H5N1 Avian Influenza: 70 Human Cases in the United States, Including One Death, Linked to Dairy Cattle

The H5N1 virus is no longer confined to birds. In the United States, farmworkers have been infected after direct contact with infected dairy cows. A new study highlights this concerning development.  

An Avian Virus Adapting to Mammals

The highly pathogenic H5N1 avian influenza virus, well known for its devastating effects on poultry farms, recently reached an unexpected milestone. In 2024, several U.S. states reported outbreaks of infection among dairy cows. This cross-species transmission—unprecedented on this scale—led to a series of human cases, now documented in a study published in *Nature Medicine*. A total of 70 human infections were confirmed between March 2024 and May 2025. This situation marks a turning point in the history of the H5N1 virus, which is moving away from its traditional host to spread among cattle, opening a new front in the fight against zoonoses.  

Who are the infected individuals?

The study reveals that the majority of infected patients had been in contact with infected animals in a professional setting. In nearly six out of ten cases, the individuals had handled sick dairy cows. About one-third of the cases were linked to industrial poultry farms. A few cases were attributed to backyard poultry, and in a handful of instances, the exposure remains unexplained. This epidemiological profile supports the hypothesis of direct transmission from animals to humans, without an intermediate host, in high-risk occupational settings. The clade identified in all cases is 2.3.4.4b, which has already been predominant in major avian epizootics in recent years.  

Mostly mild symptoms… but one death

Clinically, the infection primarily presented as conjunctivitis. Nearly 90% of patients presented with ocular inflammation, often unilateral, associated with a burning sensation, watery eyes, and persistent redness. Fever and respiratory symptoms were also common, though less consistent. Most cases had a favorable outcome, with no need for hospitalization. However, four patients had to be admitted for respiratory care, including three for viral pneumonia. One death was reported in an immunocompromised individual. These data confirm that, while the H5N1 virus remains potentially dangerous, its clinical presentation in this specific context resembles mild forms in the majority of cases, in contrast to the severe forms observed during previous human outbreaks.  

No human-to-human transmission detected

One of the most reassuring findings of this study is the absence of human-to-human transmission. To date, no secondary cases have been detected among the close contacts or colleagues of infected patients. All infections appear to result from direct or indirect contact with an infected animal or its immediate environment. Nevertheless, some cases in which no clear source of animal exposure could be identified raise questions. Environmental contamination, via aerosols or contaminated surfaces, cannot be ruled out. The absence of sustained human-to-human transmission remains, however, a major risk factor in limiting the risk of a pandemic.  

Why Dairy Cows Are a Game-Changer

Historically, mammals affected by the H5N1 virus were scavengers or predatory species—such as cats, foxes, or sea lions—following the ingestion of infected prey. The case of dairy cows is fundamentally different. As herbivores, not carnivores, they do not exhibit the typical risk behaviors associated with avian influenza. Their widespread infection suggests exposure through inhalation, direct contact with contaminated feces or bodily fluids, or even environmental contamination on dairy farms. This phenomenon transforms dairy farms into potential viral hotspots, a risk that has been largely underestimated until now. The issue is no longer solely avian; it has become an agricultural one in the broadest sense and requires a revision of biosecurity protocols.  

Clear Recommendations for Professionals

The study calls for an immediate strengthening of preventive measures for agricultural workers. Wearing goggles, masks, and gloves while milking, cleaning barns, or handling sick animals is essential. Hand hygiene, work clothing management, and surface decontamination must be standardized. Active surveillance must now also include cattle in early detection systems, alongside poultry. A “One Health” approach appears to be the only effective way to anticipate potential mutations of the influenza virus.