Atlanta, 26 February 2025 — The US Centers for Disease Control and Prevention (CDC) reported that the multistate outbreak of avian influenza A(H5N1) virus in dairy cows, poultry, and other animals has continued to spread, with 70 confirmed human cases and 989 affected dairy herds across 17 states as of February 2025. Three new human cases identified in early 2025 — in Nevada, Ohio, and Wyoming — included two hospitalisations, underscoring the ongoing risk to agricultural workers.
The CDC continues to assess the immediate risk to the general public as low, but the outbreak's persistence across livestock populations has raised significant concerns for the agriculture sector, food safety, and pandemic preparedness.
The Outbreak's Scope
Since the first detection of H5N1 in US dairy cattle in March 2024, the outbreak has expanded to unprecedented scale:
The outbreak represents the first time H5N1 has been detected in dairy cattle on a widespread basis, a development that has surprised virologists and reshaped understanding of the virus's host range. The CDC, USDA, FDA, and state partners are coordinating the response through a One Health approach that integrates human, animal, and environmental health surveillance.
New Human Cases in 2025
As of February 24, the CDC had confirmed three human cases of H5 bird flu in people who became ill in 2025:
Nevada — Dairy Worker
A dairy worker with exposure to infected dairy cows presented with conjunctivitis (eye redness and irritation) and has recovered. Most infections associated with US dairy cows to date have involved mild respiratory symptoms or conjunctivitis. Genetic sequencing identified the virus as clade 2.3.4.4b, genotype D1.1, with a PB2 D701N mutation previously associated with more efficient virus replication in mammalian cells — the same mutation identified in a human case in Chile in 2023.
Ohio — Poultry Worker
A poultry worker who participated in culling activities on a farm with infected poultry experienced respiratory symptoms and was hospitalised. The person has since been discharged and is recovering. Initial upper respiratory specimens could not be confirmed as positive, but a subsequent specimen was confirmed positive for avian influenza A(H5) virus at CDC.
Wyoming — Backyard Flock Owner
The owner of an infected backyard poultry flock experienced respiratory symptoms and was hospitalised. The individual had underlying health conditions that increase vulnerability to severe influenza illness. Direct contact with poultry that died on their property was the exposure source. The virus was identified as clade 2.3.4.4b, genotype D1.1, with a PB2 E627K mutation previously identified in a human case in Texas during 2024.
Agricultural and Economic Impact
The H5N1 outbreak has had significant consequences for US agriculture:
- Poultry sector: The culling of more than 90.9 million birds to contain the outbreak has disrupted egg and poultry meat supply chains, contributing to record egg price increases in late 2024 and early 2025
- Dairy sector: Infected herds have experienced reduced milk production, and movement restrictions on cattle between states have imposed logistical and financial burdens on dairy operations
- Trade implications: Several countries have imposed restrictions on US poultry and dairy product imports, affecting export markets
The USDA has implemented testing programmes for dairy cattle and milk surveillance, while the FDA has confirmed that the commercial milk supply remains safe due to pasteurisation, which inactivates the virus.
Research Findings
By March 2025, several studies published in peer-reviewed journals provided new insights into the outbreak:
A CDC study published on February 21, 2025, in Emerging Infectious Diseases found that ferrets previously infected with seasonal influenza A(H1N1) virus had developed cross-reactive antibodies to components of H5N1. When later exposed to H5N1, these ferrets exhibited reduced viral replication and decreased onward spread compared to ferrets without prior seasonal influenza infection. These findings suggest that pre-existing immunity from seasonal flu may partially explain why most US H5N1 cases have been mild.
A second study, published on March 17, 2025, in The Lancet Microbe, reported similar findings and additionally demonstrated that ocular (eye) exposure to H5N1 can produce severe and transmissible disease in ferrets — highlighting the importance of eye protection for workers handling infected animals.
A report published on March 7, 2025, in Emerging Infectious Diseases assessed immune responses in two dairy farm workers in Michigan who tested positive for H5N1. One worker, who had mild conjunctivitis, developed neutralising antibodies — the first study in people to assess immunity to clinically mild H5 illness.
Worker Protection and Recommendations
The CDC has emphasised that people with job-related exposures to infected animals — including dairy workers, poultry workers, and backyard flock owners — remain at the greatest risk of infection. The agency recommends:
- Use of personal protective equipment (PPE) including N95 respirators, eye protection, and gloves when working with infected or potentially infected animals
- Monitoring for symptoms for 10 days after exposure
- Prompt reporting of illnesses to public health authorities
- Early treatment with oseltamivir for confirmed or suspected cases
CDC's antiviral susceptibility testing, published in Emerging Infectious Diseases on March 7, 2025, confirmed that H5N1 viruses from the current outbreak remain susceptible to oseltamivir and other neuraminidase inhibitors, supporting current treatment recommendations.
Sources
- CDC, "A(H5N1) Bird Flu Response Update — February 26, 2025"
- CDC, "A(H5N1) Bird Flu Response Update — March 19, 2025"
- USDA Animal and Plant Health Inspection Service (APHIS), HPAI livestock detections, 2025
- Emerging Infectious Diseases, CDC studies, February–March 2025
- The Lancet Microbe, ferret study, March 17, 2025
- Nature Communications Earth & Environment, H5N1 dairy cattle economic impact study, 2025
FIRAT Editorial Board
Institutional Research Desk · Foresight Institute of Research and Translation
The collective editorial and research translation board of FIRAT, synthesising peer-reviewed evidence, policy briefs, and division milestones across our seven foundational research pillars.
