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H5N1 influenza surfaced in wastewater samples across all 10 Texas cities monitored by the at the Texas Epidemic Public Health Institute in Houston.

From March 4 through July 15, the group detected H5N1 in 10 of 10 cities, at 22 of 23 sites, and in 100 of 399 samples, Anthony Maresso, PhD, of Baylor College of Medicine in Houston, and colleagues reported in a correspondence piece in the .

They said the detections have not correlated with influenza-related hospitalizations, nor has the group picked up mutations that would signal adaptation to humans, notably the E627K mutation in the virus' polymerase basic protein 2 (PB2) gene.

Instead, the data suggest multiple animal sources, as all of their sequences "best match H5N1 genomes from birds and mammals" and are from the 2.3.4.4b clade, the investigators wrote.

Nonetheless, Maresso's group called the "widespread detection" of H5N1 in wastewater from 10 cities "troubling."

"My hope is that the next time we look, we stop seeing it," co-author Blake Hanson, PhD, of the University of Texas Health Science Center at Houston, told MedPage Today. "We think of this as a sentinel proxy of what's circulating in our community. It doesn't mean it's human, we can't tell if it's a specific animal, but it does mean it's around us in our community, and that's something we should remain vigilant about."

MedPage Today previously reported that the TexWEB team -- a collaboration between state government, Baylor, UTHealth, and municipal wastewater sources -- surveils influenza at a more granular level than the CDC's wastewater dashboard, which monitors for influenza A. It also dives deeper than other efforts that use PCR to look for the H5 hemagglutinin gene.

The potentially more sensitive methods continue to pick up an H5N1 signal even as detections from other groups have fallen off since the spring, said co-author Michael Tisza, PhD, of Baylor College of Medicine.

The decline could have something to do with "surveillance and quarantining and other preventive methods in the dairy industry," Tisza told MedPage Today, but it's not possible to say definitively.

Tisza said it's notable that most of the 15 human cases reported this year have had H5N1 genomes that have been identical to those in cows, and that it's been surprising that they haven't detected more of the hallmark mutations that would indicate adaptation to humans.

Earlier this year, the TexWEB team reported detecting H5N1 in wastewater from nine Texas cities from March 4 through April 25 in a .

Another team from Verily Life Sciences, Emory University, and Stanford University that it detected influenza A at 59 wastewater treatment plants in the U.S. this spring. Those researchers then searched for -- and found -- the H5 gene using PCR at three wastewater treatment plants with a positive flu A signal.

That team has since identified the H5 gene at 25 sites across nine states, according to the correspondence.

Heading into the fall, the TexWEB team will be working to "disentangle H5N1 from those other strains [that] might be circulating as seasonal flu starts to increase," Hanson said.

Hanson and Tisza noted that they track many other viruses, and have seen the summer COVID surge in their surveillance, as well as the uptick in parvovirus B19 that was the subject of a earlier this summer.

Their surveillance could also reveal whether cases of the more dangerous mpox clade I started to appear in the U.S., Tisza said: "We're really covering a lot of ground."

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