry and healing normally, gradual turnout with the mare and compatible healthy horses is generally preferable to prolonged confinement. Normal turnout, exercise, and social interaction with healthy mares and foals likely support both physical and immunologic development more effectively than excessive isolation or prolonged stall confinement.

The Immune Gap

Maternal antibodies are temporary. Over time, the antibodies acquired from colostrum gradually decline while the foal’s own immune system slowly matures and learns to respond independently. This creates a transitional period often referred to as the “immune gap.”

Research suggests maternal antibody levels progressively wane during the first months of life, with foals reaching a relative immune low point somewhere around 1–3 months of age while their own immunity is still developing. This may help explain why some foals appear healthy during the immediate neonatal period but become more vulnerable to respiratory disease, diarrhea, or other infections later during the nursing or weanling stages.

The immune gap also complicates vaccination timing. Maternal antibodies can interfere with early vaccine responses if vaccines are administered too soon, while delaying vaccination too long may leave the foal vulnerable during periods of increasing exposure. For this reason, many core vaccination programs begin around 4–6 months of age, though timing may vary depending on the mare’s vaccination history, regional disease risk, and veterinary recommendations. Foals born to unvaccinated mares or foals with inadequate passive transfer may require different management strategies.

Most young horses begin a vaccination program around 4-6 months of age, as maternal antibodies decline.

Weaning represents another major developmental transition during a period when both the immune system and gastrointestinal microbiome are still maturing. Abrupt separation, transport, dietary changes, overcrowding, hospitalization, surgery, or medically necessary early weaning may compound physiologic stress during this period. While some situations require earlier intervention for legitimate medical or management reasons, thoughtful weaning practices, gradual transitions when possible, stable social groups, appropriate nutrition, and minimizing unnecessary stressors may help support continued immune and gastrointestinal development.

Modern research increasingly suggests that raising a healthy foal is not simply about avoiding disease during the first 24 hours of life. It is about supporting the gradual development of a resilient and appropriately educated immune system over the first weeks and months after birth. Broodmare health, colostrum quality, early nutrition, environmental management, microbial exposure, stress reduction, turnout, social interaction, and thoughtful veterinary care all appear to contribute to that developmental process.

While no management system can eliminate all disease risk, breeders play an important role in shaping the conditions under which the neonatal immune system develops. Modern neonatal immunology reminds us that the first nursing is not the end of immune development. It is only the beginning.

References

Wong DM, Wilkins PA, eds. Equine Neonatal Medicine. 1st ed. Wiley Blackwell; 2024.

AAEP Vaccination Guidelines. American Association of Equine Practitioners.

Perkins GA, Wagner B. The development of equine immunity: Current knowledge on immunology in the young horse. Equine Vet Journal. 2015;47(3):267-274.

Lopez BS, Hurley DJ, Giancola S, et al. The effect of age on foal monocyte-derived dendritic cell maturation and function after exposure to killed bacteria. Vet Immunopathology. 2019;210:38-45.