H5N1 Bird Flu: 2 Cases Confirmed in California – CDC Update 2024

The Centers for Disease Control and Prevention (CDC) has confirmed two additional human cases of H5 bird flu in California, bringing the total number of U.S. Cases to 17 since 2022. These latest cases, identified from specimens submitted in October 2024, involve individuals with known exposure to infected dairy cows. While the emergence of human cases is being closely monitored, the CDC maintains that the overall public health risk remains low.

Dairy Cow Link and Ongoing Investigation

Both confirmed cases in California involved people whose jobs brought them into contact with dairy cows infected with the H5N1 virus. Importantly, health officials have found no evidence linking these two cases to each other, suggesting separate instances of animal-to-human transmission. An investigation led by the California Department of Public Health is currently underway to understand the extent of the spread and potential risks.

The first detection of H5N1 in U.S. Cows occurred earlier this year, with outbreaks initially reported in August 2024 in California dairy herds. This marked the first time the virus had been identified in cows within the United States. Prior to this, H5N1 was widespread in wild birds and had been causing outbreaks among poultry across the country since 2022. Human infections have been rare, typically occurring in individuals with direct contact with infected animals, such as dairy and poultry workers.

Understanding H5N1 Bird Flu

Bird flu, likewise known as avian influenza A (H5N1) or highly pathogenic avian influenza (HPAI), is a viral infection that primarily affects birds. Still, as noted by the California Department of Public Health, the virus can, in rare instances, infect mammals, including humans. Transmission to humans typically occurs through close contact with infected birds or their bodily fluids, or through inhalation of the virus. The San Diego County Health and Human Services Agency emphasizes that while the current risk to the public is low, vigilance is crucial.

Case Breakdown and Geographic Distribution

As of October 3, 2024, the CDC reports a total of 17 confirmed human cases of H5 bird flu in the United States since February 2024. These cases have been reported across several states: Texas (1), Michigan (2), Colorado (10), Missouri (1), and California (2). A significant portion of these cases – six out of the 16 reported in 2024 – have been directly linked to exposure to sick or infected dairy cows. Nine cases involved exposure to infected poultry. The source of infection for a single case in Missouri remains under investigation, with serologic testing of contacts ongoing.

Symptoms and Testing

Symptoms of H5 bird flu in humans can vary, but often include eye redness (conjunctivitis), mild flu-like upper respiratory symptoms, fever, cough, sore throat, difficulty breathing, runny or stuffy nose, and muscle aches. The CDC recommends that individuals who experience these symptoms after close contact with infected animals should contact their local health department and a healthcare provider for testing.

Preventive Measures and Guidance

The CDC has issued specific recommendations to protect individuals from H5N1 bird flu, particularly those working with dairy cattle. These recommendations include avoiding contact with wild birds and sick or dead animals, and refraining from consuming unpasteurized (raw) milk or raw cheese. For farm workers and others who may come into contact with potentially infected animals, the CDC emphasizes the importance of wearing appropriate personal protective equipment (PPE), such as gloves, masks, and eye protection. Updated PPE guidance was recently released by the CDC to enhance protection against infection.

Surveillance and Monitoring Efforts

The CDC is actively collaborating with state and local health officials to monitor the spread of H5N1 and assess the risk to public health. This includes ongoing surveillance of both animal and human populations. Data on human cases are now integrated into routine influenza surveillance systems, with monthly updates available on the CDC website. Information on animal infections is available through the U.S. Department of Agriculture (USDA).

FluView Interactive and Data Reporting

The CDC’s FluView Interactive provides a comprehensive overview of influenza activity, including data on H5N1 cases. The agency is also conducting additional characterization of the viruses identified in the recent human cases, including sequencing to track viral evolution.

What to Expect in the Coming Weeks

The CDC will continue to closely monitor the situation and provide updates as new information becomes available. Ongoing efforts will focus on supporting state-led investigations, tracking the spread of the virus in both animal and human populations, and refining public health recommendations as needed. The agency is also working to understand the potential for the virus to mutate and adapt, which could impact its transmissibility and severity. Regular reviews of the current risk assessment will be conducted to ensure that public health measures remain appropriate and effective.