CDC Confirms 3rd Human Case of H5 Bird Flu in California | CDC Newsroom

The Centers for Disease Control and Prevention (CDC) has confirmed a third human case of H5 bird flu in California. This latest case, like the two previously confirmed in the state, involves an individual with occupational exposure to infected dairy cows. All three cases identified in California to date have occurred in dairy workers on separate farms, with no known contact between them, indicating the virus is spreading from animals to people. The individual is experiencing mild symptoms, specifically conjunctivitis – commonly known as pinkeye – and has not required hospitalization. This aligns with the clinical presentation of the earlier cases in California. The CDC continues to state that the overall risk to the general public remains low.

With this new case, a total of 18 human infections with H5 bird flu have been reported in the United States since 2022, with 17 of those occurring in 2024. The CDC is currently awaiting confirmatory testing results on two additional specimens from California that have initially tested positive for the virus.

Viral Sequencing and Genetic Analysis

Preliminary sequencing data from the first two California human cases revealed the virus to be a clade 2.3.4.4b A(H5N1) virus, closely related to those detected in dairy cattle. Whole genome sequencing of one of the initial cases (A/California/135/2024) identified a B3.13 genotype. Importantly, the genetic analysis has not revealed any mutations that would suggest increased transmissibility between humans or reduced susceptibility to available antiviral medications. These sequences have been publicly shared on GISAID and GenBank for further research and monitoring. Sequencing of the third confirmed case is currently underway.

Understanding the Current Situation and Risk

The emergence of H5 avian influenza in U.S. Dairy herds, first reported in August 2024, has prompted increased surveillance and monitoring. While sporadic human cases have been linked to direct contact with infected animals, the virus has not demonstrated efficient human-to-human transmission. The CDC emphasizes that the current cases are consistent with animal-to-human spread, and the risk to the broader public remains low. Still, ongoing monitoring is crucial to detect any changes in the virus’s behavior.

What the CDC Recommends

The CDC continues to recommend specific precautions for individuals with potential exposure to infected birds or other animals, including livestock. These recommendations, detailed on the CDC website, aim to minimize the risk of infection for farmworkers and others who may reach into contact with infected animals or contaminated environments. Interim recommendations include practicing good biosecurity, using personal protective equipment, and reporting any potential exposures to public health officials.

Monitoring and Surveillance Efforts

The CDC has streamlined its reporting of H5 bird flu cases, integrating them into routine influenza surveillance data updates. Monthly updates on the number of people monitored and tested for bird flu are available on the CDC website. However, the CDC no longer reports USDA data on highly pathogenic avian influenza (HPAI) detections in animals; this information is now available directly from the USDA. The CDC will continue to report any additional confirmed human cases through its regular FluView reports and dedicated updates.

Looking Ahead: Ongoing Research and Public Health Response

The CDC is actively collaborating with state and local health departments to monitor the situation, investigate cases, and refine public health recommendations as needed. Ongoing research focuses on characterizing the virus, understanding its transmission dynamics, and assessing the effectiveness of available antiviral medications. The agency is also working to enhance surveillance systems to detect and respond to potential outbreaks quickly and effectively. Continued vigilance and proactive measures are essential to protect public health and minimize the impact of this evolving situation.