Why were colds so bad for indigenous inhabitants?

14 December 2008

Question

Why were colds so bad for indigenous inhabitants?

Answer

Kat - Presumably this is when you get stories of explorers who go to isolated communities and then all the people get really bad illnesses.

Chris - That's true and the reason is that isolated populations in distant geographical areas which are, in other words, when we say isolated were cut off from the mainstream viruses circulating and other infections circulating where there were big populations in Europe. Those individuals didn't have the same selective genetic pressure to have a more powerful immune system. If you compare the number of genes in people who are native American Indians and South Americans had for presenting information from the immune system to itself - so when a cells gets infection it presents various aspects of what it seeing washing round the body to the immune system in these things called HLA genes - well the native populations had far fewer of these than the Europeans did. Viruses that breed in Europe are much more virulent under those circumstances because they have to have all these immune evasion strategies built-in in order to outwit the much more powerful immune presenting abilities of the European population.

Kat - So they were really nasty.

Chris - They tend to be more powerful, yeah. If you take them to a population that are more vulnerable they haven't seen them before. Therefore there's no pressure on that population to evolve these defence mechanisms. Those populations are like a souped-up virus and it's overkill. They don't need to be that powerful and it makes people much iller than it would otherwise be. Earlier on in this programme we talked about how diseases spread from animals into people like the rats giving arena virus-like things to people in South Africa. It can work two ways because scientists published a paper in Current Biology where they showed that chimpanzees are dying of colds that we give them. There's a fairly recently uncovered cold virus called metapneunomvirus that we diagnose quite often in people, in humans. It causes a heavy cold and if you have children it causes wheeziness. It tends to be self-limiting and goes away. When people started studying chimpanzee populations in Africa and noticing that they were periodically getting sick and having mass die-offs they found metapneumovirus. Maybe the chimps were giving the metapneumovirus to the humans. Actually, there was in South America a few years ago there was a form of metapneumovirus with a genetic signature in it that was very specific to that strain and this then started cropping up later in Africa. This told researchers that in fact we're giving our viruses to chimps. When you put our viruses that give us a cold into a chimpanzee very often it can kill it. There's the evidence that viruses are programmed to cooperate with the immune system of the host they naturally affect. If you put them in a different context they can be much worse because they are over-optimised for that particular host.

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