A major international research group has today released a paper suggesting that Ebola viruses may emerge from five years of latency to cause a new outbreak of infections. While this is not the first case in which Ebola has re-emerged from a previously infected person, the new results substantially extend the risk period.
Currently, we have little idea how and where the virus persists in the human body. But now there are tens of thousands of people who have survived previous infections, so it is an area where more research is urgently needed.
A regrowth
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, which allowed the virus to spread. Fortunately, a major outbreak in the same region between 2013 and 2016 caused local health authorities to obtain sophisticated diagnostic equipment, including real-time RT-PCR machines used for COVID testing. 19. This eventually allowed authorities to determine that Ebola was the cause of their illness, identify an additional 15 cases and take action to stop the outbreak. In all, 12 of the 16 infected died.
In order to better understand the origin and spread of the outbreak, samples from these patients were used to obtain the virus genome behind the outbreak. This process makes it possible to compare the sequence of the genome with that of previous outbreaks and samples taken from bats, which can also carry the virus. An evolutionary analysis may suggest how the first patient became infected.
But in this case, the analysis produced a strange result. All cases were grouped into a tight group that fell within 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 were not found in any bat population.
By itself, this result is not entirely shocking. It is possible that the virus can circulate at low levels in isolated populations without drawing the attention of health authorities. If he did, however, he would continue to pick up mutations. But the tension behind the 2021 outbreak didn’t seem very different from what had been circulating in 2016. It’s as if much of the intermediate period frozen in time has passed.
Suspended animation
Because the 2021 strain had captured so few mutations over time since the 2013-2016 outbreak, its normal mutation rate would have had to decrease by a factor of five. The alternative is that, as in the case mentioned above, the virus would remain latent in someone recovering from an infection in the previous outbreak. The virus has been found in seminal fluid up to 500 days after clearing infections, and there has been at least one case of transmission after that time. But the new outbreak would require a latency of more than three times as much.
Past studies suggest that this type of persistence would be uncommon. But there are currently more than 17,000 survivors of the previous outbreak, so there is certainly a chance that a rare event will occur.
For now, however, we have no idea what tissue could hide Ebola, let alone the mechanism that allows it to sleep. The only RNA viruses known to cause long-term infections (called retroviruses) do so by integrating a DNA copy of themselves into their host’s genome. But it seems that Ebola does not have any of the genes needed to do so.
The obvious solution is to work with Ebola survivors to check for persistent infections, which could be integrated into a more general control program given the apparent risk of long-term sleep. But this poses its own challenges. Ebola survival has a stigma in many of the communities affected by the virus, and people who survived its infections often lose their jobs and housing. Therefore, it will not necessarily be easy to recruit people to work with the research community on this project.
The situation may be changing, however, as two Ebola vaccines have recently been approved and others are being tested; have been deployed to help contain outbreaks in recent years. Along with changing the public health situation in Africa, these vaccines may also begin to change the social perception of those infected.
Nature, 2021. DOI: 10.1038 / s41586-021-03901-9 (Regarding DOIs).