One Health in Action: A Collaborative Approach to Highly Pathogenic Avian Influenza Viruses  

This is part of a series about topics presented during APHL ID Lab Con 2025, a forum to discuss the latest developments on the detection and characterization of infectious diseases of public health concern.   

What do cows, birds and cats have in common? They’re all susceptible to highly pathogenic avian influenza (HPAI) viruses, the most well-known of which is influenza A (H5N1).  

HPAI viruses infect animals, who can then infect people, making them zoonotic diseases. And because of their different hosts and various modes of transmission, zoonotic diseases can be challenging to control. One Health is an approach that recognizes the many important factors that impact human health and the role of interactions between animals, people and the environment they exist in.  

During APHL ID Lab Con 2025, one session on the current HPAI H5N1 outbreak in the US demonstrated the impact of a One Health approach in action. The session highlighted the collaboration between public health laboratories, clinical laboratories, animal health professionals and food safety laboratories and showed how these agencies have responded to the recent HPAI outbreaks in cows, cats and people.   

Birds, cows and cats, oh my! 

While HPAI viruses have circulated in birds in the US for many years, the recent viral strain of H5N1 started to gain traction in 2022. In March 2024, H5N1 was reported for the first time in dairy cattle in the US. In public health, there is always the question of what’s next? And in this case, the what’s next occurred when a cat with severe illness tested positive for H5N1. 

One speaker at ID Lab Con, Dr. Anthony Tran with the California Department of Public Health, talked about the role California’s public health system played in testing cats for HPAI. Cats are particularly susceptible to severe disease from H5N1 and could be our “canary in the coal mine,” acting as our early warning sign that outbreaks are occurring and that additional testing may be needed. With neurological symptoms in H5N1-positive cats resembling rabies symptoms, it became important to expand the testing for rabies-negative cats to include HPAI testing. 

The session also featured Dr. Julie Breher, a veterinarian with the San Diego County Public Health Laboratory in California. Dr. Breher talked about the laboratory’s animal influenza A testing program and the effective coordination between public health laboratories and commercial veterinary diagnostic laboratories to identify influenza A in animals.  

The San Diego County Public Health Laboratory collaborates with the local wildlife rehabilitation center and a commercial veterinary laboratory to identify and test for H5N1 in animals who had exposure to humans. The public health laboratory coordinates with their epidemiology units to identify submissions and report testing results. This allows for the public health laboratories to monitor people who were exposed to HPAI-positive animals, highlighting the efficient coordination of animal testing and the downstream impact on monitoring exposure in people to control the spread of an H5N1 outbreak. 

What’s in the milk anyways? Raw milk testing for influenza 

California is one of the largest producers of milk in the US and, as such, has many dairy herds that have been impacted by the H5N1 outbreaks. To help with rapidly identifying diseases and monitoring trends, California counties ramped up milk testing in 2024.  

Dr. Brandon Bonin from the Santa Clara County Public Health Laboratory shared that starting in June 2024, the laboratory had started testing three samples per week. By November 2024, they had detected their first H5N1-positive raw milk sample. As a result of collaboration with California Animal Health & Food Safety Laboratory and the National Veterinary Services Laboratory, Santa Clara County was able to confirm their milk testing results and a voluntary recall was issued. Thanks to this initiative and collaboration, the recall came before any human cases associated with the positive milk samples were identified.  

In addition to California, 45 states are enrolled in the United States Department of Agriculture’s (USDA) National Milk Testing Strategy, which began in December 2024. Laboratories testing for influenza in milk and other sample types are required to report positive results to USDA to improve the coordination of a response and outbreak control. 

Breaking the mold: Expanded testing captures the unique cases 

With milk testing ongoing, there were simultaneous efforts to increase testing to identify the influenza A strains responsible for HPAI virus outbreaks across California.  

Similar to other emerging infectious disease outbreaks, testing to identify H5N1 was initially limited to individuals who met a specific case definition or were considered to have high-risk exposure, such as dairy or poultry farm workers. While this helped increase the probability of capturing positive cases, it did leave a chance for missing spillover cases. Dr. Benjamin Pinsky, a pathologist with Stanford Health Care, spoke about the expanded testing algorithm implemented within their laboratory network in the summer of 2024 to screen influenza A-positive patients for H5N1, even if they did not meet current case definitions. 

By November 2024, the first H5N1-positive pediatric case in Alameda County, CA, was identified due to the expanded testing efforts of Stanford Health Care. This case was unusual: A 14-month-old presented to urgent care with flu-like symptoms and tested positive for influenza A. Follow-up testing to identify the influenza A strain determined that this child was positive for H5N1. Since this child was not exposed to infected animals and did not consume any raw milk products, this case would have been missed if it were not for additional screenings to identify H5N1 in influenza A- positive patients. Public health laboratories across California also increased their testing initiatives and were able to focus their resources on recruiting specimens from clinical laboratories that were not already testing for H5N1. 

Infectious disease outbreaks are dynamic; they can rapidly change depending on the pathogen itself, the host it infects and the environment that facilitates its spread. When multiple hosts enter the equation, as with zoonotic diseases, the outbreak dynamics can shift even quicker. Thanks to the multidisciplinary One Health system that has been tackling the HPAI outbreaks, there is a better ability to understand transmissions between hosts and learn how to best prevent additional spread across animal and human populations.