Investigations on the carrier rate of Pasteurella multocida in healthy commercial poultry flocks and flocks affected by fowl cholera.

Twenty flocks of web-footed birds (Pekin and Muscovy ducks and geese) and eight flocks of chickens raised under intensive management were examined for the presence of carriers of Pasteurella multocida. Five hundred and seventy-eight web-footed birds and 240 chickens from healthy flocks, as well as from flocks affected by fowl cholera, were investigated. A total of 135 isolates (80 from healthy flocks and 55 from flocks affected by fowl cholera) were obtained from the pharyngeal and cloacal mucosae after mouse passage (134 isolates) and culture in selective medium (one isolate). Thirty-five percent (7/20) of the flocks of web-footed birds and 38% (3/8) of chicken flocks had birds carrying P. multocida in the pharynx and/or cloaca. Birds from flocks affected by fowl cholera carried P. multocida at a significantly higher prevalence in the mucosa of the cloaca (P < 0.001) compared with the pharynx, while the opposite was observed in birds from healthy flocks. Extended phenotypic characterization confirmed the presence of P. multocida ssp. multocida, P. multocida ssp. septica and P. multocida ssp. gallicida in the flocks examined. P. multocida ssp. gallicida was exclusively isolated from Pekin ducks, while P. multocida ssp. multocida and P. multocida ssp. septica were obtained from chickens as well as web-footed birds. Each flock was shown to be infected by a single phenotypic clone, but some clones were found in more than one flock. A different clone was found in each of four outbreaks of fowl cholera on one of the farms in the preceding 2 years. Two genotypic and phenotypic clones each of P. multocida ssp. multocida and P. multocida ssp. septica were found. This observation indicated that outbreaks are usually clonal and that elimination of P. multocida from infected farms is possible. The results suggest that healthy poultry, in addition to convalescent carriers, may also be carriers of P. multocida. However, the virulence of P. multocida isolates and resistance of carriers to clinical infection needs to be examined. This is the first report of isolation of P. multocida from the cloacal mucosa of apparently healthy domestic poultry. Sampling of the cloaca appeared to be more sensitive for detecting carriers of P. multocida. Although selective medium was used only to a limited extent, the results suggested that mouse inoculation was a more efficient method of isolating P. multocida from poultry than the use of selective media.