A genetic analysis of samples from the Louisiana patient hospitalized with the country’s first severe case of H5N1 bird flu reveals the virus likely mutated within the patient, potentially increasing its transmissibility to humans. However, there is currently no evidence of human-to-human transmission.
The CDC reported earlier this month that the patient likely contracted the virus after contact with sick and dead birds in a backyard flock. A subsequent analysis on Thursday identified mutations in samples taken during the patient’s hospitalization that were absent in the birds themselves, indicating these mutations weren’t present in the virus circulating widely in wildlife. These mutations, similar to those observed in a British Columbia patient, may enhance the virus’s ability to bind to human upper respiratory tract cell receptors. The CDC clarified that these changes likely arose from viral replication within the patient during advanced disease, rather than being present at the time of initial infection. While concerning, the agency emphasized that the mutations would be more alarming if found in animal hosts or early in infection, where they might facilitate spread to close contacts.
The CDC stressed that the risk to the general public remains low. Nevertheless, the detection of these genetic mutations highlights the continued need for genomic surveillance in both humans and animals, containment of avian influenza outbreaks in poultry and dairy cattle, and preventative measures for individuals exposed to infected animals or environments. Importantly, the analysis found no changes affecting the effectiveness of antiviral drugs and noted the viral samples are closely related to strains usable for vaccine production, if necessary. Further, no changes were observed in genes associated with mammalian adaptation. The patient was infected with the D1.1 strain, closely related to viruses circulating in U.S. wild birds and poultry, unlike the widely spreading B3.13 strain found in dairy cows, which hasn’t caused severe human illness in the U.S.
Despite the seemingly positive aspects of the analysis, virologist Dr. Angela Rasmussen cautioned that the H5N1 situation remains serious, citing a surge in human cases. She emphasized the unknown combination of mutations that could trigger a pandemic, highlighting that increased human infections raise the probability of a pandemic strain emerging. In 2024, the CDC confirmed 65 human cases of H5N1 bird flu; 39 linked to dairy herds, 23 to poultry farms and culling operations, and two with unknown sources. The Louisiana case remains the only one associated with backyard flocks.
Dr. Paul Offit, a vaccine scientist, noted the CDC’s cautious phrasing regarding the mutations’ ability to enhance cellular binding, stating a need for clearer evidence of this effect. He further emphasized the lack of clinical evidence of human-to-human transmission as a more critical point. The absence of widespread transmission significantly reduces the immediate public health concern, despite the concerning mutations detected in the Louisiana patient.