San Diego’s wastewater treatment plant in Point Loma is among five statewide that has detected the H5 influenza virus, often called bird flu, since May.

San Diego’s positive result came from a sample taken Sept. 1 by WastewaterSCAN, a private monitoring service based at Stanford University. Subsequent samples collected and tested Sept. 2 and Sept. 8 have not shown similar indications.

Likewise, a county program that routinely tests dead birds for a range of pathogens, including avian influenza, has not found any evidence of the virus locally.

Dr. Erik Berg, interim medical director of the county’s epidemiology and immunization services branch, said Monday afternoon that a public health notification sent out in the early evening Monday was undertaken out of an abundance of caution.

While bird flu can be deadly for humans, those who work in agriculture are at much higher risk than the general population.

“For the general public, the CDC’s risk assessment is that the threat to the general public is low, and we agree,” Berg said.

He said that it would be a mistake to assume that the virus found in local wastewater, which was estimated to be of low concentration, necessarily came from a human. While sanitary sewer systems are designed, built and operated to be self-contained systems, there are ways, such as pipe breaks, that can allow infiltration from the surrounding environment.

“We have the closed sewage system, so storm water runoff is not supposed to intermix with the wastewater, but, you know, we all know that’s not perfect,” Berg said, adding that viral genetic material was detected in such a low volume that further testing to determine what type of host may have been infected could not be performed.

Kristian Andersen, a virologist at Scripps Research in San Diego, said in an email Monday that it is an open question why H5 flu viruses are showing up sporadically in some municipal wastewater treatment samples.

“It could be human infections; however, it could also be people discarding milk that contains the virus itself or genetic material from it,” Andersen said.

It is clear that this problem goes far beyond San Diego County.

The U.S. Centers for Disease Control and Prevention recently characterized the virus as a “public health challenge posed by a multistate outbreak of avian influenza virus … in dairy cows, poultry and other animals.”

San Diego County’s public health department notes in its statement that there have been no human cases of bird flu in the region thus far, and dead birds tested for H5 have also come back negative.

Why is the county publicizing the finding now, more than two weeks after it occurred? Discussions with the state and notification of local farms, a county official said, were prioritized after local public health officials received notification from SCAN on Sept. 9.

While the virus is of particular concern for beef and poultry producers, the virus can be deadly when it crosses into human populations. The CDC indicates that since March 24 it has confirmed 14 human cases among 240 people who have been exposed.

The CDC recently detailed a particularly concerning case that popped up in Missouri in a person “without a known occupational exposure to sick or infected animals.” On Friday, Sept. 13, the CDC noted that the other 13 cases detected since April have all had confirmed exposure to animals.

“Four of these cases were associated with exposure to sick dairy cows, and nine were associated with exposure to avian influenza A (H5N1) virus-infected poultry,” a CDC statement said.

In addition to San Diego, wastewater bird flu detections in California included west Contra Costa County on June 10, Turlock on Sept. 4, southeast San Francisco on June 18 and June 26 and Palo Alto on July 4, July 9 and June 19.