Pathogenesis of Escherichia Coli in Previously Mycoplasma Gallisepticum Infected Layer Chicks

Two weeks-old seventy five layer chicks were divided into 2 groups of 60 (A) and 15 (B) birds. Chicks of group A were inoculated subcutaneously and intratracheally with Mycoplasma gallisepticum while the chicks of group B were left as uninoculated controls. At the age of 3, 4 and 5 weeks, 15 chicks each from group A were inoculated with pathogenic strain of E. coli. Variable clinical signs were recorded in different groups. In Mycoplasma infected birds, coughing, sneezing and mucoid nasal discharge were observed during initial stages and moist rales by the end of 2nd week of inoculation. In mixed infections, onset was rapid with more severe clinical manifestations. In Mycoplasma infected birds, tracheitis and cloudiness of airsacs were the main gross pathological lesions observed. In mixed infections, there were more advanced lesions, including airsacculitis, fibrinous pericarditis and perihepatitis. Airsacs were filled with caseous material in few birds. Serious involvement of the liver, heart, lungs and trachea were also noted. Histopathological studies in Mycoplasma infected birds revealed mononuclear cellular infiltration and epithelioid cells in tracheal mucosa. Mucosal glands of the trachea were also enlarged. In birds inoculated with E. coli after Mycoplasma, there was severe cellular infiltration and sloughing of tracheal mucosa. In lungs, congestion, focal areas of necrosis and emphysema were seen, almost in every case. Congestion, hemorrhages and leukocytic infiltration were recorded in sections of liver from many birds.