Bird flu continues to appear to pose a "low risk to the general public" for now, the Centers for Disease Control and Prevention says. But the agency's scientists ran into roadblocks investigating a human case of this "pandemic potential" virus this year, they said in a new report.

Epidemiologists from the agency were ultimately unable to access a Texas dairy farm where a human was infected with the virus in March, they disclosed in attachments to the report published Friday by the New England Journal of Medicine. That prevented investigators from being able to investigate how workers might have been exposed to the virus on the farm. 

That is because the dairy worker who came to a Texas field office for testing "did not disclose the name of their workplace," said Lara Anton, a spokesperson for the Texas Department of State Health Services. 

They also were unable to collect follow-up samples from the dairy farm worker or their contacts, which could have revealed missed cases as well as tracking the virus and antibodies against it in the body after an infection.

The worker did not wear protective eye goggles or a face mask that could have protected them from the virus, the report said. The virus was likely transmitted through their contaminated hands or droplets of the virus from sick cows. 

H5N1 was likely spreading through dairy farms via the high concentrations of the virus found in the raw milk of infected cows, authorities said previously. 

The virus had been circulating in cows for an estimated four months before it was confirmed by labs on March 25, according to a draft report from U.S. Department of Agriculture scientists released Thursday.

A mutation to the virus in wild birds, a specific "clade" of the virus that scientists call 2.3.4.4b, appears to have enabled bird flu to jump into cows. Multiple herds were likely infected during that initial spillover before the birds migrated north, officials have said.

Since then, at least nine states have detected cow infections from the virus. Cows largely recover from H5N1, unlike the mass die-offs seen in other species. Some herds with infected cows have also remained asymptomatic and are continuing to produce milk.

Experiments run by the Food and Drug Administration show that pasteurized milk remains safe to drink, despite traces of the virus found in samples from grocery stores. The outbreak has also prompted a renewed warning not to drink raw milk, which has been linked to deaths of other animals like cats. 

The ongoing outbreak is also in stark contrast to how the virus has spread in other mammals infected by the virus, which have generally resulted in what USDA scientists called "dead end hosts."

A handful of variants with potentially worrying mutations have also since been spotted in cows, the USDA analysis found. If those variants become dominant, it could change the disease caused by H5N1 or make spread to other animals or humans more likely.

The virus from cows has also been spotted spreading out of dairy farms into nearby wild birds and poultry, likely ferried by contaminated milk droplets and surfaces.

Questions also remain about the exact origins of the virus that infected the Texas dairy worker. While the H5N1 sequence from the human case is closely related to those found in dairy herds, the agency's analysis found it also differs in some key ways.

Those genetic differences suggest the human was infected by "an early, slightly different virus" that was circulating in cows before the current cases, or that multiple spillovers may have actually occurred. 

While sequences collected from sick cows on the worker's dairy farm could have helped CDC scientists answer those questions, samples were "not available for analysis."

The human worker has since recovered after their bird flu infection. They only developed conjunctivitis, or pink eye, without fever or other common flu symptoms. The worker and their contacts were given oseltamivir, an antiviral treatment for bird flu that could also head off infections.

"While acute conjunctivitis is a clinically mild illness, HPAI A(H5N1) viruses, including those belonging to clade 2.3.4.4b, pose pandemic potential and have caused severe respiratory  disease in infected humans worldwide," the report co-authored by CDC scientists said.

Missed infections?

Reports by a local veterinarian that other workers on Texas dairy farms had symptoms of flu or conjunctivitis are true, said Anton on April 30.

But at least some of those workers with symptoms were tested and turned out to be negative for H5N1, health officials in Texas as well as neighboring New Mexico told CBS News.

"It's likely there were other people with symptoms who did not want to be tested so we cannot say with absolute certainty that no one else contracted H5N1. We can say for sure that there were people sick with other respiratory viruses working on dairy farms," Anton said in a statement..

With health authorities and experts dependent on dairy workers and their contacts volunteering for monitoring their symptoms and getting tested, those tracking the virus have turned to other data sources to look for signs of undetected spread.

One draft study of wastewater samples in a northwest Texas town found that signs of H5N1 have surged in sewers, but also that emergency room trends in the area were declining at the same time. They hypothesized dumping of waste from dairy farms with sick cows are to blame, not sick humans.

The CDC has also pointed to emergency room data for reassuring worries about undetected H5N1 cases.

"We continue to monitor flu surveillance data, especially in areas where H5N1 viruses have been detected in dairy cattle or other animals, for any unusual trends in flu-like illness, flu, or conjunctivitis. CDC flu surveillance systems at this time show no indicators of unusual flu activity in people," Dr. Demetre Daskalakis, director of the CDC's National Center for Immunization and Respiratory Diseases, told reporters Wednesday.

In:
• Bird Flu
• Avian Influenza

Alexander Tin

Alexander Tin is a digital reporter for CBS News based in the Washington, D.C. bureau. He covers the Biden administration's public health agencies, including the federal response to infectious disease outbreaks like COVID-19.

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