Salmonella Outbreak Linked to Backyard Poultry
A state health department investigates an outbreak of Salmonella enterica serotype Enteritidis infections. Over six weeks, 23 cases are identified; median age 32 years (range 1–78), 14 are female. Eleven cases report diarrhoea, fever, and abdominal cramps; five were hospitalised. Interviews reveal that 17 cases (74%) had contact with live poultry in the week before illness onset. All 17 had purchased chicks or ducklings from the same feed store chain in the previous month. The feed store sources chicks from a single hatchery. Environmental samples from the store's brooder tanks and from one patient's home coop yield S. Enteritidis with an indistinguishable pulsed-field gel electrophoresis (PFGE) pattern. The hatchery denies any illness in its flock and states that its breeder flocks are vaccinated against Salmonella. The state veterinarian is asked to advise on control measures.
1. What are the key steps in the epidemiological investigation, and what additional information would you gather to strengthen the evidence for the hatchery as the source?
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Key steps: (1) Define a case: laboratory-confirmed S. Enteritidis infection with the outbreak PFGE pattern and illness onset within the outbreak period, or an epidemiologically linked case. (2) Conduct a case-control or cohort study: compare exposures (live poultry contact, feed store, hatchery) between cases and controls. (3) Trace back: obtain purchase records from cases to identify common suppliers; the feed store chain and hatchery are already implicated. (4) Environmental investigation: sample the hatchery (hatchers, chick boxes, water, feed) and the feed store; compare isolates by PFGE and whole-genome sequencing (WGS). (5) Review hatchery records: vaccination status, Salmonella monitoring results, and any recent increases in mortality or illness. Additional information: (a) WGS of isolates from patients, store, and hatchery to confirm clonality; (b) hatchery's Salmonella testing history and vaccination protocols (serovar coverage, timing); (c) feed source and whether feed is pelleted or mash (mash may be contaminated); (d) biosecurity and cleaning practices at the hatchery; (e) whether the hatchery supplies other stores with similar outbreaks; (f) case interviews about handling practices (e.g., kissing birds, cleaning brooders indoors). The hatchery's denial of illness does not rule out asymptomatic carriage; Salmonella can persist in flocks without clinical signs.
2. What control measures would you recommend for the feed store, the hatchery, and the public?
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Feed store: (1) Stop selling chicks from the implicated hatchery until the source is controlled. (2) Clean and disinfect brooders and display areas; provide handwashing stations and signage. (3) Educate customers on hygiene (no kissing birds, wash hands after contact, keep poultry out of the house). (4) Consider treating or culling affected birds; in many jurisdictions, infected birds are not treated with antibiotics due to resistance concerns and public health risk. Hatchery: (1) Conduct a thorough investigation: test breeders, hatchers, and environment; review vaccination and monitoring data. (2) If Salmonella is found, implement control: culling infected flocks, egg sanitation, fumigation, and increased biosecurity. (3) Consider vaccination with a licensed vaccine (e.g., live attenuated) if not already used. (4) Halt distribution until negative. Public: (1) Advise anyone with live poultry to wash hands, not eat or drink around birds, and supervise children. (2) High-risk groups (children <5, elderly, immunocompromised) should avoid contact. (3) Report illness to healthcare providers. (4) Clean poultry equipment outdoors. Regulatory: (1) Notify the state veterinarian and USDA-APHIS if interstate movement is involved. (2) Coordinate with CDC and FDA (feed) as needed. (3) Consider a recall of chicks from the hatchery.