CDC Confirms Human H5 Bird Flu Case in Missouri

September 6, 2024 -- CDC has confirmed a human case of avian influenza A(H5) ("H5 bird flu") reported by the state of Missouri. The case was identified through that state's seasonal flu surveillance system. The specimen was forwarded to CDC for confirmatory testing per usual protocols and confirmed yesterday. An investigation into the potential exposure is ongoing by the Missouri Department of Health and Senior Services (DHSS).

Case Information

Missouri DHSS reports that the patient, who was hospitalized, had underlying medical conditions, was treated with influenza antiviral medications, subsequently discharged, and has recovered. There is no immediate known animal exposure. No ongoing transmission among close contacts or otherwise has been identified.

This is the 14^th human case of H5 reported in the United States during 2024 and the first case of H5 without a known occupational exposure to sick or infected animals. H5 outbreaks in cattle have not been reported in Missouri, but outbreaks of H5 have been reported in commercial and backyard poultry flocks in 2024. H5N1 bird flu has been detected in wild birds in that state in the past.

While other novel flu cases have been detected through the country's national flu surveillance system, this is the first time that system has detected a case of H5. Targeted H5-outbreak specific surveillance has been conducted as part of ongoing animal outbreaks and has identified all the other cases. In this case, the specimen from the patient originally tested positive for flu A, but negative for seasonal flu A virus subtypes. That finding triggers additional testing.

CDC continues to closely monitor available data from influenza surveillance systems, particularly in affected states, and there has been no sign of unusual influenza activity in people, including in Missouri.

Identification of the neuraminidase (the "N") in the patient specimen is pending further sequencing. Attempts to sequence the genome of the virus also are underway at CDC.

Based on available data, CDC's current assessment is that the risk to the general public from H5N1 remains low. CDC's recommendations related to H5 virus have not changed at this time. As always, circumstances may change quickly as more information is learned. The results of this investigation will be particularly important in light of the current lack of an obvious animal exposure. It is important to note that, while rare, there have been novel influenza A cases where an animal source cannot be identified. The main concern in these situations is that no onward transmission is occurring. Findings from the ongoing investigation will inform whether guidance changes are needed.