Evaluation of the Pathogenicity of A/Chicken/Iran/ZMT-173/99 H9N2 Strain of Avian Influenza Virus in Serologically Mycoplasma Galisepticum Positive and Negative Broiler Chickens

Avian influenza (AI) is the most important poultry diseases in Iran. The objective of this study was the evaluation of the Intravenous Pathogenicity Index (IVPI) of A/Chicken/Tehran/ ZMT-173/99(H9N2) strain, isolated from the kidney of the broiler chickens with 40% mortality. Two groups of chickens with Myco plasma Galisepticum (Mg) positive and negative (Mg + and Mg-) status were inoculated via intravenous (IV) rout, and two other similar groups, were inoculated through both IV and oculonasal (ON) routs with 107.5 ELD50 The pathogenicity index of the virus in all groups was below 1, indicating the low pathogenesis of H9N2 strain for chickens. The great difference between pathogenesis of H9N2 strain in Mg + and Mg- groups, were 10 percent mortality in each Mg + groups. Formation of casts in tracheal bifurcation was observed only in chickens inoculated via IV along with ON routs and died at 5th day PID. This observation suggests that neous inoculation of chickens play an important role in formation of tracheal casts. Kidney and lymphoid tissues including thymus and bursa of fabricious were more severely affected in the chickens that were inoculated via IV rout alone and died at 7th day PID. Demonstration of H9N2 tropism to lymphoid organs indicates a probable immunosuppressive effect of H9N2 strain for chickens. Based on the results obtained, it is suggested that tracheal casts and severe nephritis with urate deposition are the principal cause of mortality in commercial Mg + chickens. Eventual immunosuppresion of chickens due to H9N2 infection as well as bacterial co-infection such as M. gallisepticum and E. coli might have increased the pathogenesis of H9N2 infection.