Newborn foals are born with an immature immune system, with limited ability to fight off infections and diseases.

To build immunity, neonatal foals must absorb immunoglobulins, also known as antibodies, from their dam’s colostrum within 24 hours after birth. This process is known as passive transfer of immunity.

The foal’s ability to absorb antibodies declines quickly after birth. If a foal fails to receive enough immunoglobulins within this crucial timeframe, failure of passive transfer (FPT) can result, leaving the foal vulnerable to infections and diseases.

The best way to determine if passive transfer is successful is to test Immunoglobulin G (IgG) levels in the foal’s blood serum. Successful transfer occurs when the foal has absorbed enough antibodies to yield an IgG blood concentration greater than 8 g/L.

Close monitoring of the mare and foal are important to ensure successful passive transfer. If you are concerned about your foal’s colostrum intake or IgG test results, consult with your veterinarian.

Immunoglobulins and Passive Transfer

Immunoglobulins, or antibodies, are components of the adaptive immune system. These specialized proteins recognize and bind to bacteria or viruses, enabling the horse’s immune system to neutralize and eliminate these infectious agents.

Immunoglobulin G (IgG) is the most common type of antibody, accounting for over 80% of immunoglobulins in the horse’s colostrum and blood. [1]

When foals are newly born, their digestive system possess a unique ability to absorb IgG found in their dam’s colostrum.

This is only possible thanks to the expression of a special IgG receptor, neonatal Fc-receptor (FcR), which facilitates the transport of immunoglobulins across the intestinal barrier and into the foal’s blood.

However, these receptors gradually diminish in the small intestine, with their presence significantly reduced after the first 24 to 48 hours. [2]

It is important that your foal receives sufficient colostrum during this critical period to establish their immune system. Foals need to consume approximately 2 litres of colostrum for successful passive transfer.

Evaluating Passive Transfer

To evaluate the success of passive transfer, IgG antibody levels are measured in the blood of the foal. This assessment determines whether the foal has absorbed enough antibodies from colostrum to defend against diseases effectively.

If the IgG levels are below a certain threshold, it indicates a failure of passive transfer, leaving the foal more susceptible to infections.

Prompt intervention may be necessary in such cases to provide the foal with supplemental antibodies and support the immune system.

What IgG Levels should my Foal have?

In a foal with a successful passive transfer of immunity, IgG levels in the blood should measure at 8 g/L or higher.

Category IgG (g/L) IgG (mg/dL)
Successful Transfer ≥ 8 ≥ 800
Partial Transfer 4 – 8 400 – 800
Failed Transfer ≤ 4 ≤ 400


When to Test IgG levels

Ideally, a foal should have their IgG levels tested within 8  – 12 hours after their first few feedings of colostrum. [3]

Testing during this timeframe is best to detect a failed passive transfer while it’s still possible to intervene by feeding supplemental colostrum. During this period, the foal can still absorb antibody proteins in colostrum to effectively raise IgG levels.

While foals can still be tested 24 hours after birth, their IgG levels cannot be increased through colostrum feeding at this stage. Instead, your veterinarian will treat a failed passive transfer via plasma infusion of immunoglobulins. [3]

Even if a foal is identified with a failed or partial passive transfer after the period of IgG absorption, it is still recommended to provide supplemental colostrum. This is because colostrum is rich in nutritional factors that can benefit the foal’s overall health and well-being.

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Failed Passive Transfer (FPT)

Failed passive transfer (IgG levels below 4 g/L) is estimated to occur in 10-25% of foals. [4][5] It can occur when the foal does not consume enough colostrum or the colostrum quality is insufficient.

FPT leads to insufficient immune system development and is associated with an increased risk of infection and death in newborn foals. [1] Affected foals are also more likely to require medical treatment within the first three months of life. [6]

It is important to highlight that even with a partial transfer of IgG (IgG levels between 4-8 g/L), foals still have a heightened vulnerability to diseases.

Premature Lactation

The leading cause of a failed passive transfer in foals is premature lactation in the mare. This occurs when the mare starts to produce and leak milk before giving birth.

The first colostrum produced by the mare is the most concentrated source of IgG. However, when premature lactation occurs before foaling, the colostrum that remains after leakage contains lower levels of immunoglobulins.

This reduced immunoglobulin concentration can impact the foal’s ability to receive sufficient IgG during its first feedings. Even if the foal has adequate colostrum intake, they may not receive enough antibodies to support immune system development.

Additional Causes of Failed Passive Transfer

Additional factors that can contribute to a failed passive transfer include the following:

  • Inadequate colostrum intake in the foal
  • Delayed nursing or suckling difficulties
  • Insufficient colostrum production in the mare
  • Maternal health issues (including mastitis, other illness, or medication use) that prevent nursing
  • Placental conditions (i.e. placentitis and premature placental separation), impacting the transfer of antibodies to the foal
  • Multiple births, leading to competition for colostrum
  • Failure of colostrum absorption in the foal due to gastrointestinal disorders or abnormalities

Risk Factors

Breeders and owners should stay vigilant for early indicators and warning signs that a foal is at risk of a failed passive transfer.

The following risk factors are associated with higher rates of FPT in foals:

  • Mares that require assistance during foaling [5]
  • Mares with health events before foaling [5]
  • Foals that take over an hour to stand up [3]

How To Test for IgG Levels in Foals

The gold standard for IgG testing in foals is to send a blood serum sample to a laboratory for analysis. While laboratory results are the most accurate, it can take 1 – 2 days to receive your report.

By this time, the foal’s ability to absorb immunoglobulins from colostrum is significantly diminished, limiting the options for treatment of failed passive transfer.

For this reason, horse owners and veterinarians generally prefer IgG testing options that can provide rapid, on-farm results.

The two most commonly used screening options for IgG levels in foals are the SNAP Test and Brix Refractometry. They both serve as quick and reliable methods to assess successful passive transfer. [8][9]

Both methods require collecting a small blood sample from the foal. Your veterinarian can train you or staff at your boarding facility on how to safely take a blood sample.

SNAP Foal IgG Test

The SNAP Test is a rapid immunochromatographic assay used to assess the passive transfer of maternal antibodies from the mare to the foal. The test provides results within seven minutes.

During the test, a blood sample is taken from the foal and applied to a test strip or cartridge. The test strip contains specific reagents that react with the IgG antibodies present in the sample, producing color changes to indicate results.

A qualitative comparison between colored dots on the test strip is used to determine the IgG concentration of the blood sample. This makes the SNAP Foal IgG test susceptible to user error and variation between readings.

SNAP is a screening tool and should be followed up with confirmatory laboratory tests for more accurate results. Consult your veterinarian for appropriate testing and interpretation of results.

Brix Refractometry

Brix refractometry is another way to assess passive transfer in foals. A refractometer is used to indirectly estimate the concentration of immunoglobulins in the foal’s blood serum by measuring the refractive index of the plasma.

The test involves collecting a blood sample from the foal, applying it to the prism of the refractometer and directing light through the sample. The refractometer then measures the angle at which the light is bent or refracted as it passes through the serum.

Based on the refractive index of the sample, the concentration of total solids in the serum can be estimated and correlated with IgG levels.

Brix refractometry is prone to errors when the foal is dehydrated. However, unlike the SNAP Test, this test provides a distinct value rather than a qualitative color assessment. Its results are more repeatable and less prone to user error.

Treating Low IgG Levels in Foals

If IgG testing reveals that your foal has not received adequate maternal antibodies, your veterinarian will recommend treatment to support your foal’s immune development.

Prompt intervention through plasma transfusion or administration of supplemental colostrum can help to boost IgG levels and provide passive immunity

Here are some common approaches to treating a failed passive transfer in foals:

  • Supplemental Colostrum: If low immunoglobulin levels are identified early, providing the foal with donor colostrum can help increase IgG concentrations. Banked colostrum should be provided within the first few hours after birth before the foal’s ability to absorb IgG declines rapidly.
  • Intravenous Plasma Transfusion: This is the most effective and immediate treatment for foals with low antibody levels. Your veterinarian will administer your foal with plasma from a healthy donor to rapidly increase IgG levels in the blood.
  • Antibiotics and Supportive Care: Foals with low IgG levels are more susceptible to infections. It is important to closely monitor them for signs of illness and promptly treat any infections that may occur. Antibiotics may be prescribed by your veterinarian to address bacterial infections, while supportive care measures such as fluid therapy and nutritional support may be necessary to aid in the foal’s recovery.

Colostrum Banking

When preparing for the arrival of a foal, its recommended to have frozen colostrum on hand. This ensures that you have a backup supply in case the mare experiences colostrum leakage before birth or produces poor-quality colostrum.

A healthy broodmare will produce enough colostrum to meet the needs of her own foal and also allow for colostrum collection.

Follow these guidelines to collect, store and use banked colostrum: [3]

  1. Identify Donor Mares: Select the appropriate mare to collect colostrum from.
    • Choose a healthy, non-maiden mare that has had a healthy foal.
    • Ensure the mare did not prematurely lactate.
    • Make sure the mare has received appropriate vaccinations and boosters.
  2. Collection: Allow the mare’s foal to nurse several times before collection. Collect colostrum within three hours of birth to ensure optimal antibody levels.
    • Clean the mare’s udder with warm water prior to collection.
    • Milk approximately 500 to 600 mL of colostrum from one side of the mare’s udder, leaving the remaining side for the foal.
  3. Testing: Test the colostrum to ensure adequate quality. Only keep colostrum that tests over 23% with a Brix Refractometer.
  4. Handling: Filter the collected colostrum through a cheesecloth, milk filter, or gauze to remove debris.
  5. Storage and Labeling: Store the colostrum in one liter batches. Label the container with the date of collection, name of mare, and Brix testing results.
  6. Freezing: Freeze the colostrum at −17° C (1° F) and store it for up to two years.
    • If the colostrum being used in the short term, it can be stored in the fridge for up to 2 days.
  7. Thawing: Thaw the colostrum in a water bath at approximately 37.5° C (100° F). Use a thermometer to ensure the water bath does not get too hot, as this will denature the antibodies.

Ensure that all the tools and equipment used in the process are cleaned and sanitized.  Any contact with harmful bacteria will contaminate the colostrum.

Ensuring Successful Passive Transfer

Ensuring your newborn foal gets quality colostrum with sufficient IgG antibodies is key to giving them a healthy start in life.

While it is not always possible to prevent low IgG levels, there are proactive steps you can take to support successful passive transfer to your foal.

Here are three recommendations for improving passive transfer of immunity :

1. Test the Mare’s Colostrum Quality

Good quality colostrum must contain an adequate amount of IgG antibodies to promote the foal’s health.

Testing the colostrum immediately after birth using a Brix Refractometer is highly recommended. Carefully follow the specific instructions provided with your refractometer to obtain accurate results.

Interpreting Brix Results

Here is a general guideline for interpreting Brix results to assess colostrum quality:

  • 23% or more: Good quality colostrum with an optimal concentration of immunoglobulins to support immune development in the foal.
  • 18 – 22%: Fair quality colostrum with a moderate IgG concentration.
  • 18% or less: Poor quality colostrum with a low concentration of IgG antibodies, potentially compromising the foal’s immune system.

Supplementing Colostrum

If the mare’s colostrum is poor to fair quality, allow the foal to suckle but supplement with colostrum replacement or frozen colostrum. Test the foal’s IgG levels within 12 hours after birth and have banked colostrum available to treat a possible failed passive transfer.

When bottle feeding, provide the foal with 400 – 500 mL at a time with feedings every forty to sixty minute intervals. [3]

Nutrition for Colostrum Quality

The best way to improve colostrum quality is to start early by providing the broodmare with a healthy diet that contains proper levels of key nutrients, including protein, vitamins and minerals.

Pregnant mares have a higher requirement for several vitamins and minerals, including phosphorus, calcium, copper, and iodine. Research also shows that mares supplemented with vitamin E produced colostrum with higher IgG levels. [10]

Supplementing the mare with yeast cultures and probiotics during pregnancy has also been shown to increase IgG levels in foals. [11]

Mad Barn’s Optimum Digestive Health contains yeast, prebiotics and probiotics to support hindgut function. Providing this supplement to your pregnant mare mares will support gut health, feed efficiency, increased milk production and immunoglobulin supply to the foal.

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  • Prebiotics, probiotics & enzymes
  • Support hindgut development
  • Combats harmful toxins in feed
  • Complete GI tract coverage

You can learn more in our guide on How to Feed a Pregnant Mare.

Tips to Improve Colostrum Quality

Certain management practices can also help to improve the quality of the colostrum provided to your foal.

Regular veterinary check-ups throughout the mare’s pregnancy ensure that her overall health is monitored and any concerns are addressed promptly.

The mare should be routinely vaccinated or boosted before giving birth to support her immune health and improve IgG levels in the colostrum. [12] Consult with your veterinarian about which vaccines to give your gestating mare.

If the mare’s udder starts to leak colostrum before foaling, collect her colostrum and bottle feed it after foaling. Ensure the colostrum is stored in the fridge to prevent bacterial growth, and that the bottles and buckets used to collect colostrum are cleaned and sanitized.

If leaked colostrum cannot be collected, the foal should be supplemented with frozen colostrum alongside the maternal colostrum.

2. Ensure the Foal Receives Colostrum on Time

A newborn foal should be standing and nursing within two hours of birth to ensure they can absorb immunoglobulins from colostrum. After 12 hours, the efficiency of absorption is reduced by half. [13]

  • Make sure the foal is able to stand and nurse. If needed, provide assistance so they can stand and drink colostrum as soon as possible after birth.
  • If the dam does not have any milk production or refuses to let the foal nurse, provide the foal with frozen colostrum within six hours.
  • If the foal is unable to suckle, contact your veterinarian for further guidance.

3. Ensure the Colostrum is Clean

To support the health of your foal, the colostrum provided must be clean with low bacteria levels.

Feeding contaminated colostrum poses a risk of infection to the newborn foal. The presence of bacteria in colostrum can also hinder IgG absorption by binding to antibodies and preventing their passage through the gut lining wall. [14]

Follow these steps to reduce the risk of colostrum contamination:

  • Ensure the foaling area is clean and well-maintained.
  • Clean the foaling area between each foaling.
  • If colostrum is being banked, ensure collection equipment is cleaned and sanitized.
  • Clean the mare’s udder with warm water before collecting colostrum.
  • Freshly collected colostrum should be fed within one hour. If colostrum is not being fed immediately, promptly store it in the fridge or freezer.
  • When feeding previously collected frozen colostrum, do not thaw it at room temperature. Thawing at room temperature allows bacteria to grow in the colostrum.


The best way to give your foal a healthy start in life is to ensure they consume two liters of colostrum within a few hours after birth.

Colostrum provides a vital source of Immunoglobulin G (IgG) to build the foal’s immune defenses against common diseases.

By assessing IgG levels in the foal, veterinarians and horse owners can determine if successful passive transfer of maternal antibodies has occurred or if supplemental colostrum or plasma infusions are required.

Various methods, such as the SNAP Test and Brix Refractometry, offer practical options for testing IgG levels in foals. Consult with your veterinarian to learn more about performing IgG test and interpreting results.

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  1. Kohn, C.W. et al. 1989. Colostral and serum IgG, IgA, and IgM concentrations in Standardbred mares and their foals at parturition. J Am Vet Med Assoc. View Summary
  2. Perkins, GA. Wagner. B. The development of equine immunity: Current knowledge onimmunology in the young horse. Equine Veterinary Journal. 2014.
  3. Saunders, W.B. Colostrum Banking. Clinical Veterinary Advisor
  4. Tyler-McGowan, C.M. et al.Failure of passive transfer in foals: incidence and outcome on four studs in New South Wales. Aust Vet J. 2008.View Summary
  5. Raidal, S.L. 1996. The incidence and consequences of failure of passive transfer of immunity on a Thoroughbred breeding farm. Aust Vet J View Summary
  6. Clabough, et al. 1991. Factors associated with failure of passive transfer of colostral antibodies in Standardbred foals. J Vet Intern Med View Summary
  7. Ayala, M.S.F and Oliver-Espinosa, O. J. Risk Factors Associated With Failure of Passive Transfer of Colostral Immunoglobulins in Neonatal Paso Fino Foals. J Equin Sci. 2016.
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  9. Sievert, M. et al. Comparison of Different Methods to Determine the Absorption of Colostral IgG in Newborn Foals. J Equine Vet Sci. 2022. View Summary
  10. Bondo, T. and Jensen, S.K. Administration of RRR-α-tocopherol to pregnant mares stimulates maternal IgG and IgM production in colostrum and enhances vitamin E and IgM status in foals. J Anim Physiol Anim Nutr (Berl). 2010. View Summary
  11. Leimbach, R. Influence of a Maternal Dietary Yeast Supplement on Immunoglobulin Concentrations in Quarter Horse Foals from Birth to Four Months of Age. Ohio State University. 2015.
  12. Cauchard, J. et al. Foal IgG opsonizing anti-Rhodococcus equi antibodies after immunization of pregnant mares with a protective VapA candidate vaccine. Vet Microbiol. 2004. View Summary
  13. Osaka, I. et al. Effect of the mass of immunoglobulin (Ig)G intake and age at first colostrum feeding on serum IgG concentration in Holstein calves. J Dairy Sci. 2014.
  14. Godden, S.M. et al. Colostrum Management for Dairy Calves. Vet Clin North Am Food Anim Pract. 2019.