Why isn’t Ebola contagious before symptoms are shown?

Mudassir Ali
Mar 10, 2020 06:37 PM 0 Answers
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Mudassir Ali
- Mar 10, 2020 06:37 PM

The assumption in the question is true when we consider ebola only as a human disease. But that’s not all it is. Ebola finds a home in populations of fruit bats, and it has spread widely amongst them, apparently without causing any symptoms*.
Ebola hasn’t undergone selection very much in humans, as outbreaks have always been relatively small. In part this is due to its extraordinarily high lethality; it’s always burnt out before its human hosts could travel far and spread it widely. From the virus’s point of view, this lethal threat is a bug, not a feature (although that might not hold true in densely populated environments of course). It’s not (yet) particularly well adapted to infecting us as a host species. We’re as alien to ebola as it is to us. We are at an early stage its evolution as a human disease. The virus is certainly transmissible between people, by direct contact with bodily fluids, which the symptoms cause a profusion of. But it hasn’t evolved such an efficient mechanism as influenza or other more established human viruses. Of course, this all could change. Evolution is inevitable.
So it’s certainly possible that if the virus continues to spread amongst humans we will see strains develop that can be transmitted before symptoms appear; strains that could do this would have an advantage in their ability to spread. Fortunately this would almost certainly coincide with a reduction in lethality. The same mechanism that would allow it to reproduce in quantity whilst causing no symptoms, may well also mean it’s reproduction is not so deadly to the host.

As an important side note, asymptomatic transmission of ebola between humans is possible after recovery, in semen, for up to three months after symptoms are gone. It’s not inconceivable that ebola (or whatever we will call strains evolving from it) will evolve to exploit the niche of being a sexually transmitted infection.

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