Failure of Passive Transfer in Foals: IgG Thresholds, Diagnosis, and Plasma Treatment

Failure of passive transfer (FPT) is one of the most preventable causes of neonatal foal death, and it unfolds silently in the first 24 hours of life. A foal whose IgG levels fall below 800 mg/dL at 12-24 hours of age lacks the immune protection needed to survive the microbial world it has just entered.

For breeders managing multiple mares, tracking gestation timelines and foaling dates is the first step in ensuring you’re present and prepared when it matters most. Breedio helps you monitor every mare’s due date so no foaling catches you off guard.

What Is Passive Transfer — and Why Does It Fail?

Unlike humans, foals are born agammaglobulinemic — they have virtually no circulating immunoglobulins at birth. The equine placenta does not allow antibody transfer in utero. Instead, 100% of a foal's early immune protection must come from colostrum, the antibody-rich first milk produced by the mare in the hours immediately following foaling.

Passive transfer fails when:

• The foal does not consume enough colostrum (volume failure)
• The colostrum is of poor quality (low IgG concentration)
• The foal is too weak or premature to nurse effectively
• The mare has premature lactation before foaling, leaking colostrum
• Absorption is delayed past the critical window

The foal's intestinal wall contains specialized enterocytes capable of absorbing intact immunoglobulin molecules — but this capacity is strictly time-limited. Absorption peaks at birth, drops sharply after 6 hours, becomes negligible after 12 hours, and reaches zero at 24 hours post-birth. This process, called gut closure, is irreversible.

What Are the IgG Thresholds Breeders Need to Know?

Veterinary consensus has established clear IgG thresholds for classifying passive transfer status in foals. These are measured from a blood sample taken between 8 and 24 hours of age.

These thresholds align with published clinical guidelines. A foal with IgG above 800 mg/dL has received sufficient antibody protection from colostrum to carry it through its first 4-6 months of life, until its own adaptive immune system matures.

Partial FPT (400-799 mg/dL) requires careful judgement. In a clean, low-risk environment, some foals do well. In a hospital setting, at a busy breeding farm, or if any signs of illness appear, treatment is strongly recommended.

When Should You Test? The Diagnostic Window

Testing too early produces falsely low results — the foal may still be in the process of absorbing colostral antibodies. Testing too late means the gut closure window has passed and oral supplementation is no longer an option.

Optimal testing window: 8 to 24 hours post-birth

The two most common field-based diagnostic tools are:

1. SNAP Foal IgG Test (ELISA)

The gold standard for on-farm use. Provides a semi-quantitative or quantitative IgG result from a small blood sample in under 10 minutes. Highly reliable when used within the correct time window.

2. Brix Refractometer

A rapid, low-cost screening tool. Originally designed to test colostrum quality, a serum Brix reading of ≥8.5% in a foal’s blood correlates with IgG ≥800 mg/dL. Useful for quick field screening, though less precise than ELISA.

For colostrum quality testing before the foal nurses, a Brix refractometer reading of the colostrum itself is informative:

How Much Colostrum Does a Foal Actually Need?

A 50 kg foal must ingest a minimum of 60 grams of immunoglobulins to achieve adequate passive transfer. Given that high-quality colostrum contains approximately 60-80 g/L of IgG, this translates to:

• 1.5 to 2 liters of high-quality colostrum within the first 12 hours
• Delivered across 4 nursing sessions on average
• In foals unable to nurse, via nasogastric tube, in volumes no larger than 250-500 mL per administration

The Hôpital Vétérinaire de Sherbrooke recommends a total colostrum intake equal to 10% of body weight — roughly 3.5 L for a 35 kg foal — in the first 6 to 12 hours. While total volume matters, colostrum quality is equally critical.

What Happens When Oral Supplementation Is No Longer Possible?

If the foal is already past 24 hours of age, or if IgG testing confirms severe FPT, intravenous plasma transfusion becomes the only effective treatment.

Plasma Transfusion Protocol

Commercial equine plasma (hyperimmune plasma) is available from veterinary suppliers and is the most reliable source. Key considerations:

• Volume: Typically 1-2 liters for a neonatal foal, administered IV slowly
• Rate: Must be infused slowly (10-20 mL/kg/hour) to avoid reactions
• Monitoring: The foal must be watched closely for signs of transfusion reaction: urticaria, tachycardia, trembling, or respiratory distress
• Re-testing: IgG levels should be re-checked 12-18 hours post-transfusion to confirm adequacy
• Multiple doses: Severe cases (IgG < 200 mg/dL) may require more than one transfusion

Plasma transfusion is most effective when administered before clinical illness develops. A foal that has already developed sepsis or pneumonia from FPT faces a significantly worse prognosis even with treatment.

Hyperimmune Plasma vs. Fresh Equine Plasma

Hyperimmune plasma, collected from horses vaccinated against specific pathogens (Salmonella, E. coli, Rhodococcus equi), provides both IgG replacement and targeted antibody protection. When selecting commercial plasma, verify that it has been tested for equine infectious anemia (EIA) and other bloodborne pathogens.

What Factors Increase FPT Risk?

Some mares and foals are at significantly higher risk than average. Breeders should maintain heightened vigilance when:

• The mare is older (20+ years): Older mares have a 21% abortion risk after day 40 versus 8% for younger mares, and often produce colostrum of declining quality
• The mare experienced premature lactation: Colostrum leaks before foaling, reducing IgG concentration
• The foal is premature or dysmature: Foals born before 320 days of gestation are considered premature and may lack the energy or suckling reflex to consume adequate colostrum
• The mare was not vaccinated pre-foaling: Mares should receive vaccines 30 days before their due date (West Nile, EEE/WEE, influenza, tetanus, rotavirus) to maximize colostral antibody titers
• Maternal nutrition was poor: Studies show foals from mares supplemented with key minerals and vitamins showed significantly fewer cases of incomplete passive transfer (IgG < 8 g/L at 12-36 hours post-birth)

Building a Reliable FPT Prevention Protocol

For breeders managing multiple mares, a systematic approach prevents most FPT cases before they occur.

Pre-foaling (last trimester):

1. Vaccinate the mare 30 days before the due date
2. Optimize body condition score to 5-6 (Henneke scale) at foaling
3. Ensure adequate mineral supplementation, especially copper and zinc
4. Bank colostrum from mares with known good production
5. Keep a colotest or Brix refractometer in your foaling kit

At foaling (0-6 hours):

1. Assess colostrum quality with refractometer before or immediately at foaling
2. Ensure foal stands within 1 hour and nurses within 2 hours (the 1-2-3 Rule)
3. If foal cannot nurse, hand-milk the mare and tube-feed the foal
4. If colostrum quality is poor (< 40 g/L or < 20% Brix), supplement with banked colostrum

At 8-24 hours:

1. Test foal IgG using a SNAP test or serum Brix
2. If IgG < 400 mg/dL, contact your veterinarian immediately for plasma transfusion
3. If IgG 400-799 mg/dL, evaluate clinical picture and environment before deciding on intervention
4. Document results for future breeding records

Keeping precise records of foaling dates, mare vaccination histories, and foal IgG results across multiple mares is exactly where Breedio adds real value. You can track gestation progress, set reminders for critical milestones, and never miss the optimal vaccination or testing window. Explore our Features to see how breeders are using it to manage entire breeding operations.

The Bottom Line on Passive Transfer in 2026

Failure of passive transfer remains one of the most common and entirely preventable causes of foal mortality in the first weeks of life. The science is clear:

• Test between 8 and 24 hours of age
• Treat IgG below 400 mg/dL with plasma transfusion, not supplemental colostrum
• Oral supplementation only works before gut closure at 24 hours
• Prevent FPT with pre-foaling vaccination, good nutrition, and a banked colostrum supply

The foaling season leaves no room for guesswork. Track Your Mares with Breedio to stay ahead of every due date and foaling milestone, so you're never caught unprepared when it matters most.