A person in Missouri with no known animal contact has tested positive for H5 bird flu, the state’s department of health and senior services said Friday.

It’s the first time a patient in the US outbreak has had no known exposure to sick animals. And it is the first time someone has been hospitalized with bird flu – though it’s not clear yet if influenza was the reason for hospitalization or it was incidental.

The patient, who has underlying health conditions, was hospitalized on 22 August and tested positive for flu A. Doctors sent a sample to the Missouri state public health laboratory, where it was found to be in the H5 subtype, which is also known as highly pathogenic avian influenza – or bird flu.

The US Centers for Disease Control and Prevention (CDC) confirmed that it is H5 and said it is studying the specimen more.

In past cases, officials have sought to understand whether human cases are caused by the strain that has circulated in wild birds and mammals in the US since 2022 or whether it is more closely related to the variant in dairy cows. They will also likely look to see if it has picked up mutations to make it more transmissible or virulent among people.

In an exclusive interview with the Guardian in July, Nirav Shah, principal deputy director of the CDC, said there were several concerns officials are watching for in this outbreak.

“One is increasing severity of disease,” such as if a person were hospitalized or died from bird flu, he said.

The previous cases in the US have been mild, but they were among farmworkers, who tend to be healthy and strong. In the wider population, especially among those with other health conditions, bird flu could prove more dangerous. Globally, the mortality rate hovers at about 50% of confirmed cases – although there are likely less severe cases that go undetected.

“The second would be the nature of transmission,” Shah said. “Are we seeing person-to-person transmission?”

In the two decades since H5N1 was observed among people, onward transmission among people has been exceedingly rare, he said.

Another major cause for concern with transmission centers on who is getting sick.

“If we were to see individuals with no connection to a farm whatsoever, or chicken exposure, developing signs and symptoms – that would be greatly concerning,” Shah said. “Even if there was no other transmission route identified, that would be a sign that something else is happening.”

Officials will probably continue tracing the patient’s potential exposures and contacts with other people to see how the virus spread.

Bird flu is usually found in domestic and wild birds, but in recent years it has significantly expanded its range to circulate widely in mammals, including dairy cows and other livestock.

Two indoor cats in Colorado recently tested positive for the virus. House mice have also tested positive.

No bird flu has been detected in cows in Missouri, but it has been found in poultry and wild birds.

It “can occasionally infect humans through close contact with infected animals or contaminated environments”, according to the Missouri press release.

This is the 15^th case of bird flu among people in the US since 2022 – and the 14^th this year. This is the first known human case of bird flu in Missouri.

The patient had recovered and was discharged from the hospital, the Missouri statement said.

• This article was updated on 6 September 2024 to correct that the Missouri department of health and senior services, not the state’s agriculture department, made the announcement about a person testing positive for H5 bird flu.