There has been some controversy over the question of whether the viral hemorrhagic fever virus Ebola can --like many infectious diseases --cause subclinical illness. On the one hand, subclinical illness in which few to no symptoms are present could diminish the quoted morbidity and mortality rate of the disease or, on the other hand, provide the basis for resurgent epidemics. Prior studies of varying methodological quality have provided conflicting data. A new study published in Lancet Infectious Diseases attempts to answer this question definitively. 

Using a novel IgG Ebola glycoprotein capture assay, Glynn and colleagues surveyed survivors and household members who were treated at the Kerry Town Ebola Treatment Center in Sierra Leone during the West African Ebola outbreak of 2014. Crucially, control subjects from neighborhoods in Sierra Leone without known cases of Ebola were part of the study.

Oral swabs from the former Kerry Town patients were, as expected, positive at a rate of 97.4%, while those from controls were 99.7% negative. Household contacts without a known history of Ebola were 8.3% positive on single testing, but on repeat testing more than half of those results were negative. The positive rate in asymptomatic contacts was 2.6%. Not surprisingly, of those contacts of Ebola patients who were not formally diagnosed with Ebola, 33.3% of those with contact with an Ebola corpse were positive. An overall rate of asymptomatic infection was calculated at 2.6% in the total cohort studied. ¹ 

The findings of this study are very important and should inform future Ebola outbreak management. By using control subjects, Glynn and colleagues incorporated appropriate rigor into their low overall estimate of 2.6% asymptomatic infection. If asymptomatic infection is a rare event, as this study suggests, then outbreak control should be focused on identifying symptomatic individuals and initiating the appropriate infection control measures to halt an outbreak. 

Additionally, the novel assay used in the study, which requires just oral swabbing, is an important new tool that can be employed in similar studies and to understand where Ebola has occurred in the past. As a new outbreak of Ebola has been identified in the DRC, it will be important to use this near real-time approach to target efforts on tasks that have the highest yield. As USAMRIID's Kuhn and Bavari write in an accompanying editorial, "The biggest threat to human populations therefore remains another introduction of Ebola virus from its natural host--and not transmission from an apparently healthy person infected with Ebola virus." ^2(p571)