Introduction

Since early 2022, a highly pathogenic avian influenza A virus subtype H5N1 (HPAI H5N1) has been actively spreading in the United States. Initially detected in wild birds, the virus has caused significant outbreaks in poultry and, more recently, in dairy cattle. This blog post analyzes the current situation using data from the Centers for Disease Control and Prevention (CDC), highlighting key findings and potential public health considerations.

Widespread Bird and Animal Infections

As of May 28, 2024, the CDC reports extensive H5N1 detections across various animal populations. Wild birds are the primary reservoir, with all 50 states confirming positive cases (total: 9,373). Sporadic outbreaks have affected commercial poultry flocks as well, with over 92 million birds impacted across 48 states. A concerning development is the multi-state outbreak involving dairy cattle that emerged in March 2024. As of May 28th, there are ongoing cases in 9 states with a total of 67 affected herds.

Human Cases and Surveillance

Since April 2022, three confirmed human infections with H5N1 have occurred in the United States. The first case involved a person working directly with poultry. The two most recent cases (April and May 2024) were individuals linked to outbreaks in dairy cow herds, suggesting potential occupational exposure. Importantly, no evidence of human-to-human transmission has been identified.

The CDC has implemented a comprehensive surveillance strategy to monitor potential human spread. This strategy includes:

• Monitoring people with animal exposure for 10 days (over 9,400 people monitored since February 2022).
• Analyzing public health laboratory data and clinical trends for influenza activity.
• Tracking emergency department visits for influenza-like illness.
• Monitoring wastewater for influenza A virus levels (though not specific for H5N1).

Genetic Mutation and Public Health Risk

A recent concern arose with the detection of a mutated H5N1 virus in the second Michigan dairy worker case. This mutation exhibits characteristics that suggest adaptation to mammals, raising concerns about potential increased transmissibility between humans. However, the CDC emphasizes the similarity of this strain to existing vaccine candidates, suggesting some level of preparedness.

The CDC currently considers the public health risk associated with H5N1 to be low. However, the ongoing animal outbreaks and the recent human case with a potentially adapted strain necessitate continued vigilance and close monitoring efforts.

Conclusion

The ongoing H5N1 activity in the United States presents a complex situation with implications for animal and potentially human health. Continued surveillance efforts, research into the mutated strain, and international collaboration are essential to mitigate the spread and potential public health consequences.

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Sources

• Centers for Disease Control and Prevention (CDC). How CDC is monitoring influenza data to better understand the current avian influenza A (H5N1) situation in people. https://www.cdc.gov/flu/avianflu/index.htm Accessed May 30, 2024.
• Centers for Disease Control and Prevention (CDC). H5N1 Bird Flu: Current Situation Summary. https://www.cdc.gov/flu/avianflu/index.htm Accessed May 30, 2024.
• Centers for Disease Control and Prevention (CDC). CDC A(H5N1) Bird Flu Response Update May 24, 2024. https://www.cdc.gov/flu/avianflu/inhumans.htm Accessed May 30, 2024.