DALLAS, Texas, Oct 12 (Reuters) - A Texas health worker has contracted Ebola after treating a Liberian who died of the disease at a Dallas hospital last week, raising concern about how U.S. medical guidelines aimed at stopping the spread of the disease were breached.

The infected worker, identified as a woman but not named by authorities as they announced the case on Sunday, is believed to be the first person to contract the disease in the United States.

Health officials said the worker at the Texas Health Presbyterian Hospital had been wearing protective gear during treatment of Thomas Eric Duncan. Duncan was a Liberian who died on Wednesday after being exposed to Ebola in his home country and developing the disease while visiting the United States.

The outbreak in West Africa, the worst outbreak on record of Ebola, has killed more than 4,000 people, mostly in Liberia, Sierra Leone and Guinea.

The new case in Texas indicated a professional lapse that may have caused other health workers at the hospital to also be infected, said the director of the Centers for Disease Control and Prevention (CDC).

"We don't know what occurred in the care of the index patient, the original patient, in Dallas, but at some point there was a breach in protocol, and that breach in protocol resulted in this infection," CDC director Dr. Thomas Frieden told a news conference.

"We are evaluating other potential healthcare worker exposures because if this individual was exposed, which they were, it is possible that other individuals were exposed," he said.

The worker was in close contact with Duncan and initial testing shows that the level of virus in her system is low. The CDC will conduct a secondary test to confirm the results from a lab in Austin that showed Ebola infection, he said.

"Unfortunately it is possible in the coming days that we will see additional cases of Ebola," he said.

Frieden said there was one person who may have had contact with the infected health worker when she could possible transmit the disease and that person is being monitored.

Frieden said the intubation of Duncan and use of a dialysis machine - measures taken while trying to save his life - posed high risk for transmission of the virus.

Duncan died in an isolation ward on Oct. 8, 11 days after being admitted. More than 50 people attended to his care. The hospital said it was decontaminating its isolation unit while health officials said Duncan's body had been cremated.

'FULL CDC PRECAUTIONS'

Dan Varga, the hospital's chief clinical officer told a news conference that the infected worker "was following full CDC precautions ... so gown, glove, mask and shield."

The hospital has already faced criticism for at first turning away Duncan when he first showed up there on Sept. 25, saying he had been in Liberia and had a fever. About two days after he was discharged, he grew much sicker and was taken back by ambulance and put in an isolation unit.

None of the 10 people who had close contact with him or 38 people who had contact with that group have shown any symptoms, state health officials said.

Texas officials did not identify the health worker or give any details about the person, but CNN said it was a woman nurse.

The Texas case is not the first outside badly hit West Africa in which a health care worker contracted the disease after contact with a patient.

In Spain, a nurse who contracted Ebola after caring for two infected priests repatriated to Spain remained seriously ill but is showing signs of improvement. Teresa Romero, 44, is so far the only person who has tested positive for Ebola through a transmission in the country.

Fifteen people were being monitored in a Madrid hospital for signs of Ebola on Sunday, as the Spanish government tries to contain recriminations over how it has handled the case. None have so far shown any symptoms.

EBOLA PAMPHLETS

In Dallas, there was a yellow hazardous material drum on the lawn of the brick apartment where the Texas health worker lived and information pamphlets about the Ebola virus were stuffed in the doors in the surrounding blocks of the apartment.

Neighbor Cliff Lawson, 57, said he was woken at 6:00 a.m. by two Dallas police officers who told him "don't panic."

"I went back to bed after that. There's nothing you can do about it. You can't wrap your house in bubble wrap," Lawson said.

A team is decontaminating the patient's apartment and car, Dallas Mayor Mike Rawlings said.

The hospital said in a statement that the new patient, who had not been working for two days, had been taking her own temperature twice a day. The worker informed the hospital of a fever and was isolated immediately upon arrival there.

A union for registered nurses said the Ebola case in Dallas shows that not enough is being done to educate health workers on how to manage patients who show signs of infection.

"Handing out a piece of paper with a link to the Centers for Disease Control, or telling nurses just to look at the CDC website - as we have heard some hospitals are doing - is not preparedness," said Bonnie Castillo, a registered nurse and senior official with National Nurses United.

SCREENING AT JFK AIRPORT

News of the second patient in Dallas came as U.S. authorities step up efforts to stop the spread of the virus. New York's John F. Kennedy Airport on Saturday began the screening of travelers from the three hardest hit West African countries.

Liberia is the country worst affected by the virus with 2,316 victims, followed by 930 in Sierra Leone, 778 in Guinea, eight in Nigeria and one in the United States, the World Health Organization said on Friday. Some 4,033 people are known to have died in seven countries from the outbreak, it said.

Ebola is spread through contact with bodily fluids of an affected person or contamination from objects such as needles. People are not contagious before symptoms such as fever develop.