Pathogenicity of an avian influenza virus serotype H9N2 in chickens

A low pathogenic avian influenza virus (AIV) serotype H9N2 affected many commercial flocks in the Middle East in late 1990s and early 2000s. Due to the varying pathogenicity of AIV H9N2 reported in previous studies, this study was carried out to determine the pathogenicity of a Jordanian isolate of H9N2 in broiler and specific-pathogen-free (SPF) chickens. Mild tracheal rales were observed in the broilers but not in the SPF birds starting 3 days postinfection (DPI) and until the end of the experiment at 16 DPI. Infected chickens had gross and histologic changes limited to the respiratory system (sinuses, trachea, lungs, and air sacs) characterized by congestion and lymphoplasmacytic inflammation. However, the lesions in the broiler chickens were more severe than those in the SPF chicks. Furthermore, the virus caused significant (P 5 0.004) reduction (230 g) in average body weight of the infected broiler group compared with the uninfected broiler group. Both broiler and SPF-infected groups seroconverted, and they had a geometric mean titer of 2^8.2 and 2 ^9.3, respectively, on the hemagglutination inhibition test at 16 DPI. Cloacal virus shedding was not detected by 9 DPI and 15 DPI in broiler and SPF-infected groups, respectively. This study demonstrated the pathogenic nature of the local Jordanian H9N2 isolate and the variation from what it has been reported in other countries of the region. Regional effort should be directed to start an eradication program of this disease because of its pathogenicity for chickens, wide distribution, and possible interference with surveillance for H5N1 serotype.