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) i