Some doctors call it “walking Ebola” – symptoms of the rare Bundibugyo strain appear to worsen slowly, leaving patients sick enough to spread the virus but not so unwell to stop them moving through their communities.

The pattern emerging in the Democratic Republic of Congo resonated with virologist Corri Levine, who devoted her doctoral research to Bundibugyo years before it caused the fastest-growing Ebola outbreak on record.

Studying the virus in a biocontainment laboratory at the University of Texas Medical Branch, Levine found it replicated more slowly than the Zaire strain responsible for a devastating epidemic that swept West Africa a decade ago.

“Walking Ebola” was “an ‘aha’ moment for me”, Levine said. “If it’s growing slower in the person, you might start having some symptoms, but not enough to make you feel so crummy that you can’t move. It’s this longer, prolonged disease course.”

That slower progression carries another risk. Patients with Ebola have consistently had the best chance of survival when treated early, before rising virus levels drive the disease toward septic shock and multi-organ failure.

If Bundibugyo gives doctors a longer opportunity to intervene before reaching that point, antiviral drugs may have a better chance of altering the course of the disease.

The possibility has renewed interest in antiviral drugs such as Gilead Sciences’ remdesivir, which Levine’s lab found was more active against Bundibugyo than Zaire Ebola in Petri-dish experiments.

Researchers in July began testing remdesivir alongside Mapp Biopharmaceutical’s antibody therapy MBP134 in a World Health Organisation-sponsored clinical trial in Congo, while a separate study is evaluating Gilead’s experimental oral antiviral obeldesivir in people recently expo...