ABSTRACT: Avibacterium (Haemophilus) Paragallinarum (Av. Paragallinarum) is the causative agent of Infectious Coryza (IC) in chickens. Despite the worldwide distribution of IC, no systematic study, to the best of our knowledge, was conducted on isolation and characterization of Av. Paragallinarum in Iran. The present study aimed to isolate and perform antibacterial susceptibility testing (AST) of IC agents from suspected backyard chickens with typical symptoms of IC in avian markets. From 18 collected choanal swab samples, four (22%) isolates of Av. Paragallinarum were detected by culture methods based on satellite growth on blood agar, which was confirmed by the biochemical reaction of Catalase and Oxidase tests and species-specific PCR (HPG-2). The hypervariable region of the hemagglutinin genes of 4 isolates was amplified and obtained sequences were deposited at a gene bank for more characterization. Meanwhile, 12 (66%) positive reactions were detected by observing expected 500 bpb and using PCR (HPG-2) on swab samples. Antibiotic susceptibility testing (AST) of obtained isolates were analyzed using the Kirby-Bauer disk diffusion method on Columbia agar with horse blood. Isolates were found to be resistant to amoxicillin, oxytetracycline, streptomycin, trimethoprim/sulfamethoxazole (up to 75%) and sensitive to cefalexin, ceftriaxone, enrofloxacin, florfenicol, gentamycin, linco-spectin, neomycin, doxycycline (50%), danofloxacin (75%), flumequine (50%), ofloxacin (75%). An intermediate growth inhibitionzone has been observed around antibiotic discs for ampicillin, colistin, erythromycin, penicillin, tiamulin (75%), tylosin (75%). In summary, to the best of our knowledge, this study is the first report of isolation and identification of Avibacterium Paragallinarum from backyard chickens which may be a source of IC for commercial chicken flocks. Moreover, the prevalence of resistance to some antibacterial drugs of IC agents may impose an additional threat to the poultry industry. A more in-depth study is recommended to develop a low-cost autogenous IC vaccine for small-scale flocks of poultry to prevent and manage the disease and establish antimicrobial resistance.
