Abstract:
Avibacterium paragallinarum causes infectious coryza in chickens, an acute respiratory disease that has economic significance in worldwide poultry industry. The objectives of this study were to determine the serovars, antimicrobial sensitivity, pathogenicity, virulence of capsular extraction, lipopolysaccharide and outer membrane protein extraction of Av. paragallinarum isolated from Thailand. Eighteen field isolates of Av. paragallinarum were confirmed by PCR. Serotyping by a hemagglutination inhibition test, 10 isolates were serovar A, 5 isolates were serovar B, and 3 isolates were serovar C. The susceptibility of the isolates to 17 antimicrobial agents was tested by a disk diffusion method. All isolates were susceptible to amoxicillin-clavulanic acid and lincospectin. Most strains were resistance to streptomycin and erythromycin (>90%). All isolates were resistant to cloxacillin, lincomycin and neomycin. A study of bacterial entry into, and survival within, chicken macrophages showed variation between isolates but no clear connection to serovar. A virulence test was performed by challenging 4 weeks old layers via the nasal route with 400 µl of bacteria (108 colony forming units/ml). Clinical signs were observed daily for 7 days and the birds were subjected to a post-mortem necropsy at seven days post-challenge. All 18 field isolates caused the typical clinical signs of infectious coryza and could be re-isolated at 7 days after challenge. There was no significant difference in the clinical scores of the isolates except that two isolates of 112179 and 102984 belonged to serovar A and B, respectively, gave a significantly higher score than isolate CMU1009 (serovar A) did. No correlation between serovar and severity of clinical signs was found. The virulence study of capsule, OMP and LPS extraction at concentration 1000 µg/egg via yolk sac route, LPS of virulent isolates Av. paragallinarum could cause chicken embryo death whilst OMP and capsule could not.
