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.

Birds to cows to chickens to humans

The US Department of Agriculture confirmed in late March that the H5N1 bird flu that had been spreading globally among wild birds for years had unexpectedly jumped to dairy cows in the US. To date, at least 168 herds in 13 states have tested positive for the virus. Amid the dairy outbreak, 11 humans have contracted the virus. Four of the cases were among dairy workers: one in Texas, two in Michigan, and one in Colorado, which was reported earlier this month. The remaining seven cases were among poultry workers, all in Colorado. (There was also a human H5N1 case in a Colorado poultry worker in 2022, prior to the virus jumping to cows.)

The recent spate of human infections among Colorado poultry workers is notable, given that H5N1 has been plaguing poultry farms in the US since January 2022. To date, there have been over 1,000 outbreaks across 48 states, affecting over 100 million birds. In most of those cases, it is believed that poultry, which are highly susceptible to avian influenza, became infected directly from wild birds. Yet, cases in poultry workers are quickly ticking up only after the virus moved from wild birds to dairy cows and then to poultry.

Genetic testing so far has not flagged significant changes in the virus that could explain the recent uptick or raise new alarms. There remains no evidence of human-to-human transmission, and the CDC still assesses the risk of H5N1 to the general public as low. Further, all of the human cases identified to date have been mild and appear to respond to flu antivirals. In a press briefing last week, federal officials noted that the summer's excessive heat may be playing a role in the uptick in human cases. With temperatures reaching over 100° Fahrenheit and industrial fans blowing, poultry workers in Colorado tasked with culling birds struggled to keep masks and goggles in place on their faces.

It also remains unclear how the virus is spreading from dairy farms to poultry farms in this latest stage of the outbreak. However, it may not be a surprise that Colorado, of all states, is the one seeing a spillover from dairies to poultry farms; the state has reported 46 of the country's 168 infected herds, the most of any of the 13 states affected by the outbreak. Weld County, in particular, has reported more than two dozen infected herds.