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Failure of Passive Transfer

By: Dr. Mary Rose Paradis

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Failure of passive transfer (FPT) is defined as the failure of the foal to receive adequate colostrum from his dam. Failure of passive transfer can be complete – the foal receives no colostrum – or it can be partial – the foal received poor quality colostrum. Colostrum is the first milk that the mare makes. She begins making it about one month before the foal is to be born.

During the process of producing this milk, antibodies from the mare's blood are selectively filtered and concentrated into the mammary tissue of the udder. It is generally thicker in nature and yellow tinged compared to regular mare's milk. Specific antibodies called immunoglobulins are found in the milk. The predominant immunoglobulin is IgG, followed by IgM. A batch of colostrum is only made once per pregnancy and disappears in the first couple days as the foal suckles.

The importance of the foal receiving colostrum centers around the fact that the foal is born essentially devoid of his own protective immunoglobulins. This occurs in many of our domestic species because of placental differences and thickness. In the human placenta there is only one layer of tissue separating the maternal blood supply from the fetal blood supply. Immunoglobulins, which are large molecules, can easily cross this distance. In contrast, the mare's placental has six layers of tissue separating the maternal blood flow from the fetal blood flow. The immunoglobulins are unable to penetrate these layers. Without protective immunoglobulins the newborn foal is highly susceptible to infection from even normal environmental bacteria.

The timing at which the foal should ingest the colostrum and can absorb the immunoglobulins is crucial. It is truly a race against the bacteria and the clock to protect the newborn foal from a life-threatening bacteria infection called sepsis. The foal needs to suckle the colostrum within 2 to 3 hours after birth. Any longer period without suckling endangers the foal both immunologically and nutritionally. The foal's intestine is "open" to absorb the large immunoglobulins for a limited amount of time. The intestinal tract begins to lose this ability around 12 hours of age and it is usually totally gone by 24 hours of age. Foals greater than one day of age are unable to absorb immunoglobulins from even the richest colostrum.

Colostral "richness" in terms of the amount of immunoglobulins present varies from mare to mare and at different times of the year. Mares foaling in the spring and summer generally have a higher concentration of immunoglobulins than mares foaling in the winter. Sunny regions as opposed to overcast areas of the country appear to improve colostrum quality. Mares over 15 years of age have been reported to have a decreased quality of colostrum.

Risk factors for failure of passive transfer of colostral immunoglobulins include prematurity or postmaturity. Because the mare only makes colostrum once, any leakage of milk from the mare before the foal is born should be taken as a matter of grave concern. In premature lactation the mare may never build up good concentrated colostrum or she may lose the best portion of the colostrum, diluting the rest out with normal milk that is practically devoid of immunoglobulins. If the mare rejects the foal or is aggressive toward him, the foal may also be deprived of colostrum. Other risk factors include problems that may occur in the foal to prevent him from nursing. If the foal is weak at birth or has limb deformities that prevent him from standing, the foal may also not receive adequate colostrum.

The immunoglobulin level can be measured on the farm by the specific gravity of the colostrum. There is an instrument called a colostrometer, which simply measures the specific gravity of 5 mls of colostrum in a column of distilled water. The density of the colostrum is directly proportional to the amount of immunoglobulins present. A specific gravity of 1.060 is considered adequate for foal protection. Specific gravities over 1.080 are considered excellent. It is handy to have a way of measuring colostrum quality on breeding farms. Knowledge of which mares may have poor colostrum may help predict which foals are at risk for total or partial failure of passive transfer.

Colostrum from mare's that have excellent specific gravities can be used to feed her own foal or saved for another foal that needs colostrum such as an orphan or a foal from a mare that leaked her colostrum out. In these mares you can allow her foal to nurse one teat while you milk the other. It is safe to save about 1 cup of this high specific gravity colostrum without depriving her own foal of the needed antibodies. The milked colostrum should be strained through a piece of cloth, such as gauze, to eliminate large dirt particles. It can then be placed into plastic containers or freezer bags and placed into the freezer. The container should be labeled with the name of the mare, the date and the specific gravity. It will remain good up to 1 year in the freezer.

Some people feel that lack of colostrum is not the only cause of infection in foals. The fact that the intestine is "open" to absorb colostrum in the first hours after birth also means that it may be "open" to absorb more bacteria during this time. The act of eating hastens the closure of the intestine, therefore, receiving colostrum early not only provides immunoglobulins, it decreases the amount of time that bacteria can be absorbed. Some people advocate milking the mare immediately after the birth of the foal and bottle feeding him before he stands to expedite the process of intestinal absorption of colostrums and closure of the gut to harmful bacteria.

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