Half of isolated Ebola cases have gone undetected since the disease was first discovered in 1976, according to an analysis of outbreaks by scientists published last week (June 13) in PLOS Neglected Tropical Diseases.
The virus circulates in wildlife all the time, and the rate of such spillover is intimately linked with its public health consequences. But it’s unclear how often it happens. By modelling patterns of Ebola outbreaks, the team estimated that many spillover events are overlooked entirely.
“We estimate that outbreaks are unlikely to be detected within their first few cases, and this has important public health consequences,” says lead author Emma Glennon, a disease ecologist at the University of Cambridge.
People with undocumented cases of Ebola most likely die of the disease as they don’t receive care at health facilities, says Yap Boum, regional representative of the Uganda-based Epicenter Africa, the research arm of Doctors Without Borders. Such patients also transmit the disease to people in their communities, says Boum, who was not involved in the study. “However, the [models] are difficult to generalize and therefore need to be used with care,” he adds.
The study comes out against the backdrop of three Ebola cases, which spread from the ongoing outbreak in the DRC to Uganda.
Using several published estimates of Ebola cases from the 2014–2016 West African outbreak, Glennon and her colleagues simulated hundreds of thousands of early-stage Ebola outbreaks. The researchers examined the size and distributions of the Ebola outbreak sizes they expected to see and formulated a model on how likely an outbreak of a given size is to be detected. They compared the West African outbreaks sizes to those that have happened over the last few decades, and estimated that there is less than a 10 percent chance of detecting an isolated case of Ebola.
To validate their results, the researchers used three different datasets to inform the researchers’ simulations. Glennon says they intentionally chose ones that had more and less conservative assumptions. “We found it striking that no matter which of these datasets we chose, we estimated that more than half of spillover events have gone undetected,” she says.
Ebola is never an easy disease to manage from a clinical perspective, especially without adequate treatments, says Glennon. Preventing spillover, rather than containing outbreaks after they’ve started, is considered the ideal way of controlling Ebola, she adds; wide availability of infection control and diagnostic resources will be critical to achieving this goal.
Rangarajan Sampath, chief scientific officer at the Foundation for Innovative New Diagnostics in Geneva who was not involved in the study, agrees. This kind of study points to the importance of surveillance systems at primary health care centers that can transmit that information to the national healthcare centers before infections become outbreaks, he tells The Scientist.