Bird Flu Among Poultry Cullers: The First U.S. Cluster of H5 Cases, Colorado, July 2024

People who work closely with dairy cattle or poultry infected with highly pathogenic avian influenza (HPAI) A(H5N1) are at higher risk of catching it. In July 2024, Colorado saw the first known cluster of human influenza A(H5) cases in the United States tied to poultry.

At a glance

What happened

Facility A, a commercial egg-layer operation in northeast Colorado:

Facility B, in the same county:

Facility B reported no illness and high use of protective gear at first, so preventive oseltamivir wasn't offered. Instead CDPHE set up routine screening and, over six visits, tested and treated 44 symptomatic workers. Between visits, team leads at both facilities screened workers before shifts. Two symptomatic workers at facility B declined testing and treatment.

As culling wound down, CDPHE handed out cards in English and Spanish on the symptoms of avian flu, where to get care, and what to tell a health care provider about the exposure.

Screening results

CDPHE screened 663 workers from July 11 to 26. Their median age was 30 (range 15–56), and most spoke only Spanish. Symptomatic workers answered a short questionnaire and gave nasopharyngeal and conjunctival (eye) swabs, tested at the CDPHE lab; presumptive positives went to CDC for confirmation.

Work start, screening and symptom onset dates for symptomatic workers, by facility. Onset dates were known for 25 of 65 workers at facility A and 39 of 44 at facility B. CDC

Protective gear

Among symptomatic workers, self-reported use varied widely:

The nine cases

• Five women and four men, median age 32 (range 18–56). Two had diabetes, one asthma, and one was a long-time smoker.
• All had direct contact with sick or dead birds during culling and disposal.
• Symptoms began a median of 1 day after first exposure (range 1–8), and workers were tested a median of 2 days after symptoms began.

Breathing symptoms were less common. All nine got oseltamivir. Symptoms cleared in seven a median of 4 days after onset (range 1–8); two, last reached two days in, reported conjunctivitis that was ongoing or improving. One person sought outpatient care. Four still-symptomatic workers retested negative 1–5 days later.

By swab: both eye and nose swabs were positive in three, only the eye swab in five, and only the nose swab in one. Virus was grown from five workers, and sequencing identified it as clade 2.3.4.4b genotype B3.13 — the genotype circulating in dairy cattle.

Why it matters

Before this, the U.S. had seen five human H5 cases: one in 2022 in Colorado from poultry, and four among dairy workers from April to July 2024. HPAI A(H5N1) had turned up in Colorado's dairy herds and poultry flocks that year.

These cases looked like the dairy-linked ones: work exposure, mostly mild. But H5N1 can cause severe disease and death, so prompt investigation and treatment matter. The authors can't fully rule out contamination — virus particles in the nose or eyes without infection — but think most cases were real infections:

• four of the nine were tested on arrival in the morning, before that day's exposure;
• all had conjunctivitis and were tested within three days of symptoms, as in earlier poultry-worker cases;
• live virus was grown and full gene segments sequenced from five.

Why culling is risky

Besides handling and disposing of dead birds, the main culling method at both farms meant handling every live bird — in cage-free barns where hens roam and must be caught by hand. That increases exposure and the chance of gear slipping or tearing. Providing and training a large, hastily hired workforce in proper gear was hard; some use was inconsistent or improper, and extreme heat made wearing it difficult. Still, cases also occurred at facility B, where gear use was higher — though not 100%.

Employers should put worker safety first, with engineering, administrative and protective-equipment controls.

Reaching the workers

With a mostly Spanish-speaking migrant workforce, multilingual teams including Spanish speakers were key to building trust, screening, testing and treating. On-site screening and quick testing widened access to care and likely found more cases. Testing also mattered because many workers in the same enclosed space had symptoms from other causes — SARS-CoV-2 among them, and likely respiratory irritation from the environment.

Getting ready for more

With H5N1 still circulating, public health agencies should prepare for more human cases on dairy and poultry operations:

• stock and distribute protective equipment, and train field teams to use it;
• plan large-scale screening, sampling and lab testing that can tell H5 from seasonal viruses;
• secure oseltamivir and standard protocols for treatment and prevention;
• plan for the cultural and language needs of local farm workers;
• take a One Health approach — human, animal and environmental health together — working with industry, labor and agricultural regulators.

Sources

Based on Drehoff CC, White EB, Frutos AM, et al., "Cluster of Influenza A(H5) Cases Associated with Poultry Exposure at Two Facilities — Colorado, July 2024," MMWR volume 73, number 34, Centers for Disease Control and Prevention; a work of the United States government in the public domain.