* Bulk of cases in Guinea, Liberia, Sierra Leone
* Total includes new cases in Mali, U.S.
* WHO warns that Ebola coming closer to Ivory Coast
GENEVA, Oct 25 (Reuters) - The death toll from the Ebola epidemic rose to 4,922 out of 10,141 known cases in eight countries through Oct. 23, the World Health Organization (WHO) said on Saturday.
The virus, which reached Mali through a two-year-old girl who died on Friday, now threatens Ivory Coast, having infected people virtually all along its borders with Guinea and Liberia.
Ivory Coast is the world's biggest cocoa producer. The Ebola outbreak has hurt the economic growth that has been raising living standards in the region.
The three worst-hit countries of West Africa -- Guinea, Liberia and Sierra Leone -- account for the bulk of the world's worst Ebola outbreak, recording 4,912 deaths out of 10,114 cases, the WHO said in its update.
The overall figures include outbreaks in Nigeria and Senegal, deemed by the WHO to be now over, as well as isolated cases in Spain, the United States and a single case in Mali.
But the true toll may be three times as much: by a factor of 1.5 in Guinea, 2 in Sierra Leone and 2.5 in Liberia, while the death rate is thought to be about 70 percent of all cases.
The WHO has said that many families are keeping infected people at home rather than putting them into isolation in treatment centers, some of which have refused patients due to a lack of beds and basic supplies.
The U.N. agency, sounding an ominous note, said that out of the eight districts of Liberia and Guinea sharing a border with Ivory Coast, only two have yet to report confirmed or probable Ebola cases.
It has also said trials of Ebola vaccines could begin in West Africa in December, a month earlier than expected, and hundreds of thousands of doses should be available for use by the middle of next year.
The WHO says 15 African states including Ivory Coast are at highest risk of the deadly virus being imported.
In the last 10 days it sent teams to both priority Mali and Ivory Coast to help national authorities gear up their capacity to detect and treat potential cases. Four WHO experts are traveling this weekend to Mali to reinforce the team there.
The agency warned on Friday that many people in Mali had potentially been exposed to the virus because the little girl was taken across the country while ill. Some 43 people with whom she was in contact, including 10 health care workers, are being monitored for symptoms that include fever.
In all, 450 health care workers have been infected to date -- including one in Spain and three in the United States -- leading to the death of 244 of them, the WHO said.
"At the same time, exhaustive efforts are ongoing to ensure an ample supply of optimal personal protective equipment to all Ebola treatment facilities, along with the provision of training and relevant guidelines to ensure that all HCWs (health care workers) are exposed to the minimum possible level of risk."
A medical worker quarantined in New Jersey on her return from treating Ebola victims in West Africa was being evaluated in a hospital isolation ward on Saturday after new contagion-control safeguards were imposed for America's biggest urban center.
Isolation wards have been used for medical personnel returning from Ebola zones since Craig Spencer, a doctor who treated patients in Guinea for a month, came back to New York City infected.
"The patient is currently in isolation at Bellevue Hospital in New York City, one of eight New York State hospitals that have been designated to treat patients with Ebola Virus Disease. Possible contacts are being identified and followed up," WHO said. (Reporting by Stephanie Nebehay; Editing by Mark Heinrich/Ruth Pitchford)