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A small study in Texas suggests that human bird flu cases are being missed on dairy farms where the H5N1 virus has taken off in cows, sparking an unprecedented nationwide outbreak.

The finding adds some data to what many experts have suspected amid the outbreak. But the authors of the study, led by researchers at the University of Texas Medical Branch in Galveston, went further, stating bluntly why the US is failing to fully surveil, let alone contain, a virus with pandemic potential.

"Due to fears that research might damage dairy businesses, studies like this one have been few," the authors write in the topline summary of their study, which was posted online as a pre-print and had not been peer-reviewed.

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A second Colorado poultry farm has reported a case of bird flu in a worker, marking the state's seventh human case this month amid the ongoing outbreak among dairy cows.

Colorado health officials said the seventh case is, for now, a presumptive positive. That means that the person has tested positive at the state level while confirmatory testing is being carried out at the Centers for Disease Control and Prevention.

The presumptive positive worker was at a poultry facility in the state's northeastern Weld County. In recent weeks, six workers at another poultry farm in Weld also tested positive for bird flu. In that facility, a commercial egg layer operation with about 1.8 million birds, workers were infected as they culled chickens known to be infected with the highly pathogenic avian influenza. Genetic testing of the virus in the birds and the workers indicated that they were infected with a strain of H5N1 closely related to the virus found spreading in dairy cattle and to dairy farm workers.

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The highly pathogenic avian influenza H5N1 virus that spilled from wild birds into US dairy cows late last year may have recently seeped from a dairy farm in Colorado to a nearby poultry farm, where it then infected five workers tasked with culling the infected chickens

In a press briefing Tuesday, federal officials reported that four of the avian influenza cases have been confirmed by the Centers for Disease Control and Prevention, while the fifth remains a presumptive positive awaiting CDC confirmation.

All five people have shown mild illnesses, though they experienced variable symptoms. Some of the cases involved conjunctivitis, as was seen in other human cases linked to the H5N1 outbreak in dairy cows. Others in the cluster of five had respiratory and typical flu-like symptoms, including fever, chills, sore throat, runny nose, and cough. None of the five cases required hospitalization.