MONROVIA, Liberia — The Ebola outbreak that has killed more than 1,100 people in West Africa could last another six months, the Doctors Without Borders charity group said Friday. One aid worker acknowledged that the true death toll is still unknown.

New figures released by the World Health Organization showed that Liberia has recorded more Ebola deaths – 413 – than any of the other affected countries.

Tarnue Karbbar, who works for the aid group Plan International in northern Liberia, said response teams simply aren’t able to document all the erupting Ebola cases. Many of the sick are still being hidden at home by their relatives, who are too fearful of going to an Ebola treatment center.

Others are being buried before the teams can get to remote areas, he said. In the last several days, about 75 cases have emerged in Voinjama, a single Liberian district. “Our challenge now is to quarantine the area (in Voinjama) to successfully break the transmission,” he said.

There is no cure or licensed treatment for Ebola and patients often die gruesome deaths with external bleeding from their mouths, eyes or ears. The killer virus is transmitted through bodily fluids like blood, sweat, urine and diarrhea. A handful of people have received an experimental drug whose effectiveness is unknown.

Liberia’s assistant health minister, Tolbert Nyenswah, said three people in Liberia were receiving the ZMapp on Friday. Previously, only two Americans and a Spaniard had gotten it. The Americans are improving, but it is not known what role ZMapp played. The Spaniard died.