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Many questions remain unanswered in the case of the Missouri patient who contracted H5 bird flu without any known exposure to sick or infected animals.

"It does raise the concern level, but there is a lot of information needed to understand how much it raises concern," said Amesh Adalja, MD, of the Johns Hopkins Center for Health Security in Baltimore.

James Lawler, MD, MPH, of the University of Nebraska Medical Center's Global Center for Health Security in Omaha, echoed that experts have "very limited visibility into what's truly going on with the spread of H5," especially as surveillance of dairy herds has "quite honestly, been pathetic."

The key concern is whether there has been human-to-human transmission, Lawler said: "That's certainly a concern when you have somebody who has no discernible exposure and who develops disease with H5."

While that does not seem to be the case at this time, experts are wary about that next step in the trajectory of a virus that has already "ticked a lot of boxes" on its way to becoming a potential pandemic, including being able to infect new host species, Lawler said.

Outside of human-to-human transmission, there could have been some "overlooked animal contact," Adalja told 木瓜直播. While Missouri isn't currently reporting any infected cattle herds, "there are likely infected herds there, and in many other of the 36 states that have not officially reported cases," he said.

There's also the possibility of exposure to infected birds, either wild or poultry flocks, or to any of the other wild or domestic animals that have been infected, including cats. Indeed, six domestic cats in Colorado were infected with H5N1, and only one was directly associated with a commercial dairy facility known to have sick animals, . Two of the cats were indoor-only and had no direct exposure to the virus.

The patient may also have been exposed to unpasteurized milk, which has been shown to have very high levels of virus, Lawler said. He also raised the possibility of a failure of the milk pasteurization process.

"There's a wide range of possibilities of where this person might have been infected," Lawler said. "The most concerning would be that the person caught it from another person, and that would indicate a step up in the virus' game."

He cautioned that the CDC must first determine whether it's indeed H5N1, as the agency has only confirmed that it's an H5 virus. Sequencing would also be able to show whether it's from the clade that's currently circulating in dairy cows -- 2.3.4.4b, genotype B3.13.

CDC said the case was picked up through the state's routine seasonal flu surveillance when the patient, who had other underlying medical conditions, was hospitalized on August 22. They were treated with flu antivirals, discharged, and made a full recovery.

While this is the first U.S. case of H5 influenza without a known occupational exposure to sick or infected animals, and Missouri has not reported any infected cattle herds, it has reported H5N1-infected commercial and backyard poultry flocks this year, and infected wild birds in the past, CDC said.

Lawler noted that the information about the case appeared to be withheld for several days: "It doesn't seem to be a good idea to withhold information that might indicate a significant development in the evolution of a very dangerous virus," he said.

He noted that he is concerned that the virus continues to do things it hasn't done in the past.

"Before this instance, I would have said we're [probably] not missing cases unassociated with exposure to dairy cows, birds, or milk products," he said. "Now, I'm not so sure."

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