Foals are born with little or no immunity to infection from outside organisms. There is a minimal amount of immunoglobulin-M (IgM) produced in utero, but insufficient to ensure adequate external protection. Foals acquire that immunity, initially, through the mare's colostrum, which should be high in immunoglobulins.
There is a system in the foal's gut that allows absorption of immunoglobulins during the first 12 to 24 hours of life. The level of absorption decreases during that period, which is why it is essential to have the foal nurse as soon as possible after birth. This is also why it is important to inoculate mares against tetanus and other infectious diseases in the last thirty days of pregnancy, as this will assist in ensuring that there are suitable antibodies transferred to the foal in the colostrum. Another associated important point is that of transferring a mare to her foaling location preferably no fewer than 30 days prior to her anticipated due date. This allows the mare to develop suitable antibodies to any organisms present that the foal will be exposed to in the first few hours of its life, and these antibodies should then be available to the foal in the mare's colostrum. If the mare is remaining in the same barn or farm, the chances that different pathogens may be encountered to which the foal will fail to receive immunity is limited; if however the mare is being moved to a different farm - perhaps to be bred back live cover to a stallion located elsewhere - then the risk is more significant.
Ensuring that a foal receives adequate colostrum is not however a guarantee in itself of protection against Failure of Passive Transfer (FPT). There is the possibility that the colostrum may not contain immunoglobulins or that the foal for some reason is not absorbing them. For this reason, it is always advisable to test the foal's blood for immunoglobulin levels ideally between 9 and 12 hours after birth. Desirable levels are greater than 800 mg/dL; levels between 400 and 800 mg/dL should be considered partial FPT's, and may require supplementation depending upon circumstances; levels below 400 mg/dL are FPT's and should definitely be supplemented. Testing in this time frame allows additional oral supplementation, whereas testing after about 15 hours will require IV transfers to infer immunity if levels are found to be lacking.
Failure of passive transfer has no primary symptoms in itself, other than a predisposition to the foal developing infections. Diagnosis of FPT is by the testing of blood IgG levels. It should also be noted that FPT is not in itself a fatal condition and there are foals which experience FPT but which never become sick. In order to develop neonatal septicaemia or another threatening condition related to the lack of immunity, the foal must encounter the causative pathogen. It should also be noted that good passive transfer does not guarantee that the foal will not develop neonatal septicaemia or other conditions, but that it is only a stepping stone towards assisting in prevention.
Supplementation comes in the form of plasma transfusions, through commercially produced serum or plasma products such as "Seramune", or with colostrum from another source. It should be noted that oral supplementation must be given prior to 24 hours post foaling, and preferably prior to 12 hours. Post-supplementation IgG testing is also recommended, with a repeated treatment if required, as research has shown that multiple doses of oral serum may be required to achieve adequately elevated levels, compared to IV plasma, however the latter tends to carry a greater risk factor and more expense.
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