H5N1 Bird Flu in US Dairy Herds: CDC Surveillance Update 2026

What Is H5N1 Bird Flu and Why Is It Concerning?

Highly pathogenic avian influenza (HPAI) H5N1 belongs to the influenza A virus family and has been circulating in wild bird populations globally for decades. The current dominant strain, clade 2.3.4.4b, has demonstrated an unprecedented ability to infect a wide range of mammalian species, including dairy cattle — a host previously not considered susceptible to avian influenza. Since early 2024, the USDA has confirmed HPAI H5N1 detections in dairy herds across more than a dozen US states, marking a significant and unexpected expansion of the virus's host range.

The concern among public health officials centers on the virus's potential to acquire mutations that facilitate efficient human-to-human transmission. Historically, H5N1 infections in humans have carried a high case fatality rate — exceeding 50% in some outbreak settings, according to WHO data. However, the cases linked to the current US dairy and poultry outbreaks have been mild, predominantly presenting as conjunctivitis, suggesting this particular strain may behave differently in humans. The CDC continues to assess the public health risk as low for the general population but has urged heightened vigilance among agricultural workers.

Influenza viruses are prone to genetic reassortment, particularly when co-circulating in hosts that can harbor both avian and human influenza strains. Pigs have traditionally been considered "mixing vessels" for such reassortment, but the detection of H5N1 in cattle raises new questions about mammalian adaptation pathways. Genomic surveillance by the CDC, USDA, and international partners is focused on identifying any mutations in the PB2 gene (particularly E627K and D701N substitutions) that are associated with enhanced replication in mammalian cells.

How Is the CDC Responding to H5N1 Detections?

The CDC's response has been multi-pronged, combining active surveillance, laboratory preparedness, and risk communication. The agency expanded its influenza surveillance systems to include testing of individuals with occupational exposure to infected animals, particularly dairy farm workers and poultry workers. As of late 2025, the CDC had confirmed a limited number of human H5N1 cases in the United States, all linked to direct animal contact, with no evidence of onward person-to-person spread. State and local health departments have been equipped with H5N1-specific RT-PCR testing kits to enable rapid identification of cases.

On the vaccine preparedness front, the US government has invested in the production and stockpiling of pre-pandemic H5N1 vaccine candidates. The Biomedical Advanced Research and Development Authority (BARDA) has contracted with manufacturers including CSL Seqirus and Sanofi to produce cell-based and recombinant H5N1 vaccines. Clinical trials of updated candidate vaccines matched to the clade 2.3.4.4b strain have been underway, with preliminary immunogenicity data showing adequate antibody responses in healthy adults after two doses. The Strategic National Stockpile also maintains antiviral medications, including oseltamivir (Tamiflu), which remains effective against current H5N1 strains.

Internationally, the WHO has coordinated with the Food and Agriculture Organization (FAO) and the World Organisation for Animal Health (WOAH) under the One Health framework. Genomic data sharing through the Global Initiative on Sharing All Influenza Data (GISAID) platform has enabled real-time monitoring of viral evolution. The WHO's Global Influenza Surveillance and Response System (GISRS) continues to update candidate vaccine viruses and risk assessments as the epidemiological situation evolves.

What Are the Symptoms of H5N1 in Humans?

Human infection with H5N1 avian influenza can present with a wide spectrum of clinical manifestations. In historical outbreaks — particularly those reported from Southeast Asia, Egypt, and China — the disease often manifested as severe pneumonia with rapid progression to acute respiratory distress syndrome (ARDS), multi-organ failure, and death, with case fatality rates reported between 30% and 60% by the WHO. Common initial symptoms included high fever, cough, dyspnea, and myalgia, often progressing within days to respiratory failure requiring intensive care.

However, the clinical picture observed in the recent US outbreak has been notably milder. The majority of confirmed human cases associated with dairy herd and poultry farm exposures presented primarily with conjunctivitis (eye redness and irritation), sometimes accompanied by mild upper respiratory symptoms such as cough and rhinorrhea. Only a small number of cases developed lower respiratory tract involvement. This milder presentation may reflect differences in the viral strain, the route of exposure (ocular or mucosal splash vs. deep respiratory inhalation), or host factors. The CDC recommends that any individual with occupational exposure to potentially infected animals who develops respiratory symptoms or conjunctivitis seek testing promptly.

Treatment with neuraminidase inhibitors, particularly oseltamivir, is recommended by the CDC for confirmed or suspected H5N1 cases, ideally initiated within 48 hours of symptom onset. For severe cases, higher doses and prolonged courses may be necessary. The newer antiviral baloxavir marboxil (Xofluza) has also shown in vitro activity against H5N1, though clinical experience remains limited. Supportive care including supplemental oxygen and mechanical ventilation may be required for patients who progress to severe disease.

How Can the Public Reduce H5N1 Risk?

For the general population, the risk of H5N1 infection remains low. The virus does not currently spread efficiently between people, and routine daily activities do not pose a risk. The CDC and FDA have confirmed that the commercial milk supply is safe because pasteurization effectively inactivates influenza viruses, including H5N1. Studies conducted by the FDA demonstrated that standard pasteurization parameters (72°C for 15 seconds) are sufficient to eliminate viable H5N1 virus from milk. Similarly, properly cooked poultry and eggs (internal temperature of 74°C / 165°F) pose no risk of transmitting the virus.

Individuals who work directly with poultry, dairy cattle, or wild birds are at the highest risk and should follow occupational safety guidelines issued by the CDC and OSHA. Recommended personal protective equipment (PPE) includes N95 respirators, eye protection (goggles or face shields), gloves, and fluid-resistant coveralls. Workers should be enrolled in active monitoring programs and report any symptoms — including conjunctivitis — to occupational health services promptly. The CDC also recommends that exposed workers receive seasonal influenza vaccination to reduce the risk of co-infection with human and avian strains, which could theoretically facilitate viral reassortment.