H5N1 Avian Flu in California, 2024: 37 Dairy Workers and the First U.S. Child Case

Between September 30 and December 24, 2024, 38 people in California tested positive for highly pathogenic avian influenza (HPAI) A(H5N1). Thirty-seven were dairy farm workers who had handled sick cows. The last was a child with no known exposure — the first child with H5N1 found in the United States. Everyone had mild illness, and no spread from person to person has been found in the United States.

How the outbreak was tracked

On August 30, 2024, the California Department of Food and Agriculture detected H5N1 in cows on three dairy farms in the Central Valley, confirmed by the National Veterinary Services Laboratories. The state health department (CDPH), agriculture officials, local health departments and affected farms then worked together:

• Farm owners or managers checked workers daily for symptoms and reported them; where they didn't, the local health department called or texted workers itself.
• Workers with symptoms were swabbed — eye (conjunctival), nose or throat — depending on their symptoms, and tested for A(H5) at local or state public health labs, with CDC confirming.

The first two cases, confirmed on October 3, were workers on separate farms with no known link to each other. Both had conjunctivitis, and one had a fever; staff advised isolation and offered the antiviral oseltamivir to them and their households.

The dairy workers

Symptoms were mostly in the eyes:

No one was hospitalized, no one died, and all recovered. All 37 were offered oseltamivir; two declined. No household contacts of the workers were infected — consistent with the viruses lacking the genetic markers for efficient spread between people.

The eye swab was the test that most often came back positive — 35 of 37 patients — well ahead of combined nose-and-throat swabs (8 of 29).

The child

The child was found through routine flu surveillance, not farm monitoring. Previously healthy, they had no known contact with infected animals or people and had not consumed unpasteurized dairy. They had mild respiratory symptoms and otitis media, no conjunctivitis, were not hospitalized, and recovered quickly — unlike children with H5N1 in other countries, who had severe illness.

The child's three household members also had respiratory symptoms, but all tested negative for A(H5) four days after the child's first test; the child and two of them tested positive for adenovirus and rhinovirus. The source of the child's infection remains unknown.

What the virus sequences showed

Phylogenetic tree of the 16 fully sequenced viruses. Image from CDC's report

Viruses from 30 patients were sequenced, and all were clade 2.3.4.4b. Full genomes came from 16, partial from 14. One virus carried a change linked to reduced susceptibility to baloxavir; no other concerning changes turned up.

Protecting workers

Most of the infected workers said they used some personal protective equipment (PPE) — eye protection, gloves, gowns or boots — at work. But earlier reports found that use of the recommended gear, such as N95 respirators rather than face masks, is low among dairy workers. Education about the risks and better access to PPE, ideally worked out with farm worker organizations and producers, could raise its use.

The authors recommend that public health agencies:

• investigate flu-like illness or conjunctivitis in anyone who works with infected animals
• work with dairy and poultry farms on PPE guidance, training and distribution, and on worker monitoring and testing
• test in ways that tell A(H5) apart from seasonal flu
• give oseltamivir to patients as treatment and to close contacts as prevention
• investigate every human case's exposure, and watch the virus for changes that could allow antiviral resistance or person-to-person spread
• consider expanded A(H5) subtyping in people who meet exposure and clinical or public health criteria, which may find more cases with no known exposure

Coordinating public health, agriculture, animal, occupational and environmental health and health care — a One Health approach — is key to catching changes early.

Limits

• The type of PPE and how much of the time it was worn weren't known for every patient, and access to PPE wasn't assessed.
• Some sick workers may never have been reported, so some infections may have been missed.

Sources

Based on Sophie Zhu and colleagues (California Department of Public Health, CDC and county health departments), "Human Cases of Highly Pathogenic Avian Influenza A(H5N1) — California, September–December 2024," MMWR, volume 74, Centers for Disease Control and Prevention; a work of the United States government in the public domain. The report's table counts PPE answers over 32 workers but lists 37, so no share is given here.