(Bloomberg) -- Fragments of a potentially dangerous bird flu virus that appeared in Austin wastewater, miles from the nearest dairy farm, are sending researchers in a race to find the source. 

Texas researchers are tracing pieces of the H5N1 virus that were confirmed in Austin’s sewage last week to see if they came from infected farm animals, wild birds or dumped milk, Anthony Maresso, lead researcher of the Texas Wastewater Environmental Biomonitoring network, said in an email. Unreported human cases, while unlikely, are also a possibility, he said.  

“It is truly still a mystery,” said Maresso, who’s also a Baylor College of Medicine virologist. “The sequencing indicates there can be multiple sources.”

Health officials are struggling to keep up with the flu virus that has been reported in 90 cattle herds nationwide. Three US dairy workers with mild symptoms have tested positive this year, one of them in Texas, and the risk to the public remains low. Still, bird flu has been known to cause lethal disease in rare cases. And the more widely the virus spreads, the greater the risk of human infection, US health officials said Thursday on a call with reporters. 

“That potential cattle-to-human transmission really needs to be focused on,” said Adriane Casalotti, chief of government and public affairs at the National Association of County and City Health Officials. Health officials need to ramp up surveillance “to identify any shift in the behavior of the virus” that might signal ability for wider spread, she said. 

Testing Reluctance

Farms and dairy workers have resisted testing for H5N1, due to the risk of lost milk production or income. That’s left researchers to sift through wastewater to map out the virus’s location. While the approach can gather signals from large geographical areas, it leaves the problem of finding exactly where the virus is living, and who or what is infected. 

The Texas wastewater group is trying to answer those questions by comparing genetic material found in sewage with samples from plausible sources such as poultry, cows and milk. The wastewater samples are from two sites that encompasses 95% of Austin’s population, which could include some small agriculture farms, according to Desmar Wilkes, chief medical officer for Austin Public Health.

There are no known permitted dairy farms whose sewage goes into the Austin system, according to city officials, although there may be backyard animals such as poultry that might be sources. No human cases have been reported in the city. Either an animal source or parts of the virus in raw and commercial milk are “good and reasonable guesses,” Baylor’s Maresso said. 

H5N1 is likely spreading in cattle through contaminated equipment and clothing, researchers at the US Department of Agriculture said Thursday on a call. Raw milk continues to be a point of exposure, and farm cats have died after consuming it. Pasteurization kills the virus and humans should avoid consuming raw milk, according to the US Food and Drug Administration.  

Migrant Labor

Almost 80% of the US milk supply comes from farms employing migrant laborers, many of whom hesitate to seek medical care even when they’re ill. That reluctance poses another barrier to understanding whether more human cases are present on farms. 

Testing for influenza A, a category of viruses that includes H5, usually ends in the spring. Trying to get a handle on spread, the Centers for Disease Control and Prevention is recommending that local health officials extend that testing through the summer. The agency is also considering broadening its testing guidelines for bird flu and welcomes states to refer anyone with likely exposure to come forward for testing, Nirav Shah principal deputy director of the US Centers for Disease Control and Prevention said on the Thursday call.   

At least 45 people have been tested for H5N1 nationwide, Shah said. Two cases of bird flu have been reported in Michigan, which has invited the CDC to conduct additional surveillance.   

Each state is creating its own monitoring efforts in partnership with agriculture departments, leaving a patchwork of data that is concerning to health officials in understanding the virus outside of wastewater monitoring. The variation in efforts between states is concerning, said Deliana Garcia, chief program officer of international and emerging issues at the Migrant Clinicians Network, a nonprofit community health group based in Austin. 

“The public health system is set up in such a way it can’t act quickly,” Garcia said. “If you have something that can spread that easily through a population of animals who are then in very close quarters with human beings, I’m very worried.” 

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