Pasteurella multocida isolates associated with ovine pneumonia are toxigenic.

The P. multocida toxin (PMT), a dermonecrotic protein encoded by the toxA gene, is the major virulence factor of capsular type D P. multocida strains causing progressive atrophic rhinitis (PAR) in pigs. A high frequency of P. multocida isolates harboring the toxA gene has been found among ovine pneumonic isolates, although the ability of these isolates to express PMT has never been examined. In this study we have investigated the ability of ovine toxA+ P. multocida isolates (n = 57) to express a functional toxin by detection of PMT toxin antigen using an ELISA test and its cytopathic effect in a Vero cell assay. PMT antigen was expressed in the great majority (54/57; 94.7%) of toxA+ isolates. Moreover, the 100% toxA+ ovine isolates analyzed produced a cytopathic effect in Vero cells within 24-48 h post-inoculation, identical to that described for porcine toxigenic P. multocida isolates. These results show for the first time that, in addition to isolates associated with PAR, isolates of P. multocida associated with pneumonia in sheep are also toxigenic. In addition, we found a total agreement (Kappa = 1; C.I. 0.75-1.25) between the detection of the toxA gene and the toxigenic capability of P. multocida isolates, indicating the PCR detection of toxA would be a suitable predictive marker of the toxigenic fitness of P. multocida.