As H5N1 bird flu continues to spread wildly among California dairy herds and farmworkers, federal health officials on Thursday offered some relatively good news about Missouri: The wily avian influenza virus does not appear to have spread from the state's sole human case, which otherwise remains a mystery.

On September 6, the Missouri Health department announced that a person with underlying health conditions tested positive for bird flu, and later testing indicated that it was an H5N1 strain related to the one currently circulating among US dairy cows. But, state and federal health officials were—and still are—stumped as to how that person became infected. The person had no known contact with infected animals and no contact with any obviously suspect animal products. No dairy herds in Missouri have tested positive, and no poultry farms had reported recent outbreaks, either. To date, all other human cases of H5N1 have been among farmworkers who had contact with H5N1-infected animals.

But aside from the puzzle, attention turned to the possibility that the unexplained Missouri case had passed on the infection to those around them. A household contact had symptoms at the same time as the person—aka the index case—and at least six health care workers developed illnesses after interacting with the person. One of the six had tested negative for bird flu around the time of their illness, but questions remained about the other five.

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Two California dairy farm workers who had contact with H5N1-infected cows have contracted an H5 avian influenza virus, the Centers for Disease Control and Prevention confirmed Thursday evening.

In a news release from earlier today, California's health department announced only one presumptive positive case awaiting confirmation from the CDC. In that case, the person's illness was reported to be mild, with the only symptom being conjunctivitis (eye redness). This echoes the experience of other H5N1 human cases in this outbreak. The person is said to be staying at home and taking an antiviral flu medication.

In its media statement Thursday evening, the CDC reported confirming two cases. "At this time, there is no known link or contact between the first and second confirmed cases in California, suggesting these are separate instances of animal-to-human spread of the virus," the CDC said.

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Two California dairy farm workers who had contact with H5N1-infected cows have contracted an H5 avian influenza virus, the Centers for Disease Control and Prevention confirmed Thursday evening.

In a news release from earlier today, California's health department announced only one presumptive positive case awaiting confirmation from the CDC. In that case, the person's illness was reported to be mild, with the only symptom being conjunctivitis (eye redness). This echoes the experience of other H5N1 human cases in this outbreak. The person is said to be staying at home and taking an antiviral flu medication.

In its media statement Thursday evening, the CDC reported confirming two cases. "At this time, there is no known link or contact between the first and second confirmed cases in California, suggesting these are separate instances of animal-to-human spread of the virus," the CDC said.

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Enlarge (credit: Getty | David Paul Morris)

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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Enlarge (credit: Getty | Edwin Remsberg)

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.

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Enlarge / Fresh raw milk being poured into a container on a dairy farm on July 29, 2023, in De Lutte, Netherlands. (credit: Getty | Pierre Crom)

Despite the delusions of the raw milk crowd, drinking unpasteurized milk brimming with infectious avian H5N1 influenza virus is a very bad idea, according to freshly squeezed data published Friday in the New England Journal of Medicine.

Researchers at the University of Wisconsin-Madison squirted raw H5N1-containing milk from infected cows into the throats of anesthetized laboratory mice, finding that the virus caused systemic infections after the mice were observed swallowing the dose. The illnesses began quickly, with symptoms of lethargy and ruffled fur starting on day 1. On day 4, the animals were euthanized to prevent extended suffering. Subsequent analysis found that the mice had high levels of H5N1 bird flu virus in their respiratory tracts, as well their hearts, kidneys, spleens, livers, mammary glands, and brains.

"Collectively, our data indicate that HPAI [Highly Pathogenic Avian Influenza] A(H5N1) virus in untreated milk can infect susceptible animals that consume it," the researchers concluded. The researchers also found that raw milk containing H5N1 can remain infectious for weeks when stored at refrigerator temperatures.

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