Animal-to-human viral leap sparked deadly Marburg outbreak

The third-largest outbreak in history of the deadly Marburg virus was sparked by a single jump of the pathogen from an animal to humans, preliminary genomic evidence shows.

The outbreak began last month in Rwanda, where it has infected 63 people of whom 15 have died. Other evidence suggests that the first person to become infected in the outbreak probably contracted the disease during a visit to a cave that hosts a species of bat known to harbour the virus.

Multiple introductions from animals to humans would have raised fears that the virus is more widespread in Rwanda than was previously thought. Multiple introductions could also have raised the prospect of new outbreaks, as could an unknown provenance for the virus.

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Rwanda’s response to the virus has also kept the outbreak from being even worse, researchers say. Scientists praise the country’s efforts to control the outbreak, investigate its origins and share data with the scientific community. “As soon as they realized it was a problem, they started contact tracing, conducted a thorough epidemiological investigation, identified the [first] patient and potentially the source of the infection — and managed to roll out an experimental vaccine trial within a week,” says Angela Rasmussen, a virologist at the University of Saskatchewan in Saskatoon, Canada. This shows that, with the Marburg virus disease, “a rapid urgent response can mitigate severity of the outbreak”, she adds.

The findings, which have not yet been published in full or peer reviewed, were posted on the social-media platform X and discussed at a media briefing on 20 October.

Quick containment

The outbreak, which was declared on 27 September, is Rwanda’s first; Tanzania and Equatorial Guinea recorded their first Marburg outbreaks last year, and Ghana’s first was in 2022. Outbreaks of Marburg — which causes high fever, severe diarrhoea, nausea and vomiting, and in severe cases, bleeding from the nose or gums — now occur about once a year. Before the 2020s, they were detected at most a few times each decade.

Since the outbreak began, reports of new infections have dropped off notably. Rwandan health officials have recorded one new case and no deaths in the past 10 days, and only two people remain in isolation and treatment. A Marburg outbreak can be declared over when no new cases have been reported for 42 consecutive days.

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There is no proven vaccine or treatment for infections with the virus, which is closely related to Ebola virus, both in its symptoms and in its transmission, which is primarily through contact with bodily fluids. Health officials are offering a candidate vaccine, made by the Sabin Vaccine Institute in Washington DC, to the contacts of infected individuals. More than 1,200 doses have been administered so far.

This outbreak has one of the lowest fatality rates — about 24% — on record for Marburg; previous outbreaks reported fatality rates as high as 90%. This is probably a result of there being quick diagnoses, access to medical care and that most infections are in relatively young health-care workers.

In fact, two people who were infected with the virus and put on life support were successfully intubated and later extubated as they recovered. This marks the first time that people with Marburg virus disease have been extubated in Africa, Tedros Adhanom Ghebreyesus, director-general of the World Health Organization in Geneva, Switzerland, said at a briefing on 20 October. “These patients would have died in previous outbreaks,” he said.

Single source

To help inform outbreak control, researchers at the Rwanda Biomedical Centre in Kigali sequenced the Marburg virus genome of several infected people. They found that all samples closely resembled one another, suggesting that the virus spread rapidly in a short period of time and that they shared a common origin. They also found that the virus strain is closely related to one detected in Uganda in 2014 and to one found in bats in 2009, Yvan Butera, Rwanda’s minister of state for health, who co-led the research, tells Nature.

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Comparison of the 2014 strain with the one causing the current outbreak shows a “limited mutation rate”, Butera says, suggesting that there have probably been few changes to the virus’s transmissibility or lethality over the past decade. Generally, viruses collect mutations as they replicate over time; if it is true that the mutation rate is low, Rasmussen wonders how the virus lingers in its animal reservoir — the Egyptian fruit bat (Rousettus aegyptiacus) — without major changes.

Researchers say that environmental threats, such as climate change and deforestation, have made people more likely to encounter animals that can pass on infections. More data on how the virus persists in bats — and in which tissues it does — could help to inform surveillance efforts, which would give health officials a better picture of virus hotspots, Rasmussen adds.

Butera says that the genomic analyses are being finalized; he and his colleagues hope to share the full data by the end of the week, he says.