Failure of passive transfer in foals
November 6, 2019
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Did you know that foals are not able to produce antibodies before birth and they do not receive them from the placenta? They rely entirely on the first milk received from the mare (colostrum) for the passive transfer of immunity. The vital antibodies they receive through the colostrum will protect them for the first 2 months of life.
Antibodies, also known as immunoglobulins, play a vital role in the immune system, the body’s defence mechanism. They are the first line action against intruders like viruses and bacteria. Antibodies bind to the intruders and help the cells of the immune system to recognise and destroy them. If none or only a few antibodies are present in the bloodstream, the cells of the immune system will not be able to recognise intruding antigens, significantly increasing the risk of life-threatening disease.
There is a multitude of reasons why this transfer of antibodies might not occur, including both maternal and foal related causes
1) The concentration of antibodies in the first milk rapidly decreases within the first 24 hours after lactation begins. Therefore it is imperative to recognise premature lactation in the mare as a risk factor even if the foal is alert and drinking early after birth.
2) In some cases, the quality of the mare’s colostrum is not good enough, as mainly happens in maiden mares and in older mares. You can evaluate the quality of the colostrum by appearance and by using a refractometer (typically used for assessment of wine). Good quality colostrum should be yellow and sticky in appearance and have a reading of ≥ 1.060 on the refractometer.
3) It is very important to ensure that the foal gets up and suckles quickly after birth; within the first hour of life. In some instances, however, the foal does not do this because of weakness, limb deformities or other diseases that make the process of getting up to suckle difficult.
Whatever the cause, inadequate absorption of antibodies by the foal, also called failure of passive transfer, happens in up to 38% of foals. Failure of passive transfer is not a disease on its own, but it predisposes to the development of the disease. A significant number of foals with failure of passive transfer (up to 35- 50%) develop disease requiring medical intervention within the first 3 months of life.
Therefore, it is highly recommended to determine whether the concentration of antibodies in the foal’s blood is adequate. This is done by taking a blood sample from the foal when it is approximately 24hrs old and measuring the concentration of a common type of antibodies in the blood (IgG). IgG levels ≤400mg/dl signify complete failure of passive transfer (FPT), IgG levels of ≥400mg/dl and ≤800mg/dl signify partial failure of passive transfer (pFPT) and IgG levels ≥800mg/dl signify the adequate passive transfer of immunity.
Management of FPT and pFPT depends on how high the infection pressure is in the foal’s environment and how good the level of protection is (FPT versus pFPT). Lowering infection pressure can be achieved by for example stabling the mare and foal away from yearlings and competition horses, keeping the stall and equipment clean, washing your hands before handling of the mare and foal and making sure the mare has been adequately dewormed prior to foaling. Adequate vaccination of the mare prior to foaling can also lower infection pressure and provides increased levels of antibodies in the colostrum.
When medical treatment of FPT is considered necessary, this consists of administering additional colostrum to the foal during the first 12 hours of life. When the foal is more than 12 hours of age, the antibodies cannot be sufficiently absorbed from the gut into the blood, and intravenous administration of plasma can then be used to increase the level of antibodies.