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Ebola May Not Always Have Symptoms

A new study finds about 25 percent of cases in one Sierra Leone village went undetected.

By Nathan Collins


(Photo: John Moore/Getty Images)

The recent Ebola outbreak in West Africa was by far the worst on record: According to the World Health Organization, 28,616 people were infected, and 11,310 died. But new research suggests the epidemic may have been even larger than those statistics suggest, because some people never develop symptoms despite being infected with Ebola virus.

“This study provides further evidence that Ebola, like other viruses, causes a spectrum of clinical manifestations that may include minimally symptomatic infection,” Stanford University anthropology graduate student Eugene Richardson and his collaborators write in PLoS Neglected Tropical Diseases. “The findings also suggest that many episodes of human-to-human transmission of Ebola virus in West Africa may have gone undetected in the recent outbreak.”

The idea that some people infected with Ebola might never develop symptoms is certainly plausible, but, to confirm that, researchers would have to go out and find blood samples that tested positive for the virus. Richardson and his team began their search in Kono District, Sierra Leone. There, they gathered blood samples from 30 people known or strongly suspected to have been sick with the disease, and 132 others who had no contact with Ebola. The team used those samples to validate a standard Ebola blood test, which turned out to be accurate about 97 percent of the time.

The team went next to Sukudu, a village of about 900 people and a hot spot for the outbreak in Kono District. The researchers collected blood from 187 people who’d been quarantined after possible contact with Ebola virus—either because they shared a public latrine or living quarters with someone who got sick—but who did not themselves appear to be ill.

Of those tested, 14 came back positive for Ebola virus antibodies. Two of those reported having had a fever while they were quarantined, “while the remaining 12 denied any signs or symptoms during quarantine,” Richardson and his colleagues write.

Sukudu had 34 previously reported Ebola cases—16 people who died as a direct result of Ebola, 12 who died and were suspected to have been carrying the disease, and six who survived. Counting the two who’d had only a fever as symptomatic cases, that suggests about 25 percent of all people who carried the virus never got sick.

Now, the people who participated may have had symptoms they didn’t want to report, “given the considerable denial of [Ebola virus disease] during the outbreak due to stigma and the fear of being admitted to an [Ebola treatment center] where those admitted were seldom discharged,” the researchers write. The study also focused on just one village, so this research may not apply elsewhere, although it is consistent with estimates that go back to a 1976 outbreak in southern Sudan.

The results could have an effect on who’s considered an Ebola survivor and who therefore might be eligible for free social services and medical care. The results could also affect future vaccine studies. Since their effectiveness is measured in part by whether recipients develop antibodies to a virus, a large population of people who contracted Ebola virus might make it seem like a vaccine is more effective than it is in reality.