AN INHERITED PLATELET-FUNCTION DEFECT IN A SIMMENTAL CROSSBRED HERD

An inherited bleeding disorder, resembling Simmental hereditary throma bopathy (SPIT), has been identified in a Simmental crossbred herd. In an affected bull calf, initially evaluated because of excessive bleedi ng from a vaccination site, the platelet aggregation response to the a gonist, adenosine-diphosphate (ADP) was essentially absent and the agg regation response to platelet activating factor (PAF(16)) was reduced by at least 70%, The initial laboratory assessment of platelet functio n in the dam and sire yielded results which were within normal limits, The sire was mot available for further testing, The dam, also a daugh ter of this sire, was subsequently shown to have a partially reduced a ggregation response to ADP. Of 18 other offspring of the sire evaluate d, 6 were also identified as having a partially impaired aggregation r esponse, The maximum aggregation response to ADP and PAF(16) in these 6 calves was approximately 50% of the level exhibited by unaffected an imals, In contrast, the coagulation profiles were normal for all anima ls except for a heifer calf which also exhibited a partially impaired aggregation response, The plasma level of the coagulation protein, fac tor XI, was reduced in this heifer calf which suffered a fatal hemorrh age following dehorning, This report appears to be the Ist to have ide ntified animals putatively heterozygous for SHT on the basis of the in vitro platelet aggregation response to ADP.