La. Cheryk et al., ALTERATIONS IN BOVINE PLATELET-FUNCTION AND ACUTE-PHASE PROTEINS INDUCED BY PASTEURELLA-HAEMOLYTICA A1, Canadian journal of veterinary research, 62(1), 1998, pp. 1-8
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.