SUMMARYThis is the first extensive report on the identification and characterization of Avibacterium paragallinarum (AVP) isolates obtained from outbreaks of infectious Coryza (IC) in layer flocks from Sonora State in Mexico with an IC vaccination history. Isolates obtained from IC outbreaks during the years 2007 up to and including 2019 were identified by conventional PCR test and 16S rRNA gene analysis, serotyped by Page serotyping and genotyped by the recently described partial sequence analysis of the HPG2 region. Furthermore, antimicrobial susceptibility profiles were determined by a recently improved MIC test. The conventional PCR test and the 16S rRNA analyses confirmed the isolates as AVP. Serotyping results showed the involvement of isolates belonging to serotypes A, B and C in the IC outbreaks. Genotyping of the HPG2 region revealed the presence of Sequence Type (ST)1, ST4 and ST11 of which the latter is also identified in Europe. The Minimal Inhibitory Concentration (MIC) susceptibility test showed that all tested isolates were susceptible for the majority of tested antimicrobials including erythromycin and tetracycline which are important antibiotics for the treatment of IC. Vaccination failures due to inadequate vaccination application procedures should be ruled out in the control of IC in Sonora State.

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