Haemostatic mechanisms of the newborn foal: reduced platelet responsiveness.

Whole blood platelet counts, coagulation profiles and in vitro platelet function tests were monitored in newborn foals during the first week of life. Platelet counts, mean platelet volumes and thrombin-induced malondialdehyde production were not different from adult mares. Prothrombin and partial thromboplastin times were slightly, but not significantly, longer for neonatal blood samples than for mare samples. Platelet aggregation responses to serotonin, arachidonic acid or adrenaline did not change during the study. On the other hand, adenosine diphosphate-induced aggregation and collagen-induced aggregation increased progressively over the first week of life. Adrenaline exposure diminished adenosine diphosphate-induced aggregation only during the first 12 h of life. The results of this study indicate that the haemostatic mechanisms of equine neonates are immature at birth and that, during the maturation period, the equine neonate may be at risk of platelet-associated haemorrhagic disorders.