The outbreak began in the village of Boloko after three children ate a bat and died within 48 hours, the Africa office of the World Health Organization said on Monday.
After the second outbreak of the mystery disease began in the village of Bomate on February 9, samples from 13 cases were sent to the National Institute for Biomedical Research in Congo's capital, Kinshasa, for testing, the WHO said.
The interval between the onset of symptoms - which include fever, vomiting and internal bleeding - and death has been 48 hours in most cases and "that's what's really worrying," said Serge Ngalebato, medical director of Bikoro Hospital, a regional monitoring centre.
An unknown illness first discovered in three children who ate a bat has rapidly killed more than 50 people in northwestern Congo over the past five weeks, health experts say.
There have long been concerns about diseases jumping from animals to humans in places where wild animals are popularly eaten.
