The Ebola virus is highly infectious but not very transmissible. That may sound to a lay person’s ear like a contradiction. What this means is that very little virus
– in animal experiments, as few as 10 virus particles (virions) – can potentially lead to a fatal infection. That’s the “infectious” part of the equation.
But it’s not easy for that virus to be transmitted. Ebola is much less contagious than measles or influenza. It’s not an airborne virus. It’s transmitted through bodily fluids. The overwhelming majority of people who have been infected with Ebola are people who have directly cared for a person who is actively sick with the disease or have handled the body of someone who has died from it.
Experts call it “a caregivers disease,” and note that children don’t typically get it even though they are running around touching many objects and surfaces. They don’t care for sick people as a rule.
“Ebola is terrifying but not as easy to catch as people think,” says Michael Kinzer, a medical epidemiologist with the Centers for Disease Control and Prevention, who flew to West Africa in late July and spent five weeks in Guinea helping to respond to the Ebola crisis.
“The cases we see have had one of the Big Three of direct, intimate interactions with sick people or bodies: taking care of someone at home, treating a patient without protection, or preparing the body of an Ebola victim for burial. We’re talking about very sick people with vomit and stool on their skin and clothes in a house without running water or a toilet, not someone with a runny nose on a public bus. Bodies are dangerous because the virus inside is protected for a few days, and the cleaning and preparation for burial of a body requires very close interaction with it. And we see relatively few kids with Ebola, when any parent knows that kids run around touching everything and never washing their hands. If it was on surfaces or as contagious as something like measles then we’d see a lot more sick kids,” Kinzer said by email.
One reason Ebola is not as contagious as some viruses is that the virus is relatively fragile. That’s in contrast to, say, smallpox, a virus that can remain infectious for long periods even when outside of a host. But Ebola virus left alone, outside a host, on a surface, will begin to disintegrate rather quickly.
“Ebola is an ‘enveloped’ virus, which means it is surrounded by a lipid membrane. That membrane protects it from its surroundings, but even more importantly is essential to its ability to fuse with and enter living cells. And it’s pretty poor protection, as it is vulnerable to light, heat, dryness, and almost any detergent or alcohol you care to name. The virus in saliva on a counter top will be inactive in minutes,” Kinzer said by email.
How long will it remain infectious? Hard to say, exactly, though there have been scientific studies on just that question.
In one laboratory experiment, scientists couldn’t recover Ebola virus that had contaminated a surface kept at room temperature. In another study, Ebola virus kept at cold temperature was recovered from plastic and glass surfaces after more than three weeks.
But Peter Jahrling, director of the Integrated Research Facility of the National Institute of Allergy and Infectious Diseases in Frederick, Md., doubts that in the real world the virus would survive nearly that long.
“Obviously, the more virus you start with, the longer it’s going to last. All those stability studies usually start out with the highest concentration of virus they can come up with. That’s probably 10,000 times as much virus as would naturally be deposited on a surface,” Jahrling said Friday.
He said that a HazMat team ought to be able to clean up a room where an Ebola patient had been living.
“This stuff isn’t volatile, it’s not floating around the air,” Jahrling said. “You go in there with rubber gloves and a pail full of disinfectant, and have at it.”
He suggested other protective equipment as well, including a face mask and goggles. Jahling isn’t fazed by Ebola because he studies the virus, and other dangerous pathogens, in a BSL-4 (biosafety level 4) laboratory. So does his colleague Lisa Hensley, a virologist and Associate Director for Science.
Hensley said the speed at which the virus will degrade depends on many variables. Is the virus contained within dried blood or some other fluid? Is it exposed to sunlight?
Anyone dealing with the virus should be extremely careful, she said – because the consequences of a mistake are potentially so dire.
“Because the outcome is so potentially dangerous – the case fatality is extremely high with this virus – you have to be extremely cautious. You need to take that extra step, because if you do have an exposure and if you are exposed to even a small quantity of the virus, the potential for a negative outcome is extremely high,” said Hensley.
Laboratory experiments suggest that even a single virus particle (virion) can cause a fatal infection, but Hensley cautions, again, that this is not a real-world scenario. For one thing, viruses are so small it’s hard to come up with a one-virion dose. Moreover, says Hensley, “Folks tend to mix exposure and infection. That single virus particle has to find the right cell, get inside, and start to replicate before it is destroyed.”
In laboratory experiments, scientists inject the virus directly and are “tipping the odds for the virus to find that one cell it needs,” Hensley said. “By using a needle to inject virus intramuscularly we are bypassing some of the natural barriers (challenges) that the virus must overcome to infect a host. Injecting the virus increases the likelihood that the virus will find a cell that it can infect and start producing copies of itself.”
This is a way of modeling an accidental “needle stick” by a health care worker or scientist. It’s not a good model for, say, catching Ebola from touching another person.
[This post has been update.]
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