Between 2013 and 2016, tens of thousands of people in West Africa contracted the Ebola virus, making the epidemic the largest and longest on record.
But the number of reported cases — roughly 28,000 — may have been underestimated, since it only accounts for people who manifested symptoms of disease. Researchers reporting in PLOS Neglected Tropical Diseases have discovered people in Sierra Leone who showed no signs of Ebola but have evidence of prior infection in their immune systems.
An estimated 10-80% of people who show symptoms of Ebola virus die, depending on their access to intensive care. However, researchers have also seen some evidence that a minority of people may catch the Ebola virus without developing symptoms. During the 2013-2016 outbreak, however, these so-called "minimally symptomatic infections" were not considered epidemiologically relevant to models, projections or intervention efforts so were not studied or tracked in detail.
From October 2015 through January 2016, a year after the peak of the Ebola epidemic, Eugene Richardson, of Partners in Health, and colleagues conducted a survey of people living in Sukudu, Kono District, Sierra Leone — a village of 900 inhabitants that had been a major hotspot of Ebola.
The team identified 193 adults and children older than 4 years of age who had lived with or shared a latrine with a confirmed case of Ebola during the period of active transmission but were never identified as EVD cases. The researchers collected blood from 187 of these individuals. They then tested the blood samples — as well as positive and negative controls from known Ebola cases and people not exposed to the virus — for Ebola glycoprotein antibodies, which indicate a past Ebola infection.
Of the 187 exposed individuals, none of whom were previously known to have EVD, the researchers identified 14 who tested positive for Ebola antibodies. Two of these 14 admitted having a fever while being quarantined, while the other 12 denied any signs or symptoms. However, the study could not pinpoint the timing of any past infection, nor guarantee that any individuals were not prevaricating about a lack of symptoms. The results also can't be extrapolated to other villages affected by the outbreak without more data.
"The findings provide further evidence that Ebola, like many other viral infections, presents with a spectrum of clinical manifestations, including minimally symptomatic infection," the researchers write. "These data also suggests that a significant portion of Ebola transmission events may have gone undetected during the outbreak."