A Texas health care worker who treated the Liberian-American Ebola victim has tested positive for the deadly tropical fever, dealing a blow to the worldwide battle to stem the outbreak.

More than 4,000 people have died of Ebola in seven countries since the start of the year, according to the World Health Organization, and the epidemic appears to be outpacing efforts to fight it.

Thermal Ebola screening device at JFK Airport. Reuters

If preliminary test results are confirmed, the Texas patient would be the second person diagnosed with the illness and apparently the first to contract it on US soil, a day after US airports began screening travelers from epidemic-hit west Africa.

“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.”

The health care worker from Texas Health Presbyterian Hospital in Dallas reported a low-grade fever Friday night and was isolated and referred for testing, Texas health services said in a statement early Sunday. They did not further identify the worker nor detail how exposure to the virus occurred.

The hospital had treated Thomas Eric Duncan, the Liberian man who died on Wednesday. Duncan was believed to have been infected with Ebola before he left Liberia and boarded a plane to visit family in Texas.

The latest Texas case underlines United Nations fears and growing concerns in the United States about Ebola, for which there is no vaccine or widely available treatment.

“The virus is far ahead of us and every day the situation gets worse,” the head of the United Nations’ emergency Ebola mission, Anthony Banbury, told UN leaders after a tour of Liberia, Guinea and Sierra Leone, the nations worst affected by the worst-ever outbreak of the disease.

Passengers from the three countries arriving at John F Kennedy International Airport in New York will have their temperatures taken and be screened for signs of illness and answer questions about possible exposure, the US Centers for Disease Control and Prevention (CDC) said.