Risk factors for detection of bronchial casts, most frequently seen in endemic H9N2 avian influenza infection, in poultry flocks in Iran

During the last decade, low pathogenic avian influenza (H9N2 subtype) outbreaks with high mortality have been reported in broiler chicken in various countries especially in some Asian and Middle East countries including Iran. Tubular bronchial cast extending to the lower bronchi (BC) is one of the most frequently observed post-mortem lesions in affected broiler chicken during H9N2 low pathogenic avian influenza virus outbreaks. This study was conducted to find out risk factors for high mortality in chickens suffering from respiratory symptoms and showing BC in post-mortem examination. History and general information of the flocks as well as vaccination programs, mortality rate and necropsy findings such as formation of BC in airways were collected for 563 broiler flocks in central part of Fars province, South-West of Iran, during 2001-2006. Results showed that overall mortality rate was 13.52% (95%CI: 12.2-14.8), decreasing from 18% in 2000 to 11% in 2006 (P < 0.01). Corresponding measures for BC were 22% (95%CI: 18-26) with no significant decreasing trend during the study period (P = 0.14). Multivariable logistic regression analysis showed that infectious bronchitis (IB) vaccination, flock size and geographical location were significantly associated with the observation of BC (P < 0.05). Odds of BC observation in flocks with history of IB vaccination was seven times higher than flocks without vaccination, and in small flocks was nearly half of the large flocks. In conclusion, IB vaccine can be one of the candidate risk factors for enhancing the virulence of low pathogenic H9N2 virus in the fields.