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.