The toll from the Ebola virus outbreak spreading in West Africa has risen to 1,663 cases and 887 deaths, the World Health Organization said in an update Monday.

The week ending Aug. 1 saw 163 new cases and 61 deaths, the health agency said.

The newest numbers mean the overall death rate stands at 53%. Some previous Ebola outbreaks have had death rates up to 90%, and medical experts have speculated that faster, improved medical care may play a role.

There is no approved medicine or vaccine specifically for Ebola, but supportive care, including maintaining fluids, can help.

Two Americans stricken with the virus in Liberia may be showing signs of improvement after receiving additional experimental treatments there.

Physician Kent Brantly and missionary Nancy Whitebol each "received a dose of an experimental serum while still in Liberia," Brantly's organization Samaritan's Purse said in a press release. "Dr. Brantly also received a unit of blood from a 14-year-old boy who had survived Ebola under his care."

An especially detailed report from CNN, quoting unnamed sources, says Whitebol and Brantly received infusions of antibodies tested only in monkeys.

Writebol remains in serious but stable condition as she prepares to return to the USA for further treatment Tuesday, her sponsoring organization, SIM USA, said in a statement Monday.

Writebol, who is likely to join Brantly in a special isolation unit at Emory University Hospital in Atlanta, showed at least one sign of improvement over the weekend, says Bruce Johnson, president of the charity group: "Her husband, David, told me Sunday her appetite has improved, and she requested one of her favorite dishes – Liberian potato soup – and coffee."

Brantly also "seems to be improving," the director of the Centers for Disease Control and Prevention, Tom Frieden, said Sunday on CBS' Face the Nation.

Emory wouldn't comment on the condition of the patient in isolation, to protect the patient's privacy, says Vince Dollard, a hospital spokesperson.

Infection with the Ebola virus causes fever and headache in early stages but can lead to hemorrhaging, liver failure and kidney failure and death in rapid progression.

CNN said the experimental treatment give to Brantly and Writebol is an antibody drug called ZMapp and was developed by Mapp Biopharmaceutical Inc.

That is one of several experimental therapies under development and previously untested in humans, says Thomas Geisbert, a professor at the University of Texas Medical Branch in Galveston.

Brantly is the first American to be treated for the illness in the USA. All of the cases in the current outbreak have originated in Guinea, Liberia, Sierra Leone and Nigeria.

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