A large international research group released a paper today suggesting that the Ebola virus may have emerged from five years of inactivity to trigger a new outbreak of infection. Although this is not the first instance in which Ebola has re-emerged from an already infected person, the new results substantially extend the time frame for exposure.
At present, we know very little about how and where the virus persists in the human body. But now there are thousands of people who have survived past infections, so this is an area where more research is urgently needed.
The African nation of Guinea experienced a small Ebola outbreak that began in January of 2021 when a nurse fell ill. Due to a misdiagnosis, he was not immediately isolated, which led to the spread of the virus. Fortunately, a major outbreak occurred in the same region from 2013 to 2016, resulting in local health officials receiving sophisticated diagnostic equipment, including real-time RT-PCR machines used for COVID-19 testing goes. This eventually allowed officials to determine that Ebola was the cause of his illness, identify 15 additional cases, and take measures that could contain the outbreak. Of the total 16 infected, 12 died.
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 allows the sequence of the genome to be compared with samples taken from prior outbreaks and from bats, which may also carry the virus. An evolutionary analysis can then suggest how the earliest patient became infected.
But in this case, the analysis gave a strange result. All cases were clustered in a tight cluster that fell within the cluster of viral variants that led to the 2013–2016 outbreak in the same region. These cases included some mutations that occurred only during earlier outbreaks and were not found in any of the bat populations.
In itself, this result is not entirely shocking. It is possible that the virus could spread at low levels in isolated populations without the attention of health officials. Even if it were to do so, it would keep picking up mutations. But the tension behind the 2021 outbreak was not much different from the one that circulated in 2016. Looks like it spent too much time in the middle.
For the 2021 strain to have had so few mutations in the time since the 2013–2016 outbreak, its normal mutation rate would need to drop by a factor of five. The alternative is that, as mentioned earlier, the virus remains dormant in someone who has recovered from infection in an earlier outbreak. The virus has been detected in seminal fluid up to 500 days after the infection has cleared, and there has been at least one instance of transmission after that time. But the new outbreak would require more than three times as long inactivity.
Previous studies suggest that such persistence would be unusual. But there are currently more than 17,000 left from the earlier outbreak, so it certainly has the opportunity to be a rare event.
At this time, however, we do not know in which tissue Ebola may be hiding, much less the mechanism that allows it to lie dormant. The only RNA viruses known to cause long-lasting infections (called retroviruses) do so by integrating their own DNA copy into their host’s genome. But it appears that Ebola has no genes required to do so.
The obvious solution is to work with Ebola survivors to screen for frequent infections – something that could be integrated into a more general surveillance program given the apparent risk of prolonged inactivity. But it has its own challenges. The existence of Ebola bears a stigma in many communities hard hit by the virus, with people who survive their infection often losing their jobs and housing. So it is not necessarily easy to recruit people to work with the research community on this project.
The situation may change, however, as two vaccines against Ebola have recently been approved for use, and others are in testing; They have been deployed to help contain outbreaks over the years. As the public health situation in Africa changes, these vaccines may also begin to change the social perception of those infected.
Nature, 2021. DOI: 10.1038/s41586-021-03901-9 (About DOI).