Science Questions

Why were colds so bad for indigenous inhabitants?

Sun, 14th Dec 2008

Part of the show Emerging Diseases


Andrew D asked:

Why were colds so bad for indigenous inhabitants?


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 - Thatís 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 didnít 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 Ė 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 Ė 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 havenít seen them before. Therefore thereís no pressure on that population  to evolve these defence mechanisms. Those populations are like a souped-up virus and itís overkill. They donít 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.  Thereís 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 weíre giving our viruses to chimps. When you put our viruses that give us a cold into a chimpanzee very often it can kill it. Thereís 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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Andrew Douch asked the Naked Scientists: Hi I love the show... my kids and I listen to it when we are in the car. It always gets us talking about interesting things. I have a question. When caucasians colonised Australia, many indigenous Australians died because of the common cold and influenza viruses which they had not been exposed to previously. But surely the immune systems of indigenous Australians would work in the same way as those of caucasians. Yet when we get the flu or a cold, we don't die from it (usually) - because our B and T cells can respond and make antibodies etc to fight it off. So why did that not happen in those Australian populations? Does having had exposure to a virus with SIMILAR antigens somehow help the immune cells more quickly identify and respond to a new (but somewhat similar) set of foreign antigens? It almost seems like exposure to (say) a flu virus - leads the B and T cells to make versions of themselves that are slightly different in case an altered form of the antigen should enter again. Do you see what I am asking? It's almost like we are partially immune to a new strain of the influenza virus???? Cheers, Andrew What do you think? Andrew Douch , Fri, 2nd May 2008

There are three factors as I see it:

1) As Andrew suggests, exposure to related diseases does confer some cross protection.  An example of this was the use of the cowpox virus to confer protection against smallpox.

2) A disease caught in childhood may be very different from the same disease caught in adulthood (before the use of cowpox to vaccinate children against smallpox, there was a time that very young children were inoculated using live smallpox virus.  This was dangerous, and children died, but their risks were less than if the caught the disease later in life, so it was seen as a valid trade-off).

3) Lastly, maybe not often talked about, and maybe not very politically correct to talk about it, but there is the effect of evolution.  If a population is exposed to a disease over many generations, then those families which are most susceptible to the disease will be killed off by the disease, and the remaining families will have a better natural resistance to the disease.  If a population has never been exposed to the disease then there will be many families within that population that will be particularly susceptible to that disease.

An example of this is that in areas where malaria is prevalent there is a rise in the number of families that carry the gene for sickle cell anaemia and for thalassemia, because these genes confer some protection against malaria; but in areas where malaria is uncommon, the cost of carrying these genes outweighs their advantages. another_someone, Fri, 2nd May 2008

Indeed, George

...and there is more evidence for natural selection in malaria.

Malaria selective 'push' went further, and the very 'handle' on human red cells, the surface antigen called Duffy, somewhere has been lost over the centuries, to counteract parasite infestation:

more from: iko, Fri, 2nd May 2008

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