A study in monkeys offers the first evidence that a leading drug developed to fight Ebola works against the strain causing the current outbreak in West Africa.

Six animals were infected with a very high dose of the virus and then, three days later, half were given the drug, TKM-Ebola-Makona, which was designed specifically to fight the West African strain. The monkeys that received the drug survived, but all three untreated monkeys died, researchers reported on Wednesday in the journal Nature.

TKM-Ebola-Makona is already being tested in Ebola patients in Sierra Leone, but results are not yet available. An earlier version of the drug, created to treat a slightly different strain, was given to several Ebola patients in the United States, but it was impossible to tell whether it helped them because they also received other treatments at the same time.

The drugs are given intravenously and can cause flulike symptoms, including headaches, chills and fever. They belong to a category called short interfering RNAs, or siRNAs, which work by blocking certain genes in the virus, impairing its ability to replicate. The design of these drugs varies depending on the genetic sequence of the virus. But viruses can mutate, and new strains can evolve. Researchers have wondered whether a drug made to treat one strain would also work against other strains of the same virus.