Anticytotoxin activity of bovine sera and body fluids against Pasteurella haemolytica A1 cytotoxin.

Toxin neutralizing activity of bovine sera and body fluids against Pasteurella haemolytica type A1 cytotoxin was evaluated by 51Cr release assay using bovine peripheral blood mononuclear leukocytes as the target cells. Sera collected from precolostral calves did not exert anticytotoxin activity at 10(-1) or higher dilutions, whereas randomly selected complement fixing antibody-negative sera neutralized on average over 90% of cytotoxin activity at the 10(-1) dilution and less than 50% of the toxin activity at 10(-2) or higher serum dilutions. Nasal secretions and lung washings of some of the cattle tested also contained cytotoxin neutralizing activity. The antibody nature of the cytotoxin neutralizing activity was demonstrated by its neutralization with bovine immunoglobulin G2 purified from pooled seropositive sera. Sera from a group of cattle which were vaccinated with a potassium thiocyanate extract of P. haemolytica, but which subsequently developed fibrinous pneumonia after aerosol challenge with bovine herpesvirus 1 and P. haemolytica, had significantly lower anticytotoxin activity than sera from another group of cattle which did not develop the disease after similar vaccination and challenge. Cattle which survived a natural outbreak of shipping fever had higher anticytotoxin activity than those having fibrinous pneumonia in the aforementioned experimental group, although there was no statistical difference between them and a randomly selected CF seronegative group. It is probable that this cytotoxin neutralizing antibody exerts a beneficial effect in protection of cattle against pneumonic pasteurellosis.