IgG Testing

IgG testing measures whether your newborn foal received adequate antibodies from the mare through colostrum. This matters because foals aren't born with their own functional immune system. They're completely dependent on passive transfer—the antibodies the mare passed through her milk in those first hours of life. If a foal doesn't get enough of these maternal antibodies, he's vulnerable to infection and disease. IgG testing tells you whether your foal got what he needed or whether you have a problem you need to fix fast.

Understanding passive transfer is about understanding how foals survive their first vulnerable months. A newborn foal has zero ability to fight infection on his own. His immune system exists but doesn't work yet. He can't make antibodies to respond to pathogens. He has no spleen function to speak of. He's essentially a blank slate immune-wise. The only thing protecting him is the antibodies he absorbed from colostrum in his first 24 hours of life. If those antibodies aren't there, that foal is at serious risk.

Mares manufacture these antibodies during pregnancy. Her own immune system identifies pathogens she's encountered, produces specific antibodies against them, and concentrates them into colostrum. When the foal nurses and absorbs that colostrum, those antibodies go to work protecting him against the diseases his mother has immunity to. It's a beautiful system when it works. The problem is that it doesn't always work, and if it fails, your foal is in trouble.

Why does passive transfer sometimes fail? The most common issue is poor colostrum quality. A mare might not have enough antibodies in her colostrum for various reasons—poor nutrition during pregnancy, stress, or just individual variation in how well she concentrates antibodies. Another problem is inadequate colostrum intake. Sometimes foals don't nurse well or soon enough. Sometimes they have problems swallowing or absorbing. Sometimes the mare's udder is so uncomfortable that the foal can't nurse effectively. Any breakdown in this system—poor colostrum or poor intake—and your foal is at risk.

IgG testing gives you data about whether passive transfer succeeded. It's a simple blood test that measures how many maternal antibodies ended up in the foal's bloodstream. Knowing this number tells you whether your foal has adequate protection or whether you need to intervene—potentially with plasma transfusion to give him antibodies he missed.

The process of passive transfer starts before the foal is born. During the last few weeks of pregnancy, antibodies accumulate in the mare's colostrum. You can actually see this happening if you strip the mare's udder in the days before foaling. Early milk is watery and clear. As the mare gets closer to foaling, it gets creamier and yellower. That color change indicates antibodies accumulating. The richest, most antibody-dense colostrum is usually produced in the last week before foaling.

Once the foal is born, he has a critical window. His intestines can absorb intact antibodies from colostrum for only about 24 to 36 hours after birth. After that, his digestive system matures and loses the ability to absorb large antibody molecules. So the entire success of passive transfer depends on what happens in that first day or so of the foal's life. If he doesn't nurse well in those first hours, or if the mare doesn't have adequate colostrum, he's missed his window to absorb protective antibodies.

The foal typically nurses within minutes to an hour of birth if everything is normal. He'll find the mare's udder, get a solid meal of colostrum, and start absorbing antibodies. Assuming colostrum quality is adequate and the foal nurses well, protective antibodies start working almost immediately. They circulate in his bloodstream, providing protection against the pathogens his mother has immunity to. This is passive transfer working as designed.

Fluids and electrolytes in colostrum matter too. Colostrum isn't just antibodies. It's rich in other protective factors and in nutrition that sets the foal up to thrive. A foal that gets good colostrum gets not just antibodies but also energy, minerals, and other compounds that support his health. Foals that don't nurse well or get inadequate colostrum miss out on all of this, not just the immune protection.

IgG testing happens after birth, usually between 24 and 72 hours old. The vet draws blood from the foal and sends it to a lab. The lab measures the concentration of IgG—immunoglobulin G, the primary antibody type that protects against systemic infection. Depending on what country you're in and which lab you use, standards vary, but generally foals should have IgG levels of 800 mg/dL or higher for adequate passive transfer. Some labs use 400 mg/dL as a cutoff. A foal testing below adequate levels has failure of passive transfer and needs intervention.

The timing of the test matters. Testing too early—before the foal has fully absorbed colostrum—can give you a false low result. Testing too late—after a few days—also isn't as useful because you've already missed the window to do anything about it. Most vets recommend testing around 24 hours old, after the foal has had time to nurse but before you've lost the opportunity to supplement if needed.

Preventing passive transfer problems starts with mare nutrition during pregnancy. A well-nourished mare with good body condition and adequate minerals and vitamins will produce better colostrum than a mare that's been undernourished. During the last trimester especially, the mare needs quality protein, minerals like copper and zinc, and good vitamin levels. Mares that are deficient in these nutrients produce colostrum that's weaker in antibodies. So good feeding during pregnancy is your first line of defense against passive transfer failure.

Managing the mare's stress in late pregnancy helps too. A stressed mare might not build up colostrum as effectively as a calm one. Keep pregnant mares in stable, consistent environments in the weeks before foaling. Minimize major changes or disruptions. While some degree of routine is good, major stress or upheaval can interfere with colostrum development.

Ensuring successful nursing is critical. Foals need to nurse within the first few hours of life and establish an effective nursing pattern. This means the mare needs to be calm and cooperative. It means the foal needs to be healthy and able to nurse. It means the mare's udder needs to be comfortable and accessible. Watch that first nursing to make sure it's happening effectively. The foal should get a substantial meal of that golden-yellow colostrum. If something looks off—the foal not finding the udder, the mare being difficult, the foal having trouble swallowing—address it immediately.

If you suspect inadequate nursing, you can milk out some colostrum and bottle feed it to the foal. Fresh colostrum from the dam is always better than anything you can buy. If the mare has no colostrum or inadequate colostrum, you'll need to use stored colostrum or commercial colostrum replacer. Frozen colostrum that's been stored properly is the next best option. Commercial replacers are less ideal but better than nothing if you have no colostrum available.

After IgG testing, if your foal tests low, your vet might recommend plasma transfusion. This is actually a plasma product—either fresh plasma from a donor horse or commercially produced plasma—that's administered intravenously to give the foal antibodies he missed. It's not a home treatment. It requires veterinary involvement and usually happens at a clinic or farm with the right equipment. It's expensive and not without risks, but for foals with severe failure of passive transfer, it can be lifesaving.

Monitoring the foal after birth is important regardless of IgG levels. Watch for signs of illness. Even foals with adequate passive transfer can get sick, though they're better protected. Foals with low antibody levels need extra vigilance. Any signs of depression, fever, difficulty nursing, joint swelling, or other illness should prompt immediate veterinary attention.

Failure of passive transfer—FPT—is the most significant complication. It happens when a foal doesn't absorb adequate antibodies from colostrum, leaving him immunocompromised. Foals with FPT are vulnerable to sepsis, pneumonia, and other serious infections. Some foals with FPT get sick within days of birth. Others might go a week or more before infection takes hold. The challenge is that foals with FPT look normal at first. You can't tell just by looking whether a foal has adequate antibodies. That's why IgG testing matters.

Marinal agalactia is when the mare doesn't produce colostrum or produces very little. This can happen due to fever, mastitis, or sometimes for no apparent reason. A mare with no colostrum leaves the foal with zero maternal antibodies. This is one of the most serious passive transfer failures because there's no colostrum to work with. If this happens, you need to act fast—either use stored colostrum from another source or get plasma transfusion as soon as possible.

Poor colostrum quality is frustratingly common. A mare might look like she's producing normal colostrum—adequate volume, normal color—but antibody concentration is low. This happens in mares with history of poor colostrum, mares with chronic infections, or mares under severe stress. Without testing, you don't know if colostrum quality is the problem until your foal tests low in IgG.

Disease in the foal during the critical absorption window can prevent proper antibody absorption. A foal that's septic or very ill might not nurse effectively or might have intestinal problems that prevent antibody absorption. In these cases, even with good colostrum available, the foal might still end up with low IgG levels.

Age at testing affects interpretation. A foal tested at 12 hours might show lower IgG than the same foal tested at 24 hours, because absorption is still ongoing. Some vets won't even test before 18 to 24 hours for this reason. Testing too early can give false low results that worry you unnecessarily.

Commercial colostrum replacers are better than nothing but not equivalent to real colostrum. They contain antibodies, but not the full spectrum of protective factors that real colostrum provides. If you have to use replacer, make sure you're using a quality product and that you're giving adequate volumes. Some foals don't absorb as well from replacer as from real colostrum.

Working with your vet around foaling and passive transfer is one of the best investments you can make in foal health. Your vet can counsel you on colostrum management, help you with that first nursing, advise on IgG testing timing, and intervene if problems arise. If you're planning to breed, having a relationship with an equine vet before foaling is essential.

Some vets recommend pre-foaling evaluation of colostrum quality using a colostrum tester or refractometer. This is a simple tool that measures the density of colostrum, which correlates with antibody concentration. Testing colostrum quality before the foal is born can alert you to potential problems. If a mare tests low in colostrum quality, you know to watch that first nursing extra carefully or to have backup colostrum on hand.

IgG testing itself requires veterinary involvement. Your vet draws the blood and sends it to a lab. Depending on which lab and what rush option you choose, you might have results within hours or within a day or two. Some vets have rapid test capabilities that give results much faster. Knowing results quickly matters because if your foal tests low, you can intervene sooner.

If plasma transfusion is needed, that's definitely a vet procedure, usually done at a clinic or well-equipped farm. Your vet will discuss risks, costs, and expected benefits. Plasma transfusion is not trivial—it costs significant money, requires appropriate venous access, and carries small risks of reaction or complications. But for foals with severe FPT, the alternative—an unprotected foal—is much riskier.

Some breeders work with vets to freeze and store colostrum from their mares, building a colostrum bank for emergencies. If you're going to do this, you need vet guidance on proper collection, storage, and handling. Improperly stored colostrum can become contaminated and unsafe to use. If you maintain a colostrum bank, work with your vet to ensure it's done right.

Costs for IgG testing range from around fifty to a hundred fifty dollars depending on your vet and whether you use rush results. Plasma transfusion costs significantly more—often several hundred to over a thousand dollars depending on the amount needed and where it's done. Colostrum management—having backup colostrum or using quality colostrum replacer—runs variable costs. The point is that intervention for passive transfer problems is expensive. Prevention through good mare nutrition and management is far more cost-effective.

IgG testing is a straightforward measure of whether your foal absorbed adequate maternal antibodies. It's not a tool that prevents passive transfer failure, but it identifies when failure has occurred so you can act. Testing is especially important if you have any reason to suspect problems—if the mare didn't produce good colostrum, if the foal didn't nurse well, if there were any complications at birth.

The real strategy for preventing IgG problems is prevention. Feed your mare well during pregnancy. Keep her healthy and stress-free. Monitor that first nursing and make sure it's effective. Have backup colostrum on hand if you have any concerns about colostrum quality. These preventive steps eliminate most passive transfer problems before they start. IgG testing is your safety net for when problems occur despite your best efforts.

Knowing whether your foal has adequate IgG gives you peace of mind or alerts you to problems early when intervention is still possible. A foal that tests high in IgG has good protection for his first months of life. A foal that tests low needs attention—maybe plasma transfusion, definitely extra vigilance for signs of infection. Either way, you have information instead of guessing.

Remember that IgG levels are only part of the immune picture. A foal with adequate IgG is protected against the diseases his mother has immunity to, but not against everything. He can still get infections that his mother didn't have immunity to. IgG levels matter, but so do general health, management, and early detection of any illness. Take all of these together, not just the test result in isolation.

Breeding successfully means thinking ahead. Plan for foaling before the mare gets to that point. Establish a relationship with your vet. Have a backup plan for colostrum if needed. Know what normal looks like so you can spot abnormal. IgG testing is just one piece of a comprehensive approach to foal health and breeding success. But it's an important piece, and it's a simple blood test that gives you valuable information.