New Study: DR Congo Ebola Outbreak Caused by Fresh Spillover from Animals to Humans

The ongoing Ebola outbreak in eastern Democratic Republic of Congo (DRC) has been traced to a new zoonotic spillover event—the transmission of the virus from an animal reservoir to humans—rather than from strains associated with previous outbreaks, according to a study published in the medical journal Nature Medicine.

The Ebola outbreak in this region has claimed more than 2,000 lives among a total of 4,449 confirmed cases, making it the fastest-spreading Ebola epidemic on record. The outbreak was officially declared on 15 May, but subsequent genetic sequencing has shown the virus had been circulating since February.

On 18 June, the World Health Organization (WHO) warned that the current Bundibugyo strain is spreading faster than health authorities can control it and risks becoming the deadliest documented outbreak.

What the New Study Says

In a report released this week in Nature Medicine, researchers from the DRC, Uganda, Belgium and several other countries analyzed the genetic characteristics of virus samples from 22 patients in the DRC and Uganda. They found that the strain driving the current epidemic is genetically distinct from the Bundibugyo strains that caused outbreaks in 2007 and 2012.

The report concludes that the outbreak began with a new animal-to-human spillover event, followed by human-to-human transmission. The specific animal source has not yet been identified, but scientists confirmed that the outbreak in Uganda is linked to the one in the DRC.

Earlier, regional health authorities had reported that the outbreak was caused by the rare Bundibugyo strain, for which there is currently no vaccine.

Dr. Krutika Kuppalli, an infectious disease specialist with experience responding to Ebola, explained that the study's finding of a “new zoonotic spillover” does not mean a new form of Ebola has suddenly emerged or that animals are transmitting the virus differently.

“The genetic data suggest that the current outbreak began with a new 'spillover' event – the virus may have independently crossed from an animal reservoir into humans – rather than from continued circulation or resurgence of virus from previous outbreaks,” she said. “After that initial event, the outbreak has been driven primarily by human-to-human transmission.”

How Dangerous Is This Strain?

On 18 June, WHO Director-General Tedros Adhanom Ghebreyesus warned that at its current pace, this Ebola outbreak could surpass the West African epidemic of 2014–2016. That epidemic killed at least 11,300 people among roughly 28,600 cases and spurred vaccine development.

The West African outbreak was caused by the Zaire strain, and the vaccines developed then are not effective against the Bundibugyo strain currently circulating.

Dr. Abdirahman Mahamud, WHO's director of health emergency response, said the medium-term forecast is for the epidemic to peak within six months. In a more severe scenario, it could last nine to twelve months.

Professor Kaja Abbas, an infectious disease epidemiologist at the London School of Hygiene & Tropical Medicine, noted that it is the human-to-human transmission dynamic that makes the Bundibugyo strain dangerous, not the initial animal origin.

Treatment Approaches

According to Dr. Kuppalli, the animal origin of the virus does not alter the necessary treatment approach. “Once a person has Ebola, management is based on the virus and the patient's clinical condition, not whether the infection came from an animal or another human. The important point is that the Bundibugyo strain currently has no licensed specific treatment as the Zaire strain does,” she said.

She stressed that supportive care—including aggressive fluid and electrolyte replacement, treatment of shock and organ dysfunction, and management of co-infections—remains critical, while experimental therapies are being evaluated.

“Past Ebola outbreaks have often started with animal spillover. The bigger concern is what this says about our preparedness: we still do not fully understand where and when these events occur, making prevention and early detection extremely difficult,” she added.

Containment Strategies

Researchers involved in the new study argue that strengthening “decentralized diagnostic capacity, including genomic sequencing and timely detection” could help prevent future outbreaks.

Professor Abbas said that although there is no licensed vaccine or treatment for the Bundibugyo strain, experimental therapies and vaccines are in clinical development targeting the strains circulating during the 2026 outbreak in the DRC and Uganda.

The WHO has sent 12 tons of medical supplies, including personal protective equipment for frontline health workers, with an additional 6 tons expected to arrive soon, according to Anne Ancia, the WHO representative in Ituri province.

More than 100 health workers have been infected, and about 35 have died. Response efforts have been hampered by strikes over unpaid health worker wages, misinformation, and traditional practices such as open-casket funerals, which can increase the risk of transmission.

Dr. Kuppalli called for stronger Ebola preparedness, not just efforts to halt spread after an outbreak is detected. “We need more robust surveillance at the human-animal interface, broader diagnostic tests that can detect different ebolaviruses, and medical countermeasures that work across multiple species,” she said. “This outbreak shows the consequences when a virus has several months of a head start.”

International Response

Many countries have expressed concern over the latest Ebola outbreak. Bahrain has suspended entry for 30 days for foreign travelers arriving from South Sudan, the DRC, and Uganda. Neighboring countries such as Rwanda and Uganda have implemented measures to prevent the virus crossing their borders. Rwanda has banned entry for foreigners who have been in the DRC in the previous 30 days. The United States has warned that travelers, including U.S. citizens, who have been in the DRC within 21 days of departure will not be allowed to board commercial flights to the U.S. Several Asian governments have also begun implementing border screening and enhancing quarantine preparations.