ALTERATIONS IN BOVINE PLATELET-FUNCTION AND ACUTE-PHASE PROTEINS INDUCED BY PASTEURELLA-HAEMOLYTICA A1

Platelet function was assessed by aggregometry in 10 Holstein calves b efore and after exposure to Pasteurella haemolytica (biotype A, seroty pe 1) by intrabronchial challenge. At 24 h after exposure the platelet s had become more reactive to stimulation with known platelet agonists such as adenosine diphosphate (ADP) and platelet-activating factor (P AF) and the platelet aggregates that formed were more resistant to dis aggregation. The activation of platelets was an early response in the challenged calves as platelet function had returned to pretreatment le vels 72 h after exposure to the bacteria while the acute phase reactan t proteins, haptoglobin and fibrinogen, were approaching their peak va lues and alpha(2)-macroglobulin levels had also risen significantly (P < 0.05) at this time. The plasma levels of these proteins were still elevated and albumin levels were depressed 6 d post-treatment. At post -mortem all calves exhibited pneumonic tissue damage. When P. haemolyt ica leukotoxin was added directly to bovine platelet suspensions both spontaneous aggregation and an increase in the aggregation response to ADP and PAF stimulation were observed. The morphological appearance o f the platelet aggregates exhibited the typical pattern for bovine pla telets with 2 distinct zones of cells being visible within each aggreg ate. One zone contained platelets in which the cytoplasmic granules we re still evident and the other zone contained irregularly shaped plate lets devoid of granular content. In the latter zone, discrete gaps, or pores, were evident in the plasma membrane of numerous platelets. Thi s pore formation is characteristic of leukotoxin action and is not obs erved in ADP or PAF induced aggregates.