FREETOWN, Sierra Leone (AP) — A third top doctor has died from Ebola in Sierra Leone, a government official said Wednesday, as health workers tried to determine how a fourth scientist also contracted the disease before being evacuated to Europe.

The announcements raised worries about Sierra Leone's fight against Ebola, which already has killed more than 1,400 people across West Africa. The World Health Organization said it was sending a team to investigate how the epidemiologist now undergoing treatment in Germany may have contracted the disease that kills more than half its victims.

"The international surge of health workers is extremely important and if something happens, if health workers get infected and it scares off other international health workers from coming, we will be in dire straits," said Christy Feig, director of WHO communications.

Dr. Sahr Rogers had been working at a hospital in the eastern town of Kenema when he contracted Ebola, said Sierra Leonean presidential adviser Ibrahim Ben Kargbo on Wednesday. Two other top doctors already have succumbed to Ebola since the outbreak emerged there earlier this year, including Dr. Sheik Humarr Khan, who also treated patients in Kenema.

Rogers' death marks yet another setback for Sierra Leone, a country still recovering from years of civil war, where there are only two doctors per 100,000 people, according to WHO. By comparison, there are 245 doctors per 100,000 in the United States.

The Senegalese epidemiologist who was evacuated to Germany had been doing surveillance work for the U.N. health agency, said Feig, the WHO spokeswoman. The position involves coordinating the outbreak response by working with lab experts, health workers and hospitals, but does not usually involve direct treatment of patients.

"He wasn't in treatment centers normally," she said by telephone from Sierra Leone. "It's possible he went in there and wasn't properly covered, but that's why we've taken this unusual measure — to try to figure out what happened."

WHO said late Tuesday that it was pulling out its team from the eastern Sierra Leonean city of Kailahun, where the epidemiologist working with the organization was recently infected. The team was exhausted and the added stress of a colleague getting sick could increase the risk of mistakes, said Daniel Kertesz, the organization's representative in the country.

Canada also announced late Tuesday it was evacuating a three-member mobile laboratory team from Sierra Leone after people in their hotel were diagnosed with Ebola. The Public Agency of Canada said none of the team members was showing any signs of illness but that they would remain in voluntary isolation during the 21-day incubation period.

Health workers have been especially vulnerable because of their close proximity to patients, who can spread the virus through bodily fluids. WHO says more than 120 health workers have died in the four affected countries — Sierra Leone, Liberia, Guinea and Nigeria.

While some local health workers have lacked proper protective gear, the teams from the World Health Organization and Doctors Without Borders are usually well-equipped and trained in how to use the protective suiting.

A team of two experts was sent Tuesday to investigate whether the case occurred through straightforward exposure to Ebola patients, or something else, said Feig, the WHO spokeswoman. She said the team is checking to see if there is an infection risk in the living and working environments that had not been discovered.

There is no proven treatment for Ebola, so health workers primarily focus on isolating the sick. But a small number of patients in this outbreak have received an experimental drug called ZMapp. The London hospital treating a British nurse infected in Sierra Leone, William Pooley, said he is now receiving the drug.

It was unclear where the doses for Pooley came from. The California-based maker of ZMapp had said that its supplies were exhausted and that it would take months before more doses would be available.

Two Americans, a Spaniard and three health workers in Liberia have received ZMapp, though it is unclear if the drug is effective. The Americans have recovered and have been released from an Atlanta hospital, but the Spaniard died, as did a Liberian doctor.

Health officials in Liberia said the other two recipients of ZMapp in Liberia — a Congolese doctor and a Liberian physician's assistant, have recovered. Both are expected to be discharged from an Ebola treatment center on Friday, said Dr. Moses Massaquoi, a Liberian doctor with the treatment team.

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Associated Press Medical Writer Maria Cheng in London; Jonathan Paye-Layleh in Monrovia, Liberia; David Rising in Berlin; Krista Larson in Dakar, Senegal; and Rob Gillies in Toronto contributed to this report.