Studies describe H5N1 avian influenza cases of 2024
Almost all H5N1 cases reported to the CDC in recent months were associated with animal exposure and led to mild illness, but a Canadian adolescent got severely ill with a variant that showed significant mutations, new research revealed.
Recent cases of H5N1 avian influenza in the U.S. and Canada were analyzed in a new study, case report, and editorial.
The U.S. study included 46 patients who had laboratory-confirmed H5N1 reported to the CDC in March to October 2024; 20 were exposed to infected poultry, 25 were exposed to dairy cows, and only one patient had no identified exposure. In that patient, H5N1 was detected through routine surveillance during a hospitalization for nonrespiratory illness. None of the other patients were hospitalized. Conjunctivitis was the most common symptom, affecting 42 patients (93%), while 49% had fever and 36% had respiratory symptoms. A third of patients had conjunctivitis as their only symptom. Most patients (87%) received oseltamivir, started a median of two days after symptom onset. No cases were identified among patients' household contacts.
The study authors concluded that recent cases of H5N1 in humans have generally caused mild, short illness and that conjunctivitis was the predominant symptom. “Why recent U.S. cases have generally been clinically mild remains unclear; early detection and initiation of antiviral treatment may play a role,” they wrote. The authors called for additional efforts to increase the use of personal protective equipment among those exposed to animals.
The case report, which like the CDC study was published by the New England Journal of Medicine on Dec. 31, 2024, described a 13-year-old girl in British Columbia. She initially presented to an ED with a two-day history of conjunctivitis in both eyes and a one-day history of fever. She was discharged home without treatment but returned three days later in respiratory distress with hemodynamic instability. She recovered after being treated with extracorporeal membrane oxygenation, renal replacement therapy, and a combination of amantadine, baloxavir, and oseltamivir. H5N1 was cultured from the patient's respiratory specimens and was found to have significant mutations.
An accompanying editorial by CDC officials highlighted some lessons about H5N1 from both research publications, including the need for collaboration among human and veterinary health experts, vigilant surveillance of emerging mutations, development and testing of medical countermeasures, and increased use of precautions to prevent infection, especially personal protective equipment in occupational settings.
“The CDC still designates the risk of [highly pathogenic avian influenza] A(H5N1) to most Americans as low. We do have candidate vaccines and antivirals available to try to mitigate severe influenza in the case of wider spread. That said, a balance between enhanced vigilance and ‘business as usual’ is needed,” the editorialists wrote.