Wastewater surveillance detects influenza and H5 viruses from human and animal sources

By Dr. Chinta SidharthanReviewed by Susha Cheriyedath, M.Sc.Sep 24 2024

Early wastewater monitoring identifies significant influenza virus levels, highlighting the urgent need for coordinated public health responses as human and animal infections converge during the H5N1 outbreak.

Study: Wastewater Surveillance for Influenza A Virus and H5 Subtype Concurrent with the Highly Pathogenic Avian Influenza A(H5N1) Virus Outbreak in Cattle and Poultry and Associated Human Cases — United States, May 12–July 13, 2024. Image credit: AU USAnakul / Shutterstock

A recent study published in the Morbidity and Mortality Weekly Report summarized data from early wastewater surveillance of the influenza A virus and the H5 subtype in the United States (US).

Wastewater surveillance helps monitor pathogen shedding by humans at a community level. It is independent of testing access, care-seeking behavior, and symptoms. However, current testing methods cannot distinguish between viral particles shed by humans and those from animals, which introduces complexities in interpreting results. Wastewater surveillance of the influenza virus has been performed for years at some sites, and its findings correlate with conventional surveillance measures.

The zoonotic outbreak of highly pathogenic avian influenza (HPAI) A(H5N1) virus in the US has led to 13 confirmed human cases between January and August 2024. In response, the Centers for Disease Control and Prevention (CDC) and health departments implemented wastewater surveillance of the influenza A virus and the H5 subtype. While this surveillance is valuable, it is important to note that the H5 detections in wastewater might not specifically indicate HPAI A(H5N1) virus but could reflect low pathogenic avian influenza viruses.

About the study

The present study summarized data from the first nine weeks of wastewater surveillance of influenza A and the H5 subtype in the US. Wastewater samples were collected from around 750 sites between May 12 and July 13, 2024, and tested for influenza A virus by health departments, CDC contractors, or an academic partner program (WastewaterSCAN). Viral concentrations in wastewater were quantified by polymerase chain reaction (PCR) testing.

Influenza A virus levels were categorized into five groups: minimal (< 20th percentile), low (20th to 40th percentile), moderate (40th to 60th percentile), above average (60th to 80th percentile), or high (> 80th percentile). Among the sites where high levels of influenza A were detected, a correlation was often found with community human influenza activity based on other surveillance systems.

A digital PCR assay for the H5 hemagglutinin gene was evaluated by WastewaterSCAN, which detected H5 RNA in samples from cattle outbreak locations. As such, routine H5 testing was implemented in May 2024 at 193 sites in 41 states. H5 virus data were submitted from July 2024 onwards to CDC's Data Collation and Integration for Public Health Event Response pipeline. It is important to emphasize that while wastewater testing can detect influenza viruses, the results do not definitively indicate whether the viruses originated from human or animal sources.

Findings

Among 41 states with wastewater testing, nine states (Colorado, California, Iowa, Idaho, Michigan, North Carolina, Minnesota, Texas, and South Dakota) reported one or more H5 detections between May 12 and July 13, 2024. Seven of these nine states reported an HPAI A(H5N1) virus-infected herd during this time, and one reported before it. Further, human HPAI A(H5N1) virus infection cases were confirmed in Michigan and Colorado during this period.

Among the 32 states with no H5 detections, 30 had no HPAI A(H5N1) virus-infected herds during this period, but two had before this period. About 63% of sites with H5 detections identified animal inputs within the county or sewer shed, while 33% identified milk-processing inputs within the sewer shed catchment area. This suggests that many H5 detections could be linked to animal waste rather than human cases. Other inputs were dairy operations, meat processing, wild bird inputs, domestic poultry farms, and other sources of livestock waste.

Conclusions

Taken together, most wastewater sites in the US did not have high levels of influenza A virus or any H5 detections between May 12 and July 13, 2024. At sites with high levels of influenza A virus, corresponding human influenza activity was frequently detected in the community, reinforcing the correlation between wastewater signals and traditional influenza surveillance systems. At sites with H5 detections, almost all were in a state with reported infection of dairy herds before or during the surveillance period.

Further, animal-related inputs were frequently reported among sites with H5 detections. Despite this, the current testing methods cannot distinguish between human and animal sources of the virus, complicating the interpretation of H5 detections. Notably, while influenza viruses can be detected in wastewater, existing methods cannot differentiate between animal and human sources, and the current testing approach for H5 was not HPAI A(H5N1) virus-specific. As such, wastewater H5 detections might reflect animal infections.

In addition, there is limited information about the proportion of infected humans shedding influenza viruses in feces or urine and how viral shedding varies across the course of illness and by subtype. Overall, data from these early investigations of wastewater signals can aid health officials in implementing improved measures of viral levels in wastewater and optimizing wastewater surveillance in the next respiratory illness season. This highlights the importance of a "One Health" approach, where public health, agriculture, wildlife, and environmental sectors collaborate to fully understand and manage zoonotic influenza outbreaks.