A growing awareness of how the Ebola virus can hide in parts of the body such as eyes, breasts and testicles long after leaving the bloodstream raises questions about whether the disease can ever be beaten.
Virologists said Friday's case of a Scottish nurse, Pauline Cafferkey, who had recovered from Ebola but is now suffering complications adds to signs that the virus is a long-term health risk and can lead to a "post-Ebola syndrome."
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"Over the past few years there has been mounting evidence of mental and physical health problems in Ebola survivors that can last for years after the virus is cleared from the bloodstream," said Ben Neuman, an Ebola expert and lecturer in virology at Britain's University of Reading.
"The newly discovered twist on this post-Ebola syndrome is that in some cases the health problems — often including damage to the eyes and joints — are caused by live Ebola virus growing in fluids in some of the less accessible compartments of the body."
Ebola, one of the deadliest viruses known in humans, infected 28,000 people and killed more than 11,300 of them in an unprecedented outbreak in West Africa which was declared in March 2014 and is only now coming under control.
Partly because of the vast numbers involved in the epidemic, which centred on Guinea, Sierra Leone and Liberia, infectious disease experts say we are learning more every day about Ebola from cases such as Cafferkey's and thousands more survivors.
Ebola experts said in August that around half of Ebola survivors in West Africa were already reporting suffering from chronic problems, including serious joint pain and eye inflammation that can lead to blindness.
Large virus reservoir?
"Due to the sheer scale of this outbreak compared to previous ones we are going to see aspects of Ebola virus infection that we have not observed before," said Julian Hiscox, a professor of infection and global health at Britain's Liverpool University.
He was concerned that Ebola's persistence in survivors, who have no obvious symptoms of Ebola infection and so are often living and working normally and not kept in isolation as a symptomatic patient would be, means they are "a potential reservoir of the virus".
"It's why men who have had Ebola and recovered are advised to abstain or wear condoms," he noted.
The World Health Organization's advice is that all male survivors should be tested three months after the onset of symptoms and then monthly until they know they have no risk of passing on the virus through their semen.
John Edmunds, an expert at the London School of Hygiene and Tropical Medicine, said that while the risk of transmission from survivors harbouring the virus in their eye fluids and other organs "appears to be very low", it still warrants attention.
"With so many survivors in West Africa now, there is a risk that further outbreaks can be triggered, which is why authorities have to remain very vigilant," he said.
Cafferkey, a 39-year-old nurse, was back in hospital in London on Friday with doctors saying she would be treated in isolation as a precautionary measure.
The hospital said in statement it had "identified a small number of close contacts ... that we will be following up as a precaution", but added: "The risk to the general public remains low."
Cafferkey was the first person to be diagnosed with Ebola on British soil and was originally discharged in January after seemingly making a full recovery.
Neuman said the likelihood of survivors spreading Ebola depends on how much of the virus is present in the blood.
In Cafferkey's case, he said, "if her body was able to control the virus once, the chances are she can do it twice."