Scientists have discovered several mutations in the Ebola virus responsible for West Africa’s deadly outbreak that could make leading treatments less effective, according to new research published Tuesday in the journal mBio. Researchers in the U.S. identified 10 genetic drift viruses – versions of the Ebola virus that are slightly genetically distinct from the original – that they say might not respond to the most advanced Ebola drugs, including ZMapp.

"Based on our findings, the virus has changed and is continuing to change," Jeffrey Kugelman, a viral geneticist at the U.S. Army Medical Research Institute of Infectious Diseases, which led the research, said in a statement. Researchers said drugs should be tested against the strains to see if they respond, and that drug developers must account for such mutations when developing drug therapies.

Virologists have known from the beginning that the Ebola virus would mutate. Like all viruses, the Ebola virus changes a little bit every time it infects a new host. An analysis of 99 Ebola virus genomes last summer found 395 different mutations, according to a report published in August in the journal Science. Just because a virus mutates, however, doesn’t mean it won’t respond to treatment.

The new research comes at a time when the outbreak, the worst since the Ebola virus was discovered in Zaire in 1976, continues to peter out. New cases of Ebola in West Africa have been on the decline for weeks, health officials said. In Liberia, which has seen the most cases of Ebola, new infections dropped from 300 a week at the end of December to less than 10 a week in January. Guinea saw Ebola cases fall from 114 to fewer than 30 during that same time.

Reports of Ebola infections remained high in Sierra Leone, however, where there were 140 new cases last week, although that was down from 330 in the last week of 2014. “The outbreak feels different now [and is] no longer a single outbreak spreading from a central point,” U.N. special envoy for the Ebola response David Nabarro said Tuesday during a U.N. briefing. “It’s a collection of micro-outbreaks, each with its own character and specific needs.”

Many of the dire predictions some health officials made at the onset of the outbreak never materialized. Total cases have reached an estimated 21,650, a far cry from the 1.4 million victims the U.S. Centers for Disease Control and Prevention projected last September.

In Liberia, many of the Ebola treatment centers built by the U.S. military remain empty. Most of the centers were completed after the epidemic in Liberia began to wane.

The current Ebola outbreak began in March 2014 and has claimed some 8,600 lives across West Africa. Seventy percent of the those infected have died.

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