National Trends in Backyard Poultry–Associated Salmonella Outbreaks Relative to U.S. Hatchery Chick Production, 2009–2021

INTRODUCTION: Backyard poultry ownership has expanded substantially in the United States, coinciding with a persistent rise in poultry-associated Salmonella outbreaks. No prior study has computed an outbreak rate per unit of hatchery chick production, an imperfect proxy for backyard-flock chick availability examined here. METHODS: A national time-series analysis linked CDC NORS-ACOSS outbreak data for 2009-2021 with annual U.S. hatchery chick production (broiler-type and egg-type combined; USDA-NASS QuickStats), an aggregate, imperfect exposure proxy discussed in Limitations. Backyard poultry-attributed Salmonella outbreaks and illnesses were the primary outcomes (108 outbreaks; 8003 illnesses; 1636 hospitalizations; 13 deaths). Denominators were confirmed from QuickStats for 2012-2021; 2009-2011 values were estimated by backcasting (see Limitations). Outbreak and illness rates per 100 million chicks hatched were examined for secular trend and pandemic-period deviation. RESULTS: The outbreak rate increased 1.6-fold across the confirmed 2012-2019 period (1.3-fold under an egg-type-only denominator; see Limitations); using the less certain 2009 baseline, the increase was 4.6-fold, to 0.152 in 2019. In 2020, the rate rose to 0.200, a 32% increase over 2019, despite near-stable hatchery output. Illness rates increased 4.1-fold across the confirmed period (1.7-fold under an egg-type-only denominator); the 2009-baseline figure (162-fold) is highly uncertain and should be interpreted cautiously (see Limitations). Eighty-three percent of outbreaks (90/108) were multistate, accounting for 98.8% of illnesses, consistent with, though not proof of, hatchery-level contamination. CONCLUSIONS: This study presents, to the author's knowledge, the first published estimate of a backyard poultry-attributed Salmonella outbreak rate per unit of hatchery chick production in the United.