Enlarge / Healthcare workers put on protective gear before working with Ebola patients.
A large international research group today published an article suggesting that Ebola viruses may emerge from five years of inactivity to start a new outbreak of infections. While this is not the first instance that Ebola has reappeared from a previously infected person, the new results substantially extend the period of risk.
At present, we have little idea of how and where the virus persists in the human body. But now there are tens of thousands of people who have survived past infections, making it an area where more research is urgently needed.
The African nation of Guinea experienced a small Ebola outbreak that began in January 2021 when a nurse fell ill. Due to a misdiagnosis, it was not immediately isolated, allowing the virus to spread. Fortunately, a major outbreak that occurred in the same region between 2013 and 2016 prompted local health authorities to obtain sophisticated diagnostic equipment, including real-time RT-PCR machines used for COVID-19 testing. This ultimately allowed authorities to determine that Ebola was the cause of his illness, identify 15 additional cases, and take action that stopped the outbreak. In total, 12 of the 16 infected died.
In order to better understand the source and spread of the outbreak, samples from these patients were used to obtain the genome of the virus behind the outbreak. This process makes it possible to compare the genome sequence with that of previous outbreaks and samples taken from bats, which can also carry the virus. An evolutionary analysis can then suggest how the patient became infected earlier.
But in this case, the analysis produced a strange result. All cases were grouped into a small group that was included in the group of viral variants that had caused the 2013-2016 outbreak in the same region. These cases included some mutations that had only occurred during the previous outbreak and have not been found in any bat population.
On its own, this result is not entirely surprising. It is possible that the virus could circulate at low levels in isolated populations without attracting the attention of health authorities. However, if it did, it would continue to collect mutations. However, the tension behind the 2021 outbreak did not look much different than what had been circulating in 2016. It is as if much of the intervening period had passed frozen in time.
For the 2021 strain to have detected so few mutations in the time since the 2013-2016 outbreak, it would have had to reduce its normal mutation rate by a factor of five. The alternative is that, as in the aforementioned case, the virus remained inactive in someone who recovered from an infection in the previous outbreak. The virus has been found in seminal fluid for up to 500 days after infections cleared, and there has been at least one case of transmission after that period of time. But the new outbreak would require a latency three times higher.
Previous studies suggest that this type of persistence would be rare. But there are currently more than 17,000 survivors of the previous outbreak, so there is certainly the opportunity for a rare event to occur.
However, at the moment, we have no idea in what tissue Ebola might be hiding, much less the mechanism that allows it to remain inactive. The only RNA viruses known to cause long-lasting infections (called retroviruses) do so by integrating a DNA copy of themselves into their host’s genome. But Ebola doesn’t appear to have any of the genes necessary to do this.
The obvious solution is to work with Ebola survivors to check for persistent infections, something that could be integrated into a more general monitoring program given the apparent risk of long-term downtime. But that poses its own challenges. Survival from Ebola carries a stigma in many of the communities hardest hit by the virus, and those who survived their infections often lose their jobs and homes. Therefore, it will not necessarily be easy to recruit people to work with the research community on this project.
However, the situation may be changing, as two Ebola vaccines were recently approved for use and others are being tested; they have been deployed to help contain outbreaks in recent years. Along with the changing public health situation in Africa, these developments may also begin to change the social perception of those infected.
Nature, 2021. DOI: 10.1038 / s41586-021-03901-9 (About DOIs).