Infectious Disease: What Can Evolution Do For Us? forum: topic
This is a public forum
why do things cause disease?
Bill Hanage
Friday, 07 November 2008 16:06 UTC
Here’s a question. Or maybe a thought. Make of it what you will.
How many obligate pathogens can you think of? By that I mean things which cause disease with almost every infection. Actually, let’s lower that bar a little and say cause disease more than half the time.
I think they’re actually rather rare. In the bacteria (my favourites) a lot of the most dangerous and feared infections only cause disease a tiny fraction of the times that they encounter humans. Streptococci (of the pneumococcal or group A variety), meningococci, staphs (yes including MRSA), E. coli. All these and more are what some have called accidental pathogens. They do kill a LOT of people. But they infect many many more, of whom most do not notice, but are well able to transmit.
And yet you’d probably not get that impression from a survey of the literature, which is full of ‘virulence determinants’ etc. As humans we perhaps understandably fetishise virulence – seeming to almost adopt a stance that these bugs are out to get us. The reason that they produce a virulence factor, is to cause disease. I have heard this sort of attitude from people who really ought to know better.
We should adopt a bug’s eye view. From the point of view of a parasite, all you ‘want’ to do is get transmitted. Yes, it is possible that in some cases evolution may, over a period of infection, lead to the emergence of things which grow faster with bad things for the host being the result (HIV is a good eg). But in most cases things evolve too slowly for this, and the duration of infection is too short.
So what are your favourite virulence factors? And what do you think their ‘real’ function might be. Remember, you are only allowed to say disease if the bug has to cause disease to get transmitted. For example, something which enables meningococci to survive in the brain would not count. There’s not a lot of transmission going on from cerebrospinal fluid…
-
Replies
-
Well, from my bug’s point of view, it is the ability to (a) survive higher temperatures – that ensures passage through non-human hosts and emission through fecal matter, (b) melanize using dihydroxy-phenylalanine containing substances, either from the environment (bark of certain trees) or from a mammalian hosts brain; melanin ensures survival against UV exposure in the environment and oxidative attacks inside the host (human and non-human).
Now guess what my bug is? :)
Perhaps we should stop anthropomorphizing these bugs by ascribing a ‘purpose’ to them! For example, my bugs are ubiquitous in nature, but humans (or mammals) are the end-hosts. The infectious forms are inhaled in and they actively fight the immune system, often overwhelming it to reach the brain and other organs. In immune suppressed individuals, the bugs have it easier; they just go in, multiply and kill. There is no ‘purpose’ here as such.
-
I find the Norovirus quite interesting since it spreads very rapidly and isn’t lethal (usually). The virus certainly would have something that makes you throw up, thus spreading the virus even more. I am not sure on why (it’s not my real area) but it strikes me as a good idea for a virus that wants to be spread.
Another [general] virulence factor that I like is the capsule/mammalian-like proteins that hide the bacteria from detection in a host. Not always considered a virulence factor …. but still very determinant.
(And I can’t think of that many obligate pathogens by that definition. There are a few viruses and bacteria that always cause disease though, like some strains of the H5N1 virus.)
-
