Gallibacterium anatis and Mycoplasma gallisepticum are bacterial pathogens affecting the respiratory tract of broiler chickens. Tracking the mixed experimental infection effect with G. anatis and M. gallisepticum in broiler chickens regarding clinical signs, performance, PM and histopathological lesions was the aim of this work. Birds inoculated with G. anatis (G2) or M. gallisepticum (G4) alone showed less severe clinical signs and gross pathological lesions in the respiratory tract. While, co-infection with G. anatis and M. gallisepticum (G3) exaggerated the clinical signs like rales, sneezing, nasal discharge, coughing, lacrimation and open mouth breathing and also gross and microscopical lesions which appeared as severe conjunctivitis with hemorrhagic congestive tracheitis, pneumonia and hyperplasia of the epithelium lining of trachea as compared to other groups which were infected with single pathogen or control negative group (G1). In conclusion, G. anatis produce less severe clinical signs and lesions regarding respiratory system than coinfection with M. gallisepticum which may be a most important factor enhancing the pathogenicity of G. anatis under field conditions.
Key words: Key words: Gallibacterium anatis, Mycoplasma gallisepticum, chickens, pathogenicity, PCR, coinfection