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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More than a month after a person in Missouri mysteriously fell ill with H5-type bird flu, investigators in the state are still identifying people who became ill after contact with the patient, raising questions about the diligence of the ongoing health investigation.

On September 6, Missouri's health department reported the state's first human case of H5-type bird flu, one that appears closely related to the H5N1 bird flu currently causing a nationwide outbreak among dairy cows. But the infected person had no known contact with infected animals—unlike all of the other 13 human cases identified amid the dairy outbreak this year. Those previous cases have all occurred in dairy- or poultry-farm workers. In fact, Missouri has not reported bird flu in its dairy herds nor recent poultry outbreaks.

Given the unexplained source of infection, health investigators in the state have been working to track the virus both backward in time—to try to identify the source—and forward—to identify any potential onward spread. The bird flu patient was initially hospitalized on August 22 but recovered and had been released by the time the state publicly reported the case.

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Enlarge (credit: Getty | Matthew Ludak)

The H5N1 bird flu appears to be stampeding through dairy farms in California, the country's largest milk producer. Over the weekend, the total number of confirmed infected cow herds stunningly doubled, going from 17 last Thursday to 34 Monday morning, according to state and federal officials.

With the new tally, California now ranks second among all affected states for having the most herds with avian influenza. Only Colorado, which has adopted bulk milk-tank surveillance, has more, with 64 herds confirmed. California's high ranking is despite the fact that it only reported its first three infected herds on August 30, while the dairy outbreak was first confirmed on March 25 and thought to have begun late last year.

To date, 232 herds in 14 states have been infected with the bird flu.

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Enlarge / The influenza virus from an image produced with transmission electron microscopy. Viral diameter ranges from around 80 to 120 nm. (credit: Getty | BSIP)

A person in Missouri with no reported exposure to animals was confirmed to have been infected with H5-type bird flu, the Missouri Department of Health and Senior Services (MDHSS) announced late Friday.

MDHSS reported that the person, who has underlying medical conditions, was hospitalized on August 22 and tested positive for an influenza A virus. Further testing at the state's public health laboratory indicated that the influenza A virus was an H5-type bird flu. The Centers for Disease Control and Prevention has now confirmed that finding and is carrying out further testing to determine if it is the H5N1 strain currently causing a widespread outbreak among US dairy cows.

It remains unclear if the person's bird flu infection was the cause of the hospitalization or if the infection was discovered incidentally. The person has since recovered and was discharged from the hospital. In its announcement, MDHSS said no other information about the patient will be released to protect the person's privacy.

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