Thirty more health care workers have died of Ebola in Sierra Leone than previously thought, the World Health Organization said on Tuesday, suggesting the risk to medical staff may have been understated.

A WHO update published on Monday put the number of dead health care workers in Sierra Leone at 61, out of a total of 96 who had fallen ill with the disease.

An update last week said 74 health care workers had caught Ebola in Sierra Leone as of Sept. 14 and 31 had died.

The revised figure means almost six out of 10 health workers who caught the disease in Sierra Leone have died, rather than four out of 10 as previously thought.

WHO epidemiologist Eric Nilles said the additional deaths were found during a "retrospective evaluation" that aimed to improve the overall quality of data published by the WHO.

"WHO continues to improve the characterization of the epidemic in Sierra Leone and as we do so we may see further increases in the proportion of health care workers affected," Nilles said.

The WHO said that as of Sept. 22, a total of 348 health care workers were known to have developed Ebola and 186 of them had died. Half of the cases were in Liberia and 67 in Guinea, which along with Sierra Leone have been worst hit by the outbreak.

In Nigeria, 11 health workers have contracted the disease and five have died. In total, Nigeria has had 8 Ebola deaths and 20 cases.

United Nations and WHO officials have said the high risk for health workers has hampered efforts to find volunteers to go to the epicenter of the outbreak in West Africa.

"We have difficulty in attracting the required numbers of people who are volunteering and feel comfortable to do so," said Antonio Viglilante, head of the U.N. Development Programme in Liberia.

The Ebola epidemic in West Africa has killed more than 2,800 people since it began in Guinea earlier this year.