NAIROBI, Kenya — Congo has confirmed a case of Ebola in Mbandaka, a city of 1.2 million, marking the first urban case in the latest outbreak of the disease. The World Health Organization’s lead response official called the development “a game changer.”

Confirmed cases of Ebola had previously been limited to an extremely remote area more than 100 miles south of Mbandaka, in the rain forest of Congo’s Equateur province. The case in Mbandaka is only the third confirmed case of the current outbreak; 20 others are probable, and 21 are suspected, bringing the total of potential cases to 44. The death toll is now 23.

“This is a major development in the outbreak,” said Peter Salama, the WHO’s Deputy Director-General of Emergency Preparedness and Response. “We have urban Ebola, which is a very different animal from rural Ebola. The potential for an explosive increase in cases is now there.”

Equateur Province is the epicenter of the Ebola outbreak. Google map

The port city of Mbandaka lies on the eastern bank of the Congo River, Africa’s second longest after the Nile. Tens of millions of people live along the river, and the capital cities of Congo, Central African Republic, and the Republic of Congo lie along it and its tributaries.

Ebola is notoriously hard to contain, though recent outbreaks in Congo have been managed swiftly by the government and international global health institutions. This is the ninth Ebola outbreak in Congo since the 1970s, and the first since May of last year when five confirmed cases resulted in four deaths in a province neighboring Equateur.

An outbreak between 2014 and 2016 in West Africa was the worst ever, and killed more than 11,000. There were no cases in the Congo during that particular outbreak.

The disease causes internal bleeding and spreads rapidly through contact with small amounts of bodily fluid. Its early symptoms are not obvious, and the worst effects may take weeks to show. Ebola, endemic in Congo, is often transmitted to humans through the consumption of contaminated meat, but can also be acquired through any kind of close contact with an infected animal.

The international response to the current Congo outbreak has been substantial, and is expected to grow in size and urgency after the announcement of a confirmed urban case. On Wednesday, the WHO delivered 4,000 injections of an experimental vaccine with proven efficacy in recent trials, with more batches expected soon.

The WHO is also deploying 30 “experts” to Mbandaka to “conduct surveillance in the city and is working with the Ministry of Health and partners to engage with communities on prevention and treatment and the reporting of new cases.”

Persistent rain and lack of roads has hampered the effort to contain the outbreak so far. Before Thursday, cases had only been confirmed in Bikoro, a small town whose health clinic only has “limited functionality,” according to the WHO. Helicopter and motorcycle are the only ways to reach Bikoro from Mbandaka, but an airstrip has been rapidly cleared for small planes to land with supplies.

Part of the difficulty in deploying the vaccine is that it must be transported and stored at between -60 and -80 degrees Fahrenheit, which requires powerful refrigerators. The vaccine, produced by the pharma giant Merck, is not yet licensed, though the WHO has cleared it for “compassionate use.” Its deployment is being financed by Gavi, the Vaccine Alliance, which is Geneva-based.

“The remote location of the outbreak hampers both the information about the outbreak and interventions to control it,” said Cyrus Shahpar, director of epidemic prevention at Resolve to Save Lives, a New York-based organization. “This is evidenced by the fact that the current outbreak probably started in early April, but it was not officially declared until May 8th.”

The International Federation of the Red Cross and Red Crescent Societies said the first suspected case was a police officer who died in a health center in the village of Ikoki-Impenge, near Bikoro. After the his funeral, 11 family members got sick, and seven died. All seven had attended the man’s funeral or cared for him while he was sick.

A total of 514 people who may have been in contact with infected people have been notified by the national health authorities and are currently being monitored in Mbandaka and Bikoro, according to Doctors Without Borders, often known by its French acronym, MSF.

Several tons of supplies are en route to the region, including “protection and disinfection kits containing isolation items such as protective clothing, gloves, and boots; logistical and hygiene kits containing items such as plastic sheets, chlorine spray kits, and water treatment kits; and palliative drugs to treat Ebola symptoms, such as strong painkillers, anti-anxiety drugs, and antibiotics.”

With the confirmed arrival of the outbreak in a major city, the number of people who are likely to have interacted with infected individuals increases exponentially. Both the fact that Mbandaka is densely populated and that it is a bustling port heighten the risk of rapid spread.