The Ebola virus outbreak in West Africa has now claimed 137 lives, according to the World Health Organization. And researchers say a new strain of the virus appears to be responsible.
In a statement posted on its website Thursday, the U.N. health agency said authorities have identified more than 220 suspected or confirmed cases of the disease in Guinea and neighboring Liberia. The vast majority of the cases - nearly 200 of them - occurred in Guinea.
"The source of the virus is still not known," said Dr. Stephan Gunther of the Bernhard Nocht Institute for Tropical Medicine in Hamburg, Germany.
Gunther led an international team of researchers who studied the genetics of the virus and reported results online Wednesday in the New England Journal of Medicine.
The new research identifying this strain analyzed blood samples from 20 patients in the current outbreak and found the strain was unique.
"It is not coming from the Democratic Republic of Congo. It has not been imported to Guinea" from there or Gabon, where Ebola also has occurred, Gunther said. Researchers think the Guinea and other strains evolved in parallel from a recent ancestor virus.
Ebola causes internal bleeding and organ failure and is fatal in 30 percent to 90 percent of cases, depending on the strain. It spreads through direct contact with infected people, and some earlier cases have been linked to certain fruit bats that live in West Africa.
There is no cure or vaccine, so containing the outbreak has focused on supportive care for those infected with the virus and isolating them to limit its spread.
The Guinea outbreak likely began last December or earlier and might have been smoldering for some time unrecognized. The investigation continues to try to identify "the presumed animal source," health officials report.