A total of 212 cases have been detected in Western Kasai province, said Fortunat Tumba Tshitoka, the provincial health minister.
A tropical medicine team is working to determine the cause, he said. Victims have died from dehydration following severe diarrhea and high fever.
Three tonnes of medicines have been delivered to the village of Kampungu and Mombo, where the illness has appeared. Medical teams have been deployed in the area.
"The situation is in the process of being brought under control," the minister said, adding that the number of cases being reported has gradually diminished.
A World Health Organisation mission equipped with a mobile laboratory was sent to Western Kasai on Tuesday.
The symptoms "resemble those of typhoid fever," said Francois Nguessan, the WHO's representative in the DR Congo.
Nguessan said he was only aware of 19 deaths being registered with local authorities, but said other cases may exist.