Doctors said on Sunday that a second person had been diagnosed with Ebola on US soil: one of the health care workers who had treated the first case.

"We knew a second case could be a reality, and we've been preparing for this possibility," said Dr. David Lakey, commissioner of the Texas Department of State Health Services. "We are broadening our team in Dallas and working with extreme diligence to prevent further spread."

A statement on the health service's website said that "confirmatory testing will be conducted by the Centers for Disease Control and Prevention in Atlanta." The comments did not further identify the worker nor detail how exposure to the virus might have occurred.

The health care worker reported a low-grade fever on Friday night, after treating Thomas Eric Duncan, a Liberian man who died in the Dallas hospital on Wednesday.

After reporting signs of sickness, the staff member was isolated and referred for testing. The statement added: "Health officials have interviewed the patient and are identifying any contacts or potential exposures. People who had contact with the health care worker after symptoms emerged will be monitored based on the nature of their interactions and the potential they were exposed to the virus."

The Ebola virus spreads via contact with bodily fluids, once symptoms have emerged in the carrier. The incubation period before symptoms emerge can vary from two or three days to around three weeks, according to the World Health Organization.

The vast majority of cases in this year's outbreak, the largest on record, have been recorded in Guinea, Liberia and Sierra Leone.

msh/tj (AFP, Reuters)