Four additional healthcare workers in Missouri who came in contact with a hospitalised bird flu patient developed mild respiratory symptoms but the virus was not confirmed in any of them, Reuters reported, US health officials on Friday.

The report from the US Centers for Disease Control and Prevention brings to six the number of healthcare workers who cared for the Missouri patient and developed respiratory symptoms.

Unlike previous US bird flu cases this year, the Missouri patient, who was hospitalised on Aug. 22, had no known contact with infected animals. Scientists are watching closely for signs that the virus has begun to spread more easily in people.

Missouri is leading an investigation with CDC assisting remotely. Missouri health department spokeswoman Lisa Cox said in an emailed statement that the investigation is ongoing and there is currently no evidence of person-to-person transmission.

According to Cox, one healthcare worker got a flu test and tested negative for the virus. Symptoms in the others were not discovered until it was too late to do testing, she said.

A household contact of the patient also developed symptoms but was not tested. All have recovered from their illnesses.

The state has collected blood samples of five healthcare workers and the household contact to look for antibodies that would suggest a prior infection. Cox said the CDC is evaluating samples from the unidentified hospitalized patient and the household contact.

The state is preparing the other samples to be sent to the CDC as soon as possible, Cox said.

"These cases underscore the need to take this outbreak more seriously than it has been taken," said Amesh Adalja, a senior scholar at Johns Hopkins Center for Health Security.

Results from the antibody testing need to be promptly obtained to assess the risk of human-to-human spread of the virus, Adalja said. "This dripping out of information from Missouri... is not acceptable and the state should ask for official CDC assistance now," he added.

Cox said the state is sharing relevant updates as the investigations are completed.

The Missouri case was the 14th person in the United States to be diagnosed with bird flu this year. The other 13 cases were among farm workers and linked to outbreaks on poultry or dairy farms.