Experimental assessment of pathogenicity and infectivity of H9N2 influenza virus isolated from a natural outbreak

Abstract

The H9N2 viruses can cause asymptomatic to clinical diseases in chickens under field conditions. Therefore, the present study was designed to assess the pathogenicity of Indian H9N2 avian influenza virus in experimentally infected specific pathogen free (SPF) chickens. The intravenous pathogenicity index of H9N2 virus isolated from Bareilly, Uttar Pradesh was found to be 0.00/3.00 suggesting its low pathogenicity in chickens. For infectivity study, six SPF chickens were inoculated intranasally with 100 μl of 106 EID 50 of A/Chicken/India/04TI05/2012 (H9N2) avian influenza virus. The sham phosphate buffered saline inoculated SPF chickens served as negative control. All the six H9N2 virus inoculated chickens got infected and virus could be isolated from oropharyngeal and cloacal swabs. More viral isolations were obtained from oropharyngeal swabs (38) as compared to cloacal swabs (16). Viral RNA was detected through 1 to 9 days post infection (dpi) by RT-qPCR. High viral RNA copy number (1.56x 103-1.2x 105) was detected in oropharyngeal swabs than cloacal swabs (1.15x1034.81x103). Clinically, all the H9N2 virus and sham inoculated birds did not show any overt signs of illness. All the experimental chickens seroconverted and the geometric mean haemagglutination inhibition titers at 6 and 12 and 30 dpi were 27.5, 28.33 and 27.66, respectively. This study indicates that the H9N2 virus isolated from a natural outbreak was of low pathogenicity under experimental conditions. Continuous shedding by the infected birds by oral and cloacal routes without any clinical signs might cause undetected spread of the virus under field conditions.