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Failure of Passive Transfer

By: Dr. Mary Rose Paradis

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The veterinary care of the foal with complete or partial failure of passive transfer begins with identifying the potential risk factors and performing a physical examination of the foal. If the foal is less than 12 hours of age and has not nursed then it is assumed that the foal has FPT and is in need of immediate colostrum. If a colostrometer is available then the mare's colostrum should be checked for quality. If a colostrometer is not available then the appearance of the colostrum is helpful. It should be thick, yellowish and sticky when it dries on your fingers. The mare should be milked out and the foal should be administered the colostrum by bottle. In weak foals or foals that don't have a suckle reflex, the colostrum should be administered by the veterinarian by stomach tube. Just syringing it into the foal's mouth is not adequate, and may result in aspiration (inhalation) of the milk. If the colostrum is not of good quality then another source of colostrum should be sought.

The volume of colostrum needed obviously varies with the amount of immunoglobulins present in the colostrum. For the average 100 pound foal, approximately 32 ounces of adequate quality colostrum is needed. This amount can be divided into 3 equal feedings, given hourly for 3 feedings.

Other oral fluids should not be given before the colostrum administration because it will tend to "close" the intestine to immunoglobulin absorption sooner.

The veterinarian can measure the amount of immunoglobulin absorption in foals by a simple blood test called a CITE test. The CITE test is a semi-quantitative test that will give a range of immunoglobulin levels in the foals blood. Adequate passive transfer is considered over 800 mg/dl of immunoglobulins. Complete FPT is considered less than 200 mg/dl. Partial FPT is anywhere in between. The advantage of this test is that it can be done on the farm and the results are known within 20 minutes. This allows the treatment for FPT to begin immediately if needed. Though waiting a day for the results in a normal foal is usually safe, it is not acceptable in a high-risk foal. A high-risk foal would be a foal that may not have received adequate colostrum for whatever reason.

There are other tests that can be used to measure foal immunoglobulin levels. They include the zinc sulfate precipitation test, an agglutination test and an immunodiffusion test. All are adequate but the precipitation test takes 1 hour and the immunodiffusion test requires 12 to 24 hours before it can be read. The precipitation and the agglutination test are not as quantifiable as the CITE test.

A high risk foal should have his immunoglobulin levels determined around 12 hours of age. It is important that the foal have at least 400 mg/dl at this time to provide him with some protection against the environmental bacteria. This allows time for the majority of absorption even though the foal will continue to absorb immunoglobulins for up to 24 hours. In a non- high-risk foal, it is usually safe to wait for 24 hours to test the level of immunoglobulin.

If a foal has complete or partial failure of passive transfer at 12 hours of age you can still administer good quality colostrum and expect some absorption to take place. However after 12 hours of age the absorption is decreased and it is absent at 24 hours of age. In these cases it is important to provide the foal with immunoglobulins from another route.

The only way to increase immunoglobulin levels after 24 hours of age is to administer plasma, the immunoglobulin-containing portion of blood, to the foals intravenously. It can also be given to the younger high-risk foal with complete or partial FPT as a supplement to colostrum administration.

Your veterinarian can purchase frozen hyperimmune plasma or use a blood donor on the farm to harvest plasma. The commercially available plasma has the benefits that it can be stored for a year, it is from universal donors (that lack antibodies that would react with the foal's red blood cells) and that its immunoglobulin levels is higher than normal horses. It is also easier to administer, requiring only thawing in warm water. One should be careful not to thaw in a microwave or with boiling water. Too much heat will denature the immunoglobulins.

Harvesting plasma from a local donor is more labor intensive. Two to four liters of blood must be collected from the donor horse. This blood must sit for 1 to 2 hours allowing the red blood cells to settle out. The plasma must be siphoned off sterilely. It is difficult to collect the plasma without some red cell contamination. Red cell contamination could result in sensitizing the foal to certain blood types, which may cause a problem if the animal ever needs a blood transfusion in the future. If the foal is a filly and bred later in life it is possible that she would develop colostral immunoglobulins against her own foal's blood.

Generally an intravenous catheter is placed in a foal for the administration of the plasma. Plasma should be run through a filtered IV line in order to take out any fibrin another component of blood. The rate of plasma administration should not exceed 1 liter/hour. The foal should be monitored for any side effects. These might include trembling and an increase in heart rate. The rate of plasma administration should be slowed down until these side effects stop. The amount of plasma needed depends on the starting level of immunoglobulins that the foal has and the quality of the plasma. Generally 1 to 1.5 liters of high quality plasma is needed to bring a complete FPT foal up to adequate levels. A partial FPT may only require 0.5 liters of high quality plasma.

At the present time, the quality of commercial plasma is not regulated by the FDA or USDA, as is the quality of approved drugs or vaccines. Quality can vary considerably without the consumer knowing. However, there are currently guidelines being established for regulations of these products, and greater consumer confidence can be expected once these regulations are put in place, in the next couple years.

Follow-up

The foal with failure of passive transfer should have his immunoglobulins re-checked after receiving a plasma transfusion to ensure that it has reached 800 mg/dl. Foals with septicemia as a result of their failure of passive transfer may require more plasma than non-infected foals. It appears that the immunoglobulins in these foals are used up more quickly in fighting the infections. Rechecking the immunoglobulins in these foals 4 to 5 days later may be important. Additional plasma may need to be administered.

Colostral immunoglobulins begin to disintegrate as soon as they are absorbed. In normal foals they disappear from the blood stream around 2 to 3 months of age. As they disappear, the foal begins to make his own immunoglobulins. The foal reaches adult levels of immunoglobulins around 4 to 5 months of age.

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