Incidence of Pasteurella haemolytica in the respiratory tract of apparently healthy chickens and chickens with infectious bronchitis. characterisation of 213 strains.

The tracheal flora of healthy chickens consisted mainly of Gram-positive microorganisms. A shift to a predominantly Gram-negative flora, mainly due to Escherichia, was observed in chickens with respiratory symptoms caused by infectious bronchitis. Most of the chickens in which E. coli formed a predominant part of the tracheal flora also showed a positive serological reaction to Mycoplasma gallisepticum. A raised incidence of Pasteurella haemolytica with increase in age was noted both with regard to flocks and number of chickens examined. No differences other than those due to age were related to the incidence of P. haemolytica in trachea of chickens with and without respiratory symptoms. None of the isolated strains corresponds to earlier described biotypes of P. haemolytica. Intraperitoneal injection of P. haemolytica did not affect 1-day-old chickens, nor did simultaneous intratracheal injection of 14-day-old chickens with P. haemolytica and infectious bronchitis virus exacerbate subsequent infectious bronchitis. It can be presumed that P. haemolytica in certain environments constitutes a part of the normal tracheal flora in chickens beyond a certain age, and that invasion apparently only takes place in cases of extreme general debilitation.