Why Ebola is so dangerous
The Ebola outbreak in West Africa is the world’s deadliest to date. More than 670 people have died as health officials in Guinea, Liberia and Sierra Leone struggle to control the virus.
Are plane cabins hotbeds of disease? Frequent flyers are often on guard against sneezing or coughing co-passengers, in fear of catching what they have. And the possibility that much more serious epidemics or pandemics could spread by air travel has been a worry for years, whether it is avian flu, Sars or tuberculosis.
Now we are in the middle of one of the deadliest outbreaks of Ebola virus ever: the outbreak started in Guinea last March and soon reached Liberia, Sierra Leone and Nigeria. Concern in the West has heightened after it emerged that a man had flown on internal flights while carrying the virus, with fears growing that air travel could quickly carry the terrifying disease around the world. British Prime Minister David Cameron described Ebola as a “serious threat”.
How likely is it to catch a deadly disease mid-flight and what might stop a sick passenger carrying it between countries? To get a sense of the risks involved, BBC Future talked to infectious disease researchers to find out what we know about transmission during air travel – and what this can tell us about the risk of Ebola spreading across the world.
The risks of catching an infection from an ill passenger are not as high as you would think, says Christine Pearson, a spokesperson for the Centers for Disease Control and Prevention (CDC) in Atlanta, Georgia. “It’s not any more dangerous than any place where you are in touch with lots of people – like a shopping mall food court for example.
John Oxford, a virologist at Queen Mary University, London, agrees. He points out that the aeroplane ventilation goes from the ground to the ceiling, where the air is filtered for bacteria and viruses before it recirculates. Simulations looking at the potential spread of germs have found that they are generally confined to just a couple of rows either side of the carrier. Even then, the chances appear to be small, according to a study published in the British Medical Journal. The paper looked at a flight involving nine school children who were later found to be carrying the swine flu virus. Just two other passengers, of more than 100 questioned, later developed the illness – and they were both sitting within two rows of the ill school kids. As a result, the researchers concluded that there was just a 3.5% risk of catching the illness if you were sitting in those seats. A handful of other studies, looking at measles and TB, also suggest that in-flight transmission rates are similarly low. From studies such as these, Oxford says that “the biggest risk is not on the plane, but in the taxi on the way to the airport”.
However, John Edmunds, professor of infectious disease modelling at the London School of Hygiene and Tropical Medicine, points out that it is difficult to come to any firm conclusions, even for the more prevalent diseases. The number of studies is small, he says, meaning you can’t find absolute figures for the risk and compare that to the risk of infection in a school, say. So it is even harder to assess the risk of in-flight transmission of rare and unstudied diseases like Ebola.
Mode of transmission is clearly a factor. There are a few documented cases of norovirus spreading among passengers – probably due to people sharing the same toilet. But we know that Ebola is relatively difficult to catch: unlike respiratory diseases like colds and flu, it can only be transmitted through contact with bodily fluids like saliva, vomit, or blood, which begins to secrete out of every orifice. Although it is unlikely that someone suffering the full-blown attack would attempt to fly, it is not impossible. Patrick Sawyer, a US citizen who died from the virus last week, showed some of these symptoms on a flight from Liberia to Nigeria, raising fears for the other passengers. Airline staff and public health officials should be more aware of the dangers now, however, and should place anyone suspected of having the disease in quarantine. “Everyone is on the lookout,” says Oxford.
There is always the chance that passengers could board a flight before they had developed the full-blown symptoms. But so far, the evidence suggests you are unlikely to catch it from someone in the early stages of the disease, before they have started vomiting and bleeding. “I wouldn’t want to be a passenger next to a guy with Ebola,” admits Edmunds. “But if they are just sitting there with a fever, I can’t imagine the situation being high risk.” Oxford points out that basic hygiene measures should further reduce the chance of infection – even if it can’t eliminate the danger completely. “The Ebola virus is very easily killed; you can kill it with hot water and soap,” he says. “Alcohol rubs get rid of it in a microsecond.”
If an apparently healthy person carrying the virus lands in a country, there is still room for a second stage of contagion. This is perhaps the greater risk for a global epidemic – undetected carriers are the way that swine flu and Sars spread across the world. “The history of the spread of infectious diseases is infinitely entwined with improvements in travel,” says Edmunds. He points out that in the past, the spread of diseases was limited by the length of travel; measles develops quickly, so all the cases would have burnt out by the end of a long voyage. Since Ebola has an incubation period of up to 21 days, carriers could arrive in a country weeks before symptoms develop – potentially transmitting it to the people they know. “It is an issue,” admits Oxford, but he is optimistic that the infection could still be contained. Anyone showing the symptoms would have a rapid diagnosis before being quarantined, and health authorities would also check anyone who had been in contact with them – such as air crews, passport officials and their families. “All of that can stop it spreading – and it will be stopped.”
A spokesperson for the World Health Organization, Gregory Härtl, says its official stance currently is that a global epidemic is a “small risk”, although it is currently working with the International Air Transport Association to review their recommendations. Currently, it does not recommend screening passengers at airports – since the thermal scanners used to detect a fever are unlikely to find people incubating the first stages of the disease – and there are no recommended travel restrictions.
However; the situation is evolving fast and the recommendations may change in the coming days. At the moment, no one can yet predict whether other air travellers have been infected with the disease, but the hope is that heightened vigilance will help to minimise any potential risk. However, even if it is successfully contained and the outbreak peters out, Ebola shows, yet again, the fact that no disaster or epidemic is too distant in the highly-connected modern world.
Culled from BBC Africa