According to health officials, 31 people emerged in the Democratic Republic of Congo (DRC) three months ago.
Hundreds of cases have been identified in Ituri province, in the northeast of the country.
Patrick Karamura, the region’s health minister, told AFP on Friday: “We have more than 520 cases … of which more than 31 were fatal.”
The cases are the bulging form of the disease, except for five cases of pneumonia and two of septicemic plague, he said.
Anne Laudisoit, an epidemiologist from the New York-based NGO EcoHealth Alliance, said the cases surfaced in the province between November 15 and December 13.
The mean age of patients was 13, but it ranged between three months and 73 years.
EchoHealth Alliance warned last month that adolescents under the age of 17 are the most dangerous group, representing 78.9 percent of all patients.
Plague has continued in Ituri province since it was first confirmed there in 1926.
It is a contagious disease caused by the bacterium Yersinia pestis and is found in animals around the world, including rats, squirrels and prairie dogs. Fleas usually serve as the vector for plague.
People can also become infected through direct contact with an infected animal, through inhalation, and in the case of pneumonia, person to person.
Yersinia pestis can be treated with antibiotics if started early enough.
Ms Laudisoit, who works with a team of researchers in Ituri, said an early sign of the latest outbreak came with the mass death of rats.
The outbreak comes amid a worrying revival of Ebola in the DRC.
Two people died within a week of the disease in the eastern province of North Kivu.
The DRC’s Ministry of Health has deployed a team to the health zones of Biena and Katwa to track down more than 100 contacts of two women who died of the deadly disease.
Ebola swept through eastern Congo from 2018 to 2020 in an outbreak that killed more than 2,200 people before being declared last June.
At least three people have also been killed in southeastern Guinea on Ebola, the country that has re-emerged since the deadly 2013-2016 outbreak
A further five patients tested positive for the disease and are being isolated in treatment centers, officials said.
The last major outbreak of Ebola in West Africa began in Guinea in 2013, with a consequent death toll of more than 11,000 after the disease spread throughout the continent. The vast majority of cases were in Guinea, Liberia and Sierra Leone.