Federal health officials announced Thursday that a Missouri resident infected with bird flu did not spread the virus to others, alleviating concerns about potential human-to-human transmission during the current nationwide outbreak affecting poultry and dairy farms.
The Missouri case had raised particular alarm because human-to-human transmission could have signaled the potential start of a new pandemic. The patient's infection was especially worrying as they had no direct contact with infected animals, unlike previous cases involving farm workers.
"There was real concern that we were actually getting person-to-person transmission," Jeremy Rossman, a senior lecturer in virology at the University of Kent, told Newsweek. "However, thankfully, that didn't end up being the case."
Health officials closely monitored eight people connected to the Missouri case, including six healthcare workers who fell ill after patient contact and one household member.
Antibody screening showed five healthcare workers had not contracted H5N1, while a sixth had already tested negative. Officials concluded these workers likely had an unrelated respiratory illness.
The household member did show antibodies to avian influenza. However, because both household residents developed symptoms simultaneously, investigators determined they were likely exposed to the same external source rather than transmitting the virus between themselves.
H5N1, a highly pathogenic strain of avian influenza, has been circulating in U.S. poultry since 2022. The virus has recently established itself in dairy cattle, which have become the primary source of human infections.
The Centers for Disease Control and Prevention (CDC) has confirmed 31 cases nationwide, with more expected.
The outbreak continues to spread across multiple states. In California, where the virus has infected 137 dairy herds, health officials have confirmed 15 human cases.
Washington is the sixth state to report infections, with two confirmed cases and five presumed positive cases among workers at a commercial egg farm. The CDC dispatched investigative teams to both states to assist with the growing number of infections. Officials noted that the Washington cases appear genetically unrelated to outbreaks in dairy herds.
Rossman cautioned that the current case count likely represents just the "tip of the iceberg."
He added that surveillance varies significantly by region.
"It's occurring fairly reasonably in some areas and not at all in other areas. And just because you don't have cases, chances are, if you're not looking, you're not going to find it," he said.
While human-to-human transmission would have raised serious concerns, Rossman said the impact would depend on the virus's transmissibility and severity. So far, most human infections have been relatively mild.
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