A Retrospective Analysis to Identify Epidemiologic Patterns of the Infectious Coryza Outbreak in California 2016–22

Infectious coryza (IC) is a poultry respiratory disease caused by the bacterium <i>Avibacterium paragallinarum</i>. IC has major economic impacts in both the layer and broiler industries because this disease can cause increased mortality in affected flocks, an increase in condemnations at the processing plants, and decreased egg production in laying hens. In 2017, an increased number of IC-positive cases was seen in the Turlock Branch of the California Animal Health and Food Safety (CAHFS) Laboratory System. The objective of this study was to assess the epidemiologic patterns of IC before, during, and after this outbreak, as the number of positive cases remained elevated from 2017 to the end of 2022. Necropsy cases positive for IC in the CAHFS electronic database were analyzed from January 1, 2016, a year prior to the outbreak, to December 31, 2022. Out of 1935 submissions tested for IC during this period, the 674 cases found to be positive were then categorized by age, poultry type, season, and spatial distribution to investigate risk factors for IC. Raw data trends showed that most of the cases were from backyard poultry flocks (50.5%, 341/674), which appeared to be mainly localized to Los Angeles County and San Joaquin County. Commercial layers made up the second greatest number of cases (31.4%, 212/674) and were mostly localized to Merced County and Stanislaus County. Last, most of the commercial broiler cases (18.0%, 121/674) were in Merced County. Statistical analysis of this raw data was also performed and demonstrated that poultry in commercial egg layer operations located in the Central Valley and southern regions of California were at an increased risk for IC (odds ratio = 1.3, 95% confidence interval: 1.01-1.65, <i>p</i> = 0.039). Ultimately, this study provides updated information on IC trends in California and the risk factors associated with the outbreak.