CHICAGO (Reuters) - The third American to be treated for Ebola in the United States is showing "remarkable improvement" after receiving an infusion of plasma from U.S. Ebola survivor Dr. Kent Brantly, as well as an undisclosed experimental drug, his doctors said on Thursday.

Dr. Rick Sacra, 51, who is being treated in a special biocontainment unit at the University of Nebraska Medical Center in Omaha, received two doses of plasma from Brantly, which doctors are calling a convalescent serum, and has been given nightly doses of an undisclosed experimental drug, Dr. Phil Smith, one of Sacra's doctors, said in a news briefing.

"I don't know how much of his recovery is due to the drug, how much is due to the convalescent serum and how much to the aggressive intravenous fluids," Smith said.

Brantly's blood likely contains protective antibodies that may help buy Sacra some time while his body tries to fight off the infection, Smith said. The hospital tried a number of potential donors, but Brantly's blood type turned out to be a match for his friend and fellow missionary Sacra.

"It really meant a lot to us that he was willing to give that donation so soon after his recovery," Debbie Sacra, the patient's wife, told the briefing. "I spoke to his (Brantly's) wife. We marveled that they had the same blood," she said.

The worst-ever Ebola outbreak, which has already killed at least 2,296 people in West Africa, has triggered a scramble to develop the first drug or vaccine for a deadly disease that was discovered nearly 40 years ago in the forests of central Africa.

Smith said he has been asked not to disclose the name of the experimental drug Sacra is receiving because it is still in the early stages of development and there is no data on whether it works.

Sacra arrived at the Nebraska hospital from Liberia on Friday, Sept. 5, and has since shown a "remarkable improvement," Dr. Angela Hewlett told the briefing.

Smith said Sacra's first day was "pretty rocky" but he began improving by the third day of care. His wife, Debbie, said she is "amazed" at how quickly Sacra has "turned around since he arrived."

She said Sacra contracted Ebola on Aug. 29 while working at a hospital in Liberia on behalf of the North Carolina-based Christian group SIM USA. Sacra had worked in the obstetrics ward at the ELWA Hospital of SIM in Monrovia.

By Julie Steenhuysen