FAILURE OF PASSIVE TRANSFER

When a foal is born it has limited defence against disease because it has no antibodies to fight infection. Normally, the foal receives these antibodies in the colostrum (first milk) from its mother, this is known as ‘passive transfer of immunity’. Colostrum contains antibodies that are relatively specific for the mare’s environment. Foals that don’t receive enough antibodies are at risk of developing life threatening infections as they do not build up significant levels of their own antibodies until three to six weeks of age.

There are many reasons why a foal may not receive enough antibodies after birth. If a mare ‘runs milk’ before foaling she will lose her colostrum, some mares do not produce good quality colostrum (especially old or maiden mares), or the foal may not feed well. There is only a limited period of time of up to about twelve to eighteen hours after birth when antibodies can be absorbed. If it is thought that a foal will not receive enough good quality colostrum, we can administer donor colostrum, which has been stored frozen, by stomach tube, to a foal within the first twelve hours of life.

Even if everything appears normal, it is possible that failure of passive transfer has occurred. The only way to be sure is for us to take a blood sample from the foal to measure the level of antibodies. This test is normally performed on the second day of the foal’s life. This is done as a matter of course on all foals born at Riverdene, and a satisfactory level of antibodies is required for insurance purposes.

In foals with failure of passive transfer, antibody levels can be boosted by the administration of plasma either from the foal’s dam, a donor or from commercially produced plasma. Performing a plasma donation from the dam or another horse involves collecting a large volume of blood, separating the plasma from the cells and administering the plasma to the foal intravenously.

Foals with failure of passive transfer may appear normal for days or weeks and some foals will never show any abnormalities. Others, however, will succumb to infections. The illness will vary but may include fever, lethargy, ‘off suck’, dehydration, lameness (joint or bone infections), diarrhoea, pneumonia etc. In any of these instances your foal should be seen by us as an emergency. Treatment for these conditions may be difficult, time consuming, costly, and not always successful. This is definitely a situation where prevention is better than cure.