Ebola virus does not uniformly cause severe disease and people may be infected without showing signs of illness, new research has found.
Researchers from the Stanford University School of Medicine in the US and other institutions identified 14 individuals previously unknown to have had the disease in a Sierra Leone village that was an Ebola hot spot.
They were found to be carrying antibodies to Ebola, suggesting they had been infected at some point, though they had not been included in the original count.
Twelve of them said they had had no symptoms of the disease, which typically causes fever, unexplained bleeding, headache, muscle pain, rash, vomiting, diarrhoea, breathing problems and difficulty swallowing.
The findings also suggest that the epidemic was more widespread than previously believed. Based on the results of the study, the researchers calculated the prevalence of minimally symptomatic infection to be 25 per cent.
“The study corroborates previous evidence that Ebola is like most other viruses in that it causes a spectrum of manifestations, including minimally symptomatic infection,” said Gene Richardson from Stanford.
“It provides important evidence on that front. It also means a significant portion of transmission events may have gone undetected during the outbreak. This shows there was a lot more human-to-human transmission than we thought,” he said.
The research was done in the rural village of Sukudu in Sierra Leone in West Africa, with about 900 residents. There were 34 reported cases of Ebola in the village, including 28 deaths, between November 2014 and February 2015.
More than 28,000 cases of Ebola infection were reported in Africa during the epidemic, the largest and longest in history. More than 11,000 people are estimated to have died because of the disease.
In the aftermath, Richardson and his colleagues decided to go back to the village to try to determine whether the Ebola infection could be minimally symptomatic, as previous studies have suggested.
He worked with a local physician and two community health workers in gathering data for the study.
They used a test known as the ELISA assay, a technique that can detect the presence of an antibody. They first made sure the test was accurate by comparing results from 30 Ebola survivors in Sukudu with those of 132 people in other villages where the virus had not been reported.
Richardson said the test proved to be a reasonable measure of viral antibodies.
The researchers then recruited 187 men, women and children from Sukudu who had likely been exposed to Ebola, either because they were living in the same household or had shared a public toilet with a person confirmed to have had the disease.
The study was published in the journal PLOS Neglected Tropical Diseases.