People with Ebola may not always show symptoms
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, a process that was approved by the local village chief.
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 thirty 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.
Of these, fourteen 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, diarrhea, breathing problems and difficulty swallowing. Two recalled having had a fever at the time of the outbreak, the scientists reported.
Public health efforts not entirely effective
In combining the initial reports of thirty-four infections with the fourteen newly identified cases, the researchers calculated the prevalence of minimally symptomatic infection in the village to have been 25 percent.
Richardson said it is unknown if an asymptomatic individual is capable of transmitting the virus. Because these individuals did not have an active case of the disease, “They were not passing it along in the usual way, through vomiting or diarrhea,” he said. “It’s unclear if they can pass it along it sexually.”
The virus has been shown to hide out for months in semen, even after symptoms have subsided, with some published cases of survivors transmitting the virus through sexual contact.
Richardson said the study indicates that public health efforts to prevent infection and contain the virus during the epidemic were not entirely effective.
“It reminds us that we need to do a much, much better job in future epidemics,” Richardson said.
He and his colleagues are now working in other villages in Sierra Leone where public health surveillance was poor during the epidemic, testing and interviewing individuals to get a better handle on the true number of people affected during the crisis.
“We expect to find a lot more undocumented survivors, so we can begin to answer the question of what was the true burden of disease,” he said.
— Read more in Eugene T. Richardson et al., “Minimally Symptomatic Infection in an Ebola ‘Hotspot’: A Cross-Sectional Serosurvey,” PLOS Neglected Tropical Diseases 10, no. 11 (15 November 2016): e0005087 (DOI: 10.1371/journal.pntd.0005087)
