The number of suspected cases of Ebola has risen to 18 from nine in nearly a week in an isolated part of Democratic Republic of Congo, where three have died from the disease since April 22, the World Health Organization said on Thursday.
The W.H.O. was criticized for responding too slowly to an outbreak in West Africa in 2014 that left more than 11,000 people dead, and Dr. Peter Salama, the executive director of the organization’s health emergencies program, said at a briefing that it was essential to “never, ever underestimate Ebola” and to “make sure we have a no-regrets approach to this outbreak.”
The risk from the outbreak is “high at the national level,” the W.H.O. said, because the disease was so severe and was spreading in a remote area in northeastern Congo with “suboptimal surveillance” and limited access to health care.
“Risk at the regional level is moderate due to the proximity of international borders and the recent influx of refugees from Central African Republic,” the organization said, but it nonetheless described the global risk as low because the area is so remote.
About a week ago, in addition to the nine suspected cases, 125 patients who had come into close contact with the disease were being monitored. Now, about 400 patients are being followed, and the two newest possible cases were reported Monday and Tuesday, according to the latest report on the W.H.O. website.
The Ebola virus causes fever, bleeding, vomiting and diarrhea, and it spreads easily by contact with bodily fluids. The death rate is high, often surpassing 50 percent, particularly with the Zaire strain, which has been confirmed in two cases in this outbreak.
The outbreak was reported in a densely forested part of Bas-Uele Province, near the border with the Central African Republic. Cases have occurred in four separate parts of a region called the Likati health zone.
Aid groups and the W.H.O. have struggled to reach the affected area, which has no paved roads and can be reached only by a motorcycle ride through the forest, or by helicopter or light aircraft.
The first known case occurred on April 22, when a 39-year-old man who had fever, vomiting, diarrhea and bleeding died on the way to a hospital in the Likati zone. The person caring for him and a motorcyclist who transported him also died.
The first six months of the response to the outbreak are expected to cost the W.H.O. and aid groups $10 million, Dr. Salama said at the briefing. He said telecommunications networks would have to be established and airstrips repaired so that aid workers can provide the necessary medical care.
The W.H.O., aid groups and the Congolese government are discussing the possibility of using an experimental Ebola vaccine, made by the American pharmaceutical company Merck, that proved effective in Guinea.
The response would involve a “ring vaccination,” in which contacts of patients, contacts of contacts, and health workers would be vaccinated. There would be no mass public vaccination.
The vaccine has not yet been licensed, and its use would require permission on several fronts. Nonetheless, Dr. Salama said that if permission were granted, the vaccine could be made available in a week or so. Other experimental antiviral drugs may also be considered.
The Ebola virus is considered endemic in the Democratic Republic of Congo, where eight outbreaks, the largest involving about 300 patients, have been recorded since 1976.
The country “has considerable experience and capacity in confronting these outbreaks,” Dr. Daniel Bausch, an Ebola expert at the W.H.O., said in an email. He added, “I think there is a very good probability that control can be rapidly achieved.”
Dr. Salama said that aid workers had reached a town in the Likati zone, which was as close as they had been able to come to the epicenter of the outbreak. He said aid groups were setting up centers for treatment and isolation, and mobile labs.
The first aid group to arrive was the Alliance for International Medical Action, which was already in the region, responding to cholera.
In a telephone interview from Conakry, Guinea, the group’s executive director, Matthew Cleary, said that seven people who were believed to have contracted Ebola had been taken to a district hospital in the Likati zone that was not equipped to deal with the virus.
“It’s urgent to get them into a proper isolation center,” Mr. Cleary said, adding that the group was building treatment units. They will include windows that allow families to see patients, a response to past outbreaks in which people feared and sometimes shunned sealed-up isolation units into which patients seemed to disappear.
Brienne Prusak, a spokeswoman for Doctors Without Borders said on Wednesday that the group had sent a team of about 20 doctors, nurses and other experts to the Likati zone, and that it was still trying to figure out how to reach the epicenter.
“Transport is extremely difficult in the area, and helicopter flights may be the only way to get there,” she said by email. “We considered motorbikes but are now thinking of helicopters because we need to get so many materials there. We’re expecting to get to the epicenter by the weekend.”
The Centers for Disease Control and Prevention in the United States is also sending a renowned Ebola expert, Dr. Pierre Rollin, to Congo, along with epidemiologists, a spokeswoman said.