Health officials say the 65-year-old was hospitalized after coming into contact with an infected flock of birds in the backyard.

A 65-year-old man in the southern state of Louisiana was hospitalized in critical condition as a result of the bird flubecoming the first serious case reported in the United States.

The Centers for Disease Control and Prevention (CDC) announced Wednesday that the person had been in contact with an infected flock of backyard birds. There have been no reported cases of person-to-person transmission.

“CDC confirms first serious case of H5N1 bird flu in US. While an investigation into the source of the infection is ongoing, it has been determined that the patient had exposure to sick and dead birds in backyard flocks,” the agency said.

"This case does not change CDC's overall assessment of the immediate public health risk from H5N1 avian influenza, which remains low."

Demetre Daskalakis, director of CDC's National Center for Immunization and Respiratory Diseases, also told reporters that the Louisiana case was the first linked to a noncommercial herd.

While experts stress that the threat to public safety is currently low, the spread of the virus has raised questions about how the outbreak is being tested and tracked.

As of Wednesday, the CDC number 61 confirmed human cases of bird flu in the US, with most of them linked to exposure to sick people poultry or dairy cows.

Bird flu, or H1N1, is endemic - or frequent - among wild bird populations. But its presence among livestock and domestic animals increased the likelihood of human infections.

The CDC said that a strain of bird flu was first detected among US dairy cattle on March 25. "This is the first time that these bird flu viruses have been found in cows," it said. said.

Soon after, on April 1, Texas reported the first human transmission from exposure to a dairy cow. Since then, 37 of the 61 human bird flu infections have been linked to herds of infected dairy cattle, with California accounting for 33 of the cases.

According to the US Department of Agriculture (USDA), there are 865 infected herds in 16 states. Wednesday, California declare a state of emergency to try to stop the outbreak.

"This proclamation is a targeted action to ensure that government agencies have the resources and flexibility they need to respond quickly to this outbreak," Gov. Gavin Newsom said in a statement.

His statement emphasized that California boasts "the largest testing and monitoring system in the country to respond to the outbreak."

"While the risk to the public remains low, we will continue to take all necessary steps to prevent the spread of this virus," Newsom said.

Some farmers have resisted the push to test commercial livestock for bird flu, and the U.S. government itself has largely resisted imposing mandatory testing, relying instead on voluntary efforts.

That point of view though moved on December 6, when the US Department of Agriculture's Animal and Plant Health Inspection Service announced it would be to test samples of raw, unpasteurized milk from herds nationwide.

The federal order requires any dairy farm, milk transporter or dairy processing facility to share raw milk samples upon request.

Herd owners are also authorized to share any information related to the spread of disease among their cattle so that federal authorities can track the spread of bird flu.

But the B3.13 genotype associated with bird flu in cattle is different from the strain in the Louisiana case, known as the D1.1 genotype. The CDC confirmed this case on Friday, but did not release its findings until Wednesday.

In two cases — one from a child in California and another from an adult in Missouri — the CDC has not yet determined how the patients were infected. A Canadian teenager was also hospitalized last month with a severe case of bird flu.

Bird flu killed 123 million poultry since the outbreak first began in the US in 2022.