After nearly a week in hospital, the first Ebola patient diagnosed in the United States began to receive an experimental therapy initially developed to treat other viral diseases, according to the Dallas hospital where he lies in critical condition.

Federal health officials had said for days that the family of the Liberian man, Thomas Eric Duncan, had been told about experimental therapies to treat the deadly virus. As recently as Sunday, they said they were unaware whether or not Duncan, who fell ill about a week after arriving in Texas from Liberia on Sept. 20, was receiving any.

On Monday, however, a spokeswoman for Texas Health Presbyterian Hospital said Duncan began receiving a drug called brincidofovir, made by Durham, North Carolina-based Chimerix Inc, on Saturday afternoon. The drug, which comes in tablet form, has never been tested on laboratory animals infected with Ebola let alone in human Ebola patients.

Questions had been raised on social media about why Duncan had not received experimental treatments after he was admitted to the hospital on Sept. 28. Medical experts said the choice of the Chimerix drug may have been influenced both by the deterioration in Duncan's condition and the therapy's safety profile.

"When a patient becomes sick enough, there may be a feeling of, 'should we give it a shot?'," said bioethicist Dr. Kevin Donovan of Georgetown University Medical Center. "This alone may have tilted them toward using it, rather than having him die 'untreated.'"

At least three other Ebola patients flown to the United States from countries in West Africa where the virus has killed more than 3,400 people received experimental drugs.

The patients included two U.S. aid workers treated at Emory University Hospital in Atlanta, Georgia, who received Mapp Biopharmaceutical's ZMapp in July and August. A third U.S. aid worker treated Nebraska Medical Center in Omaha, who received Tekmira Pharmaceutical's TKM-Ebola last month.

All three recovered, but it is scientifically impossible to say whether the drugs played a role in their recovery, something only rigorous clinical trials can determine.

'AVAILABLE FOR IMMEDIATE USE'

Dr. Thomas Frieden, director of the U.S. Centers for Disease Control and Prevention (CDC), told reporters on Sunday that Duncan was apparently not receiving experimental therapies, in part because there are no more doses of ZMapp, a cocktail of three antibodies.

Asked about TKM-Ebola, Frieden said it "can be quite difficult for patients to take" and "can actually make someone sicker."

Neither supply nor administration is apparently an issue with brincidofovir, which was first developed to treat adenovirus infections in people with weakened immune systems.

"Chimerix has brincidofovir tablets available for immediate use in clinical trials," the company said in a statement. It also said on Monday that the U.S. Food and Drug Administration approved its emergency use for Ebola patients and that it was working to set up clinical trials.

TKM-Ebola is given intravenously. "IV access can be quite challenging with a medically compromised patient," Dr. Christopher Kratochvil, chief medical officer at UNeHealth, parent of the University of Nebraska Medical Center, said in an interview.

Shares in Chimerix rose 4.7 percent on Monday and shares in TKM-Ebola manufacturer Tekmira fell nearly 19 percent.

Another consideration in the choice of drugs for Duncan might have been safety. Ebola is infamous for causing massive fluid loss, which can lead to electrolyte imbalances that stress the heart and kidneys. Brincidofovir, also called CMX-001, has been tested in more than 1,000 patients without raising safety concerns, including kidney toxicity, the company said.

Chimerix faced a firestorm of criticism this year when it initially declined to provide the drug on a "compassionate use" basis to 8-year-old Joshua Hardy, a Virginia boy who developed a potentially-fatal adenovirus infection after a bone marrow transplant for kidney cancer.

The company relented and enrolled the boy in a clinical trial. He went home from the hospital in July, but the incident led Chimerix to replace its chief executive.

CMX-001 is a compound called a nucleotide analog. Its molecules behave enough like those that form the genetic material (nucleotides) of viruses such as Ebola that the microbes incorporate it into their DNA or RNA, a sister molecule.

But CMX-001 is different enough that, once incorporated, it prevent a virus's genetic material from replicating. That stops the virus from spreading throughout the body.

When West Africa's Ebola outbreak was reported last March, Chimerix sent brincidofovir to government labs, including at the CDC and National Institutes of Health, to be tested against test-tube samples of the Ebola virus. It showed "potent activity," the company said.