H5 Bird Flu: First US Case in 2024 Without Known Animal Exposure – CDC Update

The Centers for Disease Control and Prevention (CDC) has confirmed a human case of avian influenza A(H5) – commonly known as H5 bird flu – in Missouri. This marks the 14th human case of H5 reported in the United States during 2024, and notably, the first instance identified through the country’s routine seasonal flu surveillance system without a known direct exposure to infected animals. The case was initially detected by Missouri’s Department of Health and Senior Services (DHSS) and subsequently confirmed by the CDC on September 6, 2024.

Case Details and Patient Recovery

According to the Missouri DHSS, the patient was hospitalized due to underlying medical conditions. They received treatment with influenza antiviral medications and have since been discharged and fully recovered. Crucially, investigators have not yet identified any immediate known animal exposure linked to this case, nor have they detected any ongoing transmission to close contacts. This lack of a clear animal source is a key aspect of this particular case, prompting further investigation.

Although outbreaks of H5 have been reported in commercial and backyard poultry flocks in the U.S. Throughout 2024, and H5N1 bird flu has been detected in wild birds in Missouri previously, this case doesn’t fit the pattern of known exposures seen in the other 13 human cases reported this year. The CDC continues to closely monitor influenza surveillance systems, and currently, there’s no indication of unusual influenza activity in people, including within Missouri.

How This Case Was Identified

The initial detection occurred since the patient’s specimen tested positive for influenza A, but negative for the typical seasonal flu A virus subtypes. This unusual result triggered additional, more specific testing, ultimately leading to the confirmation of H5N1 bird flu. This highlights the importance of the national flu surveillance system’s ability to detect novel influenza strains, even when they don’t present as typical seasonal flu.

Investigation and Genetic Sequencing

The Missouri DHSS is currently conducting a thorough investigation to determine the potential source of the infection. The CDC is likewise working to sequence the genome of the virus, and to identify the neuraminidase (“N”) component of the strain. This genetic analysis will be crucial in understanding the virus’s characteristics and potential for spread.

What Does This Mean for Public Health?

The CDC emphasizes that, based on the available data, the risk to the general public from H5N1 remains low. Current recommendations related to H5 virus have not changed. However, the agency acknowledges that circumstances can evolve rapidly as more information becomes available. The ongoing investigation is particularly important given the absence of an obvious animal exposure, a scenario that, while rare, has been observed in previous novel influenza A cases.

The primary concern in these situations is to ensure that no further human-to-human transmission occurs. Findings from the investigation will inform whether any adjustments to existing guidance are necessary. A recent report from the CDC, detailed on their website, investigated close contacts of the patient and found no evidence of human-to-human spread. Six healthcare workers and one family member who had symptoms were tested, and all were sero-negative, meaning they showed no signs of previous H5N1 infection.

Understanding H5N1 and Risk Assessment

Avian influenza A(H5) viruses are known to circulate primarily among wild birds. While human infections are rare, they can occur when people come into close contact with infected birds or their environments. The H5N1 strain is particularly concerning due to its potential for causing severe illness in humans. However, it’s important to remember that the vast majority of human cases have been linked to direct or indirect exposure to infected poultry or wild birds.

The CDC’s assessment of risk is based on a number of factors, including the virus’s ability to infect humans, the severity of illness it causes, and the potential for human-to-human transmission. Currently, the virus does not appear to be easily transmitted between people, which is why the overall risk to the public remains low. However, ongoing surveillance is essential to detect any changes in the virus’s behavior.

Current Surveillance Efforts and Future Steps

The CDC is utilizing its existing influenza surveillance systems to monitor for H5 activity in people. This includes monitoring data from state and local health departments, as well as conducting targeted surveillance in areas affected by outbreaks in animals. As of February 18, 2026, the CDC has streamlined its reporting of A(H5) bird flu updates, integrating them into routine influenza data updates. Data on the number of people monitored and tested for bird flu are now available monthly.

the USDA data on highly pathogenic avian influenza (HPAI) detections in animals is now available on the USDA’s website, rather than the CDC’s. This shift reflects the current public health situation and allows the CDC to focus its resources on monitoring human cases and potential outbreaks.

The results of the ongoing investigation in Missouri will be critical in informing future public health guidance and surveillance strategies. The CDC will continue to monitor the situation closely and provide updates as latest information becomes available. Individuals should consult with a qualified healthcare professional if they develop flu-like symptoms, especially if they have had recent contact with birds or poultry.