A Sharifi - Department of Poultry Health and Diseases, Faculty of Veterinary Medicine, Urmia, Iran
M Allymehr - Department of Poultry Health and Diseases, Faculty of Veterinary Medicine, Urmia, Iran
A Talebi - Department of Poultry Health and Diseases, Faculty of Veterinary Medicine, Urmia, Iran

Control strategy of respiratory complex infections should address precipitating and predisposing causative agents in general and immunosuppressive agents in particular. In both clinical and subclinical forms, infectious bursal disease virus (IBDV) is one of the most immunosuppressive diseases of young chickens. This study aimed to investigate the concurrent occurrence of subclinical infectious bursal disease (IBD) and multicausal respiratory complex infections caused by Newcastle disease virus (NDV) and avian metapneumovirus (aMPV) in broilers. In this study, ۸۰۰ tissue samples (e.g., trachea, cecal tonsil, bursa of Fabricius, and spleen) and ۴۰۰ sera samples were collected from broilers with confirmed respiratory signs selected from ۲۰ broiler farms in west Azerbaijan province, Iran, from October ۲۰۱۸ to February ۲۰۱۹. Pathogens in the tissue samples were detected using RT-PCR for the VP۲ gene of IBDV, F gen of NDV, and N gene of aMPV. The amplified products were sequenced afterward. At the end of the husbandry period, sera samples were used to detect antibodies against IBDV, aMPV, and NDV using ELISA and HI tests. Molecular results showed that the ۴۵% (۹/۲۰), ۳۰% (۶/۲۰), and ۱۵% (۳/۲۰) of tissue samples were positive for IBDV, NDV, and aMPV, respectively. Regarding co-infection, ۵% (۱/۲۰) of farm isolates were positive for IBD and ND, while ۱۰% (۲/۲۰) of farms isolates were positive for IBD and aMPV. Co-infection of IBD, ND, and aMPV was not detected in farm isolates. Serological results indicated that the IBD co-infected flocks had almost higher (P<۰.۰۵) antibody titers against IBD; however, IBDV-NDV co-infected flocks and IBDV-aMPV co-infected flocks had lower antibody titer against NDVand aMPV, respectively. It can be concluded that lower antibody titer against ND and aMPV in IBD-ND and IBD-aMPV co-infections indicated suppressive effects of IBD on these diseases. Therefore, vaccination against IBD even in regions without clinical form of IBD is inevitable for the reduction of immunosuppressive effects of subclinical IBD on immune responses against these diseases.

Avian metapneumovirus, Broiler, Bursal disease virus, Newcastle disease, Respiratory complex