Twenty people have been rescued so far after the ferry sank late on Friday in the Busira River. It was travelling as part of a convoy of other vessels and the passengers were primarily merchants returning home for Christmas, said Joseph Joseph Kangolingoli, the mayor of Ingende, the last town on the river before the site of the accident.

According to Ingende resident Ndolo Kaddy, the ferry carried “more than 400 people because it made two ports, Ingende and Loolo, on the way to Boende, so there is reason to believe there were more deaths”, The Associated Press news agency reported.

Central government authorities have yet to comment on the incident and offer further details about rescue operations.

Congolese officials have often warned against overloading boats and pledged to punish those violating safety measures on rivers. However, in remote areas, many people cannot afford public transportation on the few available roads.

The incident comes less than four days after another boat capsized in the country’s northeast, killing 25 people.

At least 78 people drowned in October when a boat that was reportedly carrying hundreds of passengers capsized on Lake Kivu, in the country’s east.

Another 80 lost their lives in a similar accident on the Kwa River, about 70km (43 miles) from the city of Mushie in Mai-Ndombe province, near Kinshasa, in June.

Health authorities on Tuesday said the disease, circulating in the southwestern Kwango province, presents in the form of a respiratory illness.

 Earlier this month, local authorities said the disease had killed 143 people in the country’s Panzi health zone in November, as fears surmounted about the mysterious illness.

“The mystery has finally been solved. It’s a case of severe malaria in the form of a respiratory illness,” the Ministry of Public Health said in a statement, adding that malnutrition in the area had weakened the local population, leaving them more vulnerable to disease.

The statement said that 592 cases had been reported since October, with a fatality rate of 6.2 percent.

Provincial health minister, Apollinaire Yumba, told the Reuters news agency that anti-malaria medicine provided by the World Health Organization was being distributed in the main hospital and health centres in the Panzi health zone.

A WHO spokesperson said more health kits for moderate and critical cases were due to arrive on Wednesday.

The symptoms of the disease are fever, headache, cough, runny nose and body aches.

Most of the cases and deaths are in children under 14, according to national health authorities, with children under five representing the majority of cases.

“Respiratory distress was noted in some children and some other people who died,” Congolese Minister of Health Roger Kamba said earlier this month, noting that some patients were anaemic, which was the cause of some of the deaths linked to the disease.

The outbreak of the disease is some 700km (435 miles) away from DRC’s capital, Kinshasa, with the Panzi health zone “rural and remote”, the WHO has said, which added challenges in investigating it.

A doctor at Panzi Hospital told Al Jazeera last week that the facility was not sufficiently equipped to deal with the outbreak.

According to the Severe Malaria Observatory, the DRC has the second-highest number of malaria cases and deaths globally. Malaria is also the country’s leading cause of death, according to the observatory.

Forty-year-old Denise Kabeya lives in the Panzi area in DRC’s western Kwango province – where health authorities say most cases of the unknown disease have emerged since October.

That month, Kabeya’s 12-year-old daughter Diane fell ill.“My beloved little one had headaches, runny nose, no appetite and was physically weak,” Kabeya said. “I thought it was malaria and typhoid fever,” she shared, explaining that those diseases are endemic in the region.

When Diane developed a blocked nose, Kabeya hoped the worst was over and that she would make a speedy recovery, as was usually the case when she developed such symptoms. But the child complained about more aches and pains, and eventually passed away from her illness in late October.

Between October 24 and December 11, there were 514 cases of the undiagnosed disease reported in eight of the 30 health areas in Panzi health zone, according to figures from the local health authorities.

Earlier this week, the WHO, which has sent experts to the region to investigate the cause of the outbreak and help with the response, said more than 30 people had died in hospitals from the disease.

There were another 44 deaths recorded in remote communities in Panzi, Congolese Minister of Health Roger Kamba said last week.Kamba said the outbreak was first reported in October, with the alert level raised to high at the end of November.

Most of the cases and deaths are in children under 14, national health authorities said, with children under five presenting with the majority of cases.

Because so little is known about this mystery disease, it is difficult to know why children are so vulnerable, experts said.

Speaking at a media conference last week, Kamba said that the disease is flu-like.

“Respiratory distress was noted in some children and some other people who died,” Kamba, said, noting that some patients were anaemic, which was the cause of some of the deaths linked to the disease.

 Cases of severe malnutrition have aggravated the disease in some children, according to local sources in Panzi, information also corroborated by the national Ministry of Health.

“Symptoms are flu-like. Patients suffer from high fever, headaches, coughing, runny nose and aches and pains,” Serges Zenga, an attending physician in the Kenge health zone, near Panzi, told Al Jazeera.

This week, WHO Director-General Tedros Adhanom Ghebreyesus said laboratory samples from infected people were being collected and tested. So far, most of the samples had tested positive for malaria, he said, but added that it was possible more than one disease was involved.

Meanwhile, the remoteness of the epicentre of the outbreak, as well as a lack of infrastructure, was jeopardising the fight against the disease.

Teams of laboratory technicians, epidemiologists from the Health Ministry and some of the Congolese government’s partners, including the WHO, are already deployed in the field, with a view to identifying the disease and reinforcing efforts to combat it.But located some 700km (435 miles) away from DRC’s capital, Kinshasa, the Panzi health zone is “rural and remote” and difficult to access, the WHO said – a feat made tougher by poor road conditions.

“We sent a team into the field. The team took two days to reach the area because the road is very bad in the rainy season,” Kamba said.

Additionally, rural Panzi lacks testing capacity, meaning that samples have to be taken to laboratories in Kikwit, more than 500km (310 miles) away.

The WHO said limited diagnostics in the region have delayed the identification of the underlying cause of the outbreak. However it added that its teams were helping with patient treatment, risk communication and engagement with the community as the health crisis continued.

Meanwhile, on the ground, medics say there are huge difficulties in caring for patients who are bedridden at Panzi hospital.

“We don’t have resuscitation kits such as oxygen condensers, we don’t have treatment centres and isolation facilities, and we don’t even have electricity; [there is] scarcity of communication tools and internet,” Dr Rufin Mukuwa, a doctor at the Panzi general referral hospital, told Al Jazeera.

Speaking to Al Jazeera by phone from Kenge, the capital of Kwango province, some residents said they feared the outbreak spreading from remote Panzi into their communities.

“We’re worried that the provincial authorities haven’t taken preventive measures to prevent this disease from spreading to other areas,” resident Emile Yimbu complained.

He called for the reinforcement of epidemiological surveillance measures to contain the disease within a well-defined zone, and lessen its consequences.

Prosper Kiswemba, a local councillor in Kenge, is also worried about how end-of-year travel plans in the region may affect the potential spread of the disease.

Kiswemba said hygienic measures such as temperature sampling and the installation of hand-washing stations along arterial roads leading to Panzi needed to be reinforced, lest the disease spread further.

Meanwhile, one local health expert who spoke to Al Jazeera on condition of anonymity, said they feared the disease may be zoonotic in origin.

Even though national and global health bodies have not announced any animal-related links, the expert said: “Most of the people I’ve interviewed personally admit to having been in contact with certain wild animals a few days before falling ill.”

To be safe, he said people should be cautioned about reducing contact with wild animals.

As the mystery disease spreads, patients in Panzi health zone are also suffering from a number of other diseases, including typhoid fever and measles, doctors said.

The timing of this new outbreak has some local health facilities overwhelmed, a local medical source told Al Jazeera.

Mukuwa at Panzi general hospital, however, sees the outbreak as a worrying but surmountable challenge.

“We are receiving numerous cases, varying between 15 and 20 patients a day,” he said.

He sees some symptoms of the disease – like respiratory issues – as being similar to those of COVID-19, and said patients at the hospital are being treated according to the signs and symptoms they present with.

“For example, we use azithromycin, amoxicillin or spiramycin for respiratory complications. If patients come in with headaches, we use paracetamol to relieve them,” he explained.

This has brought some relief, he said, while investigations into the outbreak continue.

Two years ago, Panzi suffered an epidemic of typhoid fever. The area also has high levels of malnutrition and low vaccination coverage, meaning children are vulnerable to a range of diseases, the WHO’s Tedros said this week.

This new disease comes as doctors in the DRC are still tackling the ongoing mpox epidemic, with at least 40,000 cases recorded and more than 1,000 deaths reported.