Four more Missouri healthcare workers are experiencing mild respiratory symptoms after coming in contact with a bird flu patient, health officials said Friday.

A total of six healthcare workers have now developed symptoms after having contact with the patient, who is the first confirmed person to contract the disease with no known animal exposure.

The patient has recovered, but the case is raising questions about potential human-to-human transmission of bird flu, which primarily affects animals.

The only worker who was tested for the virus had negative results, the Centers for Disease Control and Prevention said.

That worker, who was given a PCR test for the virus, had what was considered high-risk contact with the patient and later developed mild symptoms.

Another worker who had high-risk contact and three others who had low-risk contact were not tested during the time they experienced symptoms, according to the CDC.

"PCR testing would have been unreliable at the time of discovery of these individuals' prior symptoms," said the agency.

PCR testing, which can give results quickly, was common during the Covid-19 pandemic.

Along with the healthcare workers, one member of the patient's household developed symptoms, but also was not tested, according to the CDC.

All of the healthcare workers with symptoms provided blood samples for antibody testing, as well as the household contact, the CDC said.

The results of those tests remain unknown.

Bird flu, also known as avian influenza, is rare in humans. There have been 14 human cases of the viral disease in the US so far this year.

Missouri's handling of the only one with no known animal exposure has come under scrutiny.

Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy, told the health publication STAT that he was concerned about how long Missouri was taking to figure out who else may have been infected by the original patient.

“Public health credibility is really on the line here,” Mr Osterholm said.

The 13 other cases in 2024 involved farm workers who had links to bird flu outbreaks on dairy or poultry farms.

The CDC continues to caution that the risk to the general public remains low.

But bird flu cases among cows have been on the rise in the US this year.

Since a March outbreak was first reported, cattle in 14 states have been affected, the CDC said.