Infection with Mannheimia haemolytica (M. haemolytica) causes severe economic losses among different animal species as sheep, goat, swine and cattle, while in poultry it is usually seen as secondary bacterial infection to other avian pathogens. The present study highlights the impact of M. haemolytica on the health and egg production in White Shaver and Hi-Sex
brown layer flocks in Egypt. The clinical signs were recorded as respiratory manifestations, edematous swelling in the comb and fluctuating decrease in egg production. The necropsy findings revealed air saculitis, pericarditis, perihepatitis, congested and flaccid ova with egg peritonitis. A representative samples of spleen, liver lung , trachea and heart were collected to detect the presence of any avian respiratory viruses by using conventional and RT-PCR as well as, testing for bacterial presence by both phenotypic and genotypic methods. The results revealed detection of M. haemolytica, in the examined samples. The antibiotic sensitivity test was performed on isolated M. haemolytica and the results showed that M. haemolytica was resistance to all tested antibiotics