Washington (AFP) - A US doctor stricken with Ebola in Liberia was offered an experimental serum but insisted that his colleague receive it instead, a Christian aid agency said Thursday.
There is no known cure for Ebola, which has killed 729 people in Sierra Leone, Guinea, Nigeria and Liberia since March as West Africa faces the largest outbreak in history.
Two Americans are among those battling the often fatal fever -- doctor Kent Brantly and healthcare worker Nancy Writebol.
They "are in stable but grave condition," said a statement from Samaritan's Purse, noting that "Brantly took a slight turn for the worse overnight."
The group did not elaborate further on his condition. Brantly was hospitalized last week after working with Ebola patients for more than a month.
SIM USA, the charity for which Writebol worked, said her condition had also "worsened," and confirmed that she has received an "experimental drug that doctors hope will better address her condition."
Samaritan's Purse said that when an "experimental serum" arrived in the capital, Monrovia, on Wednesday, there was only enough for one person and Brantly had asked for it to be given to Writebol.
"Even as he battles to survive Ebola, this heroic doctor is still focused on the well-being of others," said the group's statement.
It also noted that Brantly, 33, had been given a unit of blood from a 14-year-old boy who survived Ebola because of his care.
"The young boy and his family wanted to be able to help the doctor who saved his life," it said.
A spokeswoman for Samaritan's Purse told AFP she had no other details on the serum or where it came from, "other than it is an experimental treatment."
US Centers for Disease Control and Prevention chief Tom Frieden told reporters that there is no known cure for Ebola.
"I don't know the details of what may have been given," he said on a conference call to discuss CDC's latest warning against non-essential travel to West Africa.
"We have reviewed the evidence of the treatments out there and we don't have any treatment that has proven effectiveness against Ebola disease," Frieden said.
He also said it was unclear whether getting blood from a recovered patient would be helpful.
"This was something that was done in the past for infectious diseases, but there are so many things we don't know about why someone may recover, which antibodies may be protective and which may be harmful," Frieden said.
"It is very difficult to know how to comment on that."
The Ebola virus can be fatal in up to 90 percent of cases, though this outbreak has killed about 60 percent of those infected.
On Tuesday, the outbreak claimed the life of Umar Khan, 43, a top doctor in Sierra Leone and the West African nation's sole virologist.