KINSHASA, Congo (AP) — An unknown illness has killed over 50 people in northwestern Congo, according to doctors on the ground and the World Health Organization on Monday.

The interval between the onset of symptoms and death has been 48 hours in the majority of cases, and "that’s what’s really worrying,” Serge Ngalebato, medical director of Bikoro Hospital, a regional monitoring center, told The Associated Press.

The outbreak began on Jan. 21, and 419 cases have been recorded including 53 deaths.

According to the WHO's Africa office, the first outbreak in the town of Boloko began after three children ate a bat and died within 48 hours following hemorrhagic fever symptoms.

There have long been concerns about diseases jumping from animals to humans in places where wild animals are popularly eaten. The number of such outbreaks in Africa has surged by more than 60% in the last decade, the WHO said in 2022.

After the second outbreak of the current mystery disease began in the town of Bomate on Feb. 9, samples from 13 cases have been sent to the National Institute for Biomedical Research in Congo's capital, Kinshasa, for testing, the WHO said.

All samples have been negative for Ebola or other common hemorrhagic fever diseases like Marburg. Some tested positive for malaria.

Last year, another mystery flu-like illness that killed dozens of people in another part of Congo was determined to be likely malaria.