Scientists Only Know About Half of Ebola Outbreaks in the Last 40 Years

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Half of all Ebola spillover events—where the deadly virus jumps from wildlife to people—go undetected, warns new research from the University of Cambridge.

The discovery, which comes amidst an unfolding epidemic in the Democratic Republic of Congo and Uganda, highlights the need to improve detection and rapid response protocols to prevent future epidemics.

"From an ecological perspective, we now have evidence that spillover is much more common than the record shows. Studying the forces that leads to spillover has been difficult because there are so few historical data points, but I hope this study can help ecologists find missing data and better understand the important process of spillover," Emma Glennon from Cambridge's Department of Veterinary Medicine told Laboratory Equipment.

Ebola is a rare and deadly filovirus found in people and non-human primates that kills between 25 and 90 percent of those infected. The virus is spread through direct contact with the bodily fluids of an infected animal or person infected with Ebola.

Fruit bats are natural carriers of Ebola, which means they spread the virus without being affected by it.

The virus was first identified during two simultaneous outbreaks that occurred in South Sudan and the Democratic Republic of Congo (DRC) in 1976. The DRC epidemic broke out in a village near the Ebola River, giving the disease its name.

The World Health Organization (WHO) reports 24 outbreaks of Ebola that caused 1,590 deaths total between 1976 and 2013. The West African Ebola outbreak that began in 2013 and died down in 2016 is the largest epidemic of the disease on record, causing 11,323 deaths.

Rising fears of a new Ebola outbreak in DRC were first reported in May 2017 and has killed more than 1,300 people so far. Public officials announced the first confirmed case of Ebola in neighboring Uganda, raising fears that the outbreak will continue to spread.

Published in PLOS Neglected Tropical Diseases, the research is the first to estimate the number of undetected Ebola outbreaks in the last 40 years.

Using three independent datasets from the 2013-2016 West Africa Ebola epidemic, the research forecasted how often spillover events fizzled out versus becoming a true outbreak. They then compared these numbers with reported outbreak sizes.

Most undetected outbreaks resulted in fewer than five people getting sick with the virus, the study estimated. These isolated cases add up and could represent over 10 patient cases.

The odds of finding an isolated case of Ebola is less than one in ten, the research said.

"Although the total amount of evidence in this area is still limited, this study's main result—that at least half of Ebola virus disease outbreaks go undetected—is consistent under many different sets of assumptions," the authors wrote.

Ebola outbreaks can be easily managed with low tech interventions such as isolation or protective clothing when detected early, but can be difficult to stop once the disease has begun to spread. Early symptoms of the virus mimic malaria, typhoid and yellow fever, making Ebola difficult to diagnose in its early stages.

"Spillover is more common than we think, and we can't reliably detect or prevent it without widespread investment in clinical infection control and diagnostic tools," Glennon said. "Most Ebola spillover events are being handled locally before they're ever detected. To intervene at the critical early stages of outbreaks, it is essential that every clinic and public health department has the resources to practice strong infection control for cases with any cause of fever. National or international Ebola-specific outbreak response strategies, no matter how sophisticated or well-funded, cannot help stop an outbreak early if its first cases have been misidentified, e.g., as malaria."