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

The report from the U.S. 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 U.S. bird flu cases this year, the Missouri patient, who was hospitalized 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.