A new study in macaques sheds light on how the Ebola virus can persist in the brains of survivors even after they’ve been vaccinated, treated with FDA-approved monoclonal antibody therapies, or both.

There’s a growing body of evidence suggesting that Ebola can lurk in the body for long periods of time, evading the immune system as well as available therapeutics. Reports from Ebola outbreaks document survivors that relapse, falling ill and sometimes dying months or even years after they’ve recovered from their acute illness. Macaque and human studies have also shown that the Ebola virus can persist long-term in the eyes, brain, and testes of survivors. Research published today (February 9) in Science Translational Medicine sheds new light on this process: scientists found the virus in the ventricles—cavities in the brain that produce and circulate cerebrospinal fluid (CSF)—of several macaques that were treated for and appeared to have made complete recoveries from Ebola.

Over the course of the study, Zeng and his team infected 36 rhesus macaques (Macaca mulatta) with Ebola, treated them with monoclonal antibodies, and monitored their plasma and CSF for the presence of Ebola RNA using quantitative reverse transcription PCR (RT-qPCR). According to those RT-qPCR analyses, seven of the macaques had elevated levels of Ebola RNA and Ebola antigens in their CSF two and four weeks after initial virus exposure, indicating dormant or persistent infections. Two of those seven macaques suddenly fell ill, dying 30 and 39 days, respectively, after being infected with the virus and roughly two weeks after having made what seemed like a complete recovery. Meanwhile, most macaques in the experiment survived for about four months after infection, then were euthanized.

After the macaques were euthanized, the researchers stained samples from their brains and those of the two monkeys that had died of Ebola. They observed the presence of Ebola RNA and Ebola antigens in the brain ventricles of the seven macaques with persistent infections.

See “Ebola’s Immune Escape

In the two monkeys that died of the disease, the virus seemed to be gone about 12 days after first exposure, says lead study author Xiankun Zeng, a researcher at the United States Army Medical Research Institute of Infectious Diseases (USAMRIID), but “several days before their deaths, the monkeys got a fever.” The pathology was only in the ventricular system of the brain, he explains, where the researchers observed “massive infections” and “massive tissue damage and inflammation.” All other organs were normal.

Macaque models are considered the gold standard for Ebola research, Zeng explains, and any new therapeutics must be tested on the monkeys rather than mice or other animals due to the macaques’ close biological similarity to humans. While there has yet to be a study showing that Ebola lurks in the ventricles of a human brain, Zeng explains that at least one relapse case linked to the virus in a survivor’s brain has been reported. Thus, he says he’s confident that in humans as well as in macaques, the virus invades cerebrospinal fluid in the brain.

The discovery has important implications for both public health initiatives and clinical care for individual patients, says Zeng. He explains that multiple devastating Ebola outbreaks from the past decade have been linked to persistent infections from a previous one. For example, an Ebola outbreak in 2021 began after the disease re-emerged from a survivor who initially became sick five years prior, a recent Nature study found. A better understanding of how such relapse infections occur could lead to more effective strategies for preventing future outbreaks.

Furthermore, the macaques in Zeng’s study were treated with the same monoclonal antibody cocktails currently approved for use in humans. That could suggest that current treatment protocols need to be updated to completely root out the virus from the body.

Miles Carroll, an emerging viruses researcher at the University of Oxford who didn’t work on the study, tells The Scientist over email that the paper “provides unique insights into the cell types within the brain environment, that support [Ebola] persistence.”

However, he says there are higher priorities than individual relapsing infections when it comes to containing Ebola, noting that “the more likely route of [Ebola virus disease] survivor human-to-human transmission has been shown to be sexual transmission, mediated by infected semen.”

See “Ebola Can Persist 19 Months in Semen, Study Finds

University College Cork biochemist Anne Moore, who also didn’t work on the new study, has a different take, telling The Scientist over email that “if this persistent [Ebola] infection, that occurs in about 20% of [monoclonal antibody]–treated animals, is reflected in even a fraction of human cases, then this warrants serious attention, given that these [antibodies] are authorized for use.”

Moore adds that she’d like to see follow-up research to further clarify the molecular dynamics at play, suggesting that researchers replicate the experiment using macaque monoclonal antibodies instead of human ones to see if it alters the outcomes.

“Our study highlights the importance of careful, long-term follow-up of disease survivors to prevent, diagnose, or treat recrudescent disease that comes at an enormous individual and public health cost,” Zeng says.

Zeng adds that public health officials and others handling the follow-up work will need to take great care to avoid further stigmatizing survivors. A 2015 letter in the International Journal of Health Policy Management describes how fear and stigma represent major challenges to understanding Ebola and bringing outbreaks under control. Additionally, the authors argue that Western researchers have put local communities in Liberia, Guinea, and Sierra Leone—all nations affected by Ebola outbreaks—on edge by taking approaches rooted in colonialism and by failing to share new medical knowledge with the regions that are actually being hit by the disease. In order to conduct the long-term study that Zeng recommends, researchers would need to win public trust in regions affected by Ebola.

Down the road, Zeng suggests that a new, improved combination of therapeutics, likely monoclonal antibodies coupled with an antivirus treatment with better penetration, “may help clear Ebola persistency from the brain, eyes, and testes” and prevent the kind of relapse he uncovered in his study. Moore adds that perhaps the antibodies could be re-engineered in order to reduce the risk of persistent infections.

Carroll agrees with Zeng, saying that “there is an urgent need to develop practical in vivo models to assess the efficacy of small molecule therapeutics to penetrate immune privileged sites, especially the testes, and inactivate persistent [Ebola]. In the absence of such therapies, [Ebola virus disease] survivors may continue to be a potential source of future human-to-human transmission.”