Epidemiologist affiliated with the Center for Disease Control and Prevention (CDC) and the California Department of Public Health have published on their investigations of 46 U.S. human cases of H5N1 influenza1. Cases occurred in six states from March through October 2024 and were associated with occupational exposure among workers involved in depopulation of infected flocks and contact with dairy cattle.

The mild clinical signs including conjunctivitis and respiratory signs were documented. Among the 20 poultry-related exposures and 25 dairy workers, there were no hospitalizations and no evidence of infection among 97 direct household contacts. Only 25 percent of the patients had received a seasonal influenza vaccination over the past 12 months. Among the poultry workers, 65 percent were provided with eye protection and a face mask or respirator, and PPE use was regarded as “suboptimal”.

Inoculation of ferrets with the A/Texas/37/2024A (H5N1) virus led to severe infection and mortality. In contrast the A/Michigan/90/2024A (H5N1) virus was less pathogenic in this laboratory animal. The Texas strain had two changes in polymerase proteins that facilitate replication in mammalian tissue culture.

The severe case of unknown origin affecting a teenager in Canada and the fatal case in an elderly patient in Louisiana were characterized as a D1.1 genotype associated with wild birds. 

An accompanying editorial2 in the New England Journal of Medicine, stressed the following:-

• Collaboration is necessary among agencies and investigators in human and veterinary medicine, public health and occupational medicine.

• Mutations identified in the virus isolated from the case in Canada emphasize the need for surveillance of avian influenza viruses isolated from human to predict changes that may facilitate person-to-person transmission.

• Countermeasures including antiviral therapy and vaccines must be continually evaluated including susceptibility to single drugs or combination of antivirals.

• Prevention of occupational exposure is evident. Apart from providing masks, respirators and face shields, instruction and supervision in their correct use will reduce the probability of infection.

The editorial emphasized the need for balance between “enhanced vigilance and business as usual." Epidemiologic studies are necessary to acquire an understanding of the extent of exposure, viral evolution and transmission. Although the CDC has established that the risk of human infection is currently low the emergence of a contagious strain would potentially be catastrophic. This realization should be a major consideration in suppressing infection in herds and flocks that come into contact with infected birds and animals.

1. Garg, S. et al Highly Pathogenic Avian Influenza A (H5N1) Virus Infection in Humans. N. Engl. J. Med (2025) 392:843-854
2. Ison, M. and Marrazzo, J. The Emerging Threat of H5N1 to Human Health. N.Engl. J. Med (2025) 392:916-918