An increase in flu viruses detected at wastewater treatment plants in California in recent weeks has sparked concern that the H5N1 bird flu may be spreading more rapidly than anticipated, potentially putting the state’s 1.7 million dairy cows at risk for infection.

Health officials have observed multiple spikes in influenza A viruses, which include the H5N1 avian flu strain that has killed millions of birds worldwide and infected dozens of dairy cow herds across nine U.S. states.

The Bay Area, in particular, is a hotspot for flu activity, according to data from approximately 700 sites published by the Centers for Disease Control and Prevention. Nearly all monitored facilities in the region show moderate to high increases in influenza A viruses.

While the presence of bird flu has not been confirmed, it remains a possibility.

A spokesperson for the California Department of Public Health told the Chronicle that they are investigating the surge, which is “not following human influenza trends,” as it is not reflected in general population illness reports.

Normally, the circulation of influenza A — the type that more commonly causes flu in adults — diminishes significantly after the peak of the winter flu season. However, the current increase suggests otherwise.

Though the avian flu poses minimal risk to humans, its spread among livestock could disrupt food supplies and potentially lead to a pandemic if it mutates to enable human-to-human transmission.

As of now, no cases of infected herds have been reported in California, the leading milk-producing state in the U.S., with around 1,300 dairy farms.

The CDPH is collaborating with the CDC, local health departments, and wastewater utilities to investigate these increases, “including testing specifically for evidence of H5N1 in multiple sewer sheds throughout the state,” the agency said.

Thus far, no human cases of H5N1 infection have been detected in California.

While existing wastewater monitoring methods can detect the presence of influenza A viruses, they cannot identify their specific subtype. This limitation means that while they can detect avian influenza H5N1 viruses, they cannot distinguish them from other subtypes of influenza A viruses.

Although avian flu viruses typically do not infect humans, there have been documented cases of human infection. According to the World Health Organization, between 2003 and 2024, there were 889 reported cases of avian flu globally, with a mortality rate of approximately 50%. Whether there were additional asymptomatic infections during this period remains uncertain.

Individuals who work on dairy and poultry farms face a higher risk of infection due to direct contact with chickens and dairy cattle. While avian flu has been detected in various mammalian species, including sea lions, cows are of particular concern due to their frequent contact with humans and their significance in the food supply chain.

The exact mode of transmission of avian flu to or among cattle is not fully understood. It could occur through infected feces, including when feces are aerosolized in dust. Additionally, there have been reported cases of cats becoming infected after consuming raw milk from sick cows.

The current strain of avian flu emerged among birds in 2022 and was first detected in dairy cows in the United States in late March, marking the first known instance of bird flu viruses in cows.

Since then, it has spread to 49 herds of dairy cattle across nine states: Texas, Michigan, Kansas, Colorado, Idaho, New Mexico, North Carolina, South Dakota, and Ohio, according to the Department of Agriculture.

There have been only three reported cases of avian flu infection in people in the United States. One case occurred in Colorado in 2022 and involved an individual who worked closely with infected poultry. The other two cases were more recent — one in Texas in March and one last week in Michigan — involving people employed at dairy farms.

In the latest case, there was a “notable change” in the virus’s genetic makeup that makes it more adaptable to mammalian hosts, according to an update from the CDC published Friday.

In both earlier cases, the individuals experienced mild symptoms such as fatigue and conjunctivitis and eventually recovered. The third is too recent to know what symptoms the infected individual may experience.

“We continue to investigate and when we have something to report, we will let you know,” the CDPH said.