As least half of Ebola outbreaks may go undetected, researchers estimate - Healio

At least half of all Ebola outbreaks have gone undetected since the virus was first reported in 1976, researchers estimated.

“Emerging infectious diseases are often not investigated in rural Africa unless outbreaks involve a sizeable number of cases. A number of different Ebola virus disease (EVD) outbreaks have been reported in the literature and in surveillance reports since its discovery in 1976. The majority of the reports are of large outbreaks,” Emma E. Glennon, a PhD student in the department of veterinary medicine at the University of Cambridge, and colleagues wrote.

“Given the low reported rate of transmission of Ebola, and the high frequency with which cases infect no one else, one might expect most outbreaks to be very small (fewer than five people). This is the first study to the authors’ knowledge that quantitatively estimates the number of undetected EVD outbreaks or probabilities of EVD outbreak detection by outbreak size.”

Glennon and colleagues based their estimates on simulations made using three available datasets on the distribution of secondary infections in EVD outbreaks, including one that included all reported exposures from more than 19,000 cases in Guinea, Liberia and Sierra Leone — the three countries hit by the West African outbreak. The other two datasets came from a single district in Sierra Leone and one from the city of Conakry, Guinea.

They reported using the datasets to simulate realistic outbreak size distributions and compared them with reported outbreak sizes.

According to the findings, Glennon and colleagues estimated that there have been 67, 26 or 118 1/2 undetected spillover events since 1976 based on the large West African dataset, the Sierra Leone dataset and Conakry data set, respectively. Most of these were “dead-end zoonotic spillovers causing a single human case,” they wrote.

Using the three datasets, they estimated the proportion of detected spillover events and small outbreaks at 26% based on the full West African outbreak data, 48% based on Sierra Leone data and 17% based on Guinea data.

Based on their median estimates, they concluded that at least half of all spillover events — and maybe as many as 83% — have not been reported.

“This study’s main result — that at least half of EVD outbreaks go undetected — is consistent under many different sets of assumptions,” Glennon and colleagues wrote. “This is the most thorough estimation of EVD outbreak detection to date and corroborates the majority of more qualitative work on EVD surveillance, suggesting greater investment in primary health care and local surveillance will be important to detect EVD outbreaks early and consistently.” – by Caitlyn Stulpin

Disclosures: The authors report no relevant financial disclosures.

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