BUNIA, Congo (AP) — Confirmed cases of Ebola in Congo have reached 2,011, including 754 deaths, according to government data released overnight in what authorities say is the fastest-growing outbreak on record.

Health workers at the Bunia General Hospital went on strike on Wednesday, the latest group to have walked off their job at the epicenter over payment issues. Health professionals and other frontline workers barricaded the entrance of the hospital, lamenting they haven’t received any compensation despite working under difficult conditions.

A total of 753 patients remain in isolation or in hospitals, while 366 have so far recovered, according to data from Congo’s Ministry of Health. Contact tracing remains a challenge, with coverage of those exposed still at 67%.

The Central African nation has been battling the Ebola outbreak caused by the rare Bundibugyo virus since May 15.

Two months since the onset, the outbreak continues to spread faster than health officials can track despite an expanding response. At least 80% of new cases are emerging from unknown chains of transmission, the World Health Organization said Tuesday.

A key challenge is that health authorities have yet to identify the outbreak’s patient zero, while displacements from armed conflict as well as mining-related movements have made it difficult to trace thousands who have come in contact with infected individuals.

Many of the newly reported deaths are people who died in their communities without ever reaching a health facility and without receiving care, Dr. Chikwe Ihekweazu, the WHO health emergencies chief, said Tuesday after returning from Bunia in Ituri, the worst-hit province in the outbreak.

Health workers are also going on strike in different parts of Ituri. Some have told The Associated Press they have not received any payment since they started work at the onset of the outbreak.

Response efforts have also been challenged by the lack of approved vaccines or treatments for the Bundibugyo virus, unlike the more common Zaire virus for which there is a vaccine and which was responsible for most of Congo’s past 16 outbreaks of the disease.

Enrollment for a highly anticipated study of two possible Ebola treatments recently kicked off in Ituri.