Evolutionarily, why should wounds itch?

[CplFerro]

An itchy wound is more likely to be a scratched wound, coming into contact with long, ragged, filthy caveman fingernails, bringing in a boatload of germs. Is scratching an itchy wound the healthy thing to do? Or, if not, why wouldn't nature select the Itchy Wound gene out of the population?

Cpl Ferro

 

[Donks]

From a couple of minutes of googing, I gather that scratching produces histamine which, apart from inducing more itch, speeds up healing.

 

[Cheesejoff]

The itch also draws attention to the fact that the wound is there.

 

[CplFerro]

Dear Donks,

Doesn't that contradict the whole notion of forming a scab? Why form a scab if we can just shred the heck out of our injuries? I think cavemen were probably walking around with a bunch of suppurating injuries all over them.

Cpl Ferro

 

[Donks]

Dear Donks,

Doesn't that contradict the whole notion of forming a scab? Why form a scab if we can just shred the heck out of our injuries?

Dear CplFerro,

Does scratching must mean tearing your flesh open, or is there some sort of middle ground? Perhaps evolution also picks for organisms smart enough not to rip their flesh open every time they get a mosquito bite.

I think cavemen were probably walking around with a bunch of suppurating injuries all over them.

Good for you.

Do you have an alternate explanation you'd like to share? Do you have a point of some kind?

Cpl Ferro

Donks

 

[CplFerro]

Does scratching must mean tearing your flesh open, or is there some sort of middle ground? Perhaps evolution also picks for organisms smart enough not to rip their flesh open every time they get a mosquito bite.

Dear Donks,

Given that caveman fingernails are not going to be well-manicured, nor would these people have any concept of restraint that pure biology doesn't instill in them, I can entirely see them ripping open scabs to deal with the maddening itch, thus letting in all manner of germs. My point is that it's not a survival strategy to scratch delicately healing injuries.

Cpl Ferro

 

[Donks]

Dear Donks,

Given that caveman fingernails are not going to be well-manicured, nor would these people have any concept of restraint that pure biology doesn't instill in them, I can entirely see them ripping open scabs to deal with the maddening itch, thus letting in all manner of germs. My point is that it's not a survival strategy to scratch delicately healing injuries.

Cpl Ferro

If you want to go follow this line of though, I recomend you demonstrate that the itching and the scabbing do not work together to heal faster than scabbing alone. Give me some evidence than in fact it is a poor survival strategy to scratch, that it somehow makes the organisms less likely to breed and pass their traits to the next generation. Or would this be a better mechanism if God did it?

 

[skoob]

We are not perfect organisms. It could simply be that our itchy-scratchy mechanism is suboptimal. Perhaps if you wait a few thousand generations humans will have evolved a better system that only makes us itch if scratching actually will help.

I'd also like to point out that evolution doesn't happen for a reason. Itching hasn't evolved in order for us to scratch parts that itch. Itching and scratching just happened to provide individuals with an advantage over those who don't itch or scratch itches. That doesn't necessarily mean that it's a good idea to scratch every itch. It just means that in general, scratching itches is a good thing.

 

[Donn]

Seems we have survived despite that - scratching itches I mean.

Is is right to speak of "genes for" this and that? Is every little sensation and action and thought dictated by a gene? I mean could the itching not simply be the way the body works, no particular gene required? Perhaps it's a side-effect of the interaction of other genes as they built the flesh?
(Reading the Selfish Gene and getting muddled you see!)

ETA - Posted too slowly to be in context anymore!

 

[delphi_ote]

Dear Donks,

Given that caveman fingernails are not going to be well-manicured, nor would these people have any concept of restraint that pure biology doesn't instill in them, I can entirely see them ripping open scabs to deal with the maddening itch, thus letting in all manner of germs. My point is that it's not a survival strategy to scratch delicately healing injuries.

Cpl Ferro

Remember that natural selection only acted on things which allowed for survival to mating, not necessarily long term survival. Itching was probably an instinct to remind primitive man to get big damaging things out of their body quick (or something like this. I'm working on a hypothesis here, not quoting any kind of reference or authority.) If that hurt long term survival, so be it. As long as they lived long enough to have offspring, natural selection would favor those adaptations.

 

[Ketyk]

I have another theory as to why itchiness would be a good thing. It probably tends increase the blood supply to the wound below the scab, allowing scar tissue to be removed and letting new cells develope. I doubt that even a caveman would want to rip of a scab off because it is itchy. I've also found that itciness occurs well along the healing process, not when it is still an acute injury.

 

[Soapy Sam]

I question your postulate.

I don't think wounds itch unless they are bandaged.

Mine don't.

Are they supposed to?

 

[Dali's Ghost]

I've also found that itciness occurs well along the healing process, not when it is still an acute injury.

Itching for me only starts once a scab has formed for a couple days. And if I do scratch off the scab, reopening the wound, it usually hurts bad enough that I don't want to do it again.

Plus, everyone itches without having a wound for various reasons. And breaking the skin will always cause some amount of pain. So everyone regulates the strength of their scratch based on an instinct to avoid pain. The condition of the fingernails doing the scratching would not be a factor. The most you could assume is that an unmanicured nail has a greater likelyhood of breaking the skin from a careless scratch. But I don't think that would not happen very often.

 

[CplFerro]

I don't know, I have an unbandaged wound right now that itches a bit, and if I were to set about scratching it I could easily shred the nascent scab, possibly infecting the wound. I wouldn't underestimate the germ-spreading capacity of fingernails...

Cpl Ferro

 

[bjb]

I thought we evolved to lick our wounds, not scratch them.

 

[blutoski]

An itchy wound is more likely to be a scratched wound, coming into contact with long, ragged, filthy caveman fingernails, bringing in a boatload of germs. Is scratching an itchy wound the healthy thing to do? Or, if not, why wouldn't nature select the Itchy Wound gene out of the population?

Cpl Ferro

This is one of those things that's evidence of 'jerry-rigging'. This part of immune system is largely dedicated to parasites. When you get a cut, the body sends in cells and chemicals, and turns the damaged tissue into a sort of disaster area.

Histamines and some other cytokines and chemicals make you itch, the benefit being that if they're responding to a parasite, you'll dig it out. One side effect is that when you get a scab, it continues to irritate the wound, and the body responds the same way it would if you had a parasite.

Basically, life-over-limb: the net is that in an evolutionary environment where parasites were common, a little extra scarring from digging at scabs was a small cost to get the benefit of an anti-parasite instinct.

 

[CapelDodger]

I think cavemen were probably walking around with a bunch of suppurating injuries all over them.

Cavemen had mothers to tell them not to pick at it.

By the time a scab has formed Homeland Security has flooded the area, it is not a good entry-point for biological hazards. If a scab is dislodged before the skin has healed, blood flows - another protection against infiltration - and a new scab forms. There's not a big problem here. It's the initial wound that presents a big enough problem to exert selection pressure.

The sub-optimal evolution argument is probably sound. Itching and scratching are mostly associated with parasites such as lice and ticks, which explains the histamine-itch connection. Histamine prompts an itch-scratch reaction to remove the parasite, and at the same time alerts Homeland Security to a breach that may well have introduced much more insidious parasites that use said ticks as a vector. There's no selection pressure to distinguish between a wound and a louse-bite.

eta : Like blutoski said.

 

[athon]

Damnit, Blutoski! I hate it when somebody beats me to it.

Basically, yeah, as he said. The majority of pathogens we are exposed to today are relatively recent problems in our evolutionary history. For the most part, we have evolved alongside parasites such as ticks, worms, fleas, fungi and filiarials. Bacterial infections are 'uncommon' when compared with these parasites.

The latter stages of chemicals released during cellular trauma irritate specific surface nerve cells to make them itch. Scratching the area affected can dislodge parasites, stimulate blood-flow to the area, remove foreign objects pushed out of the body, or release pus from a healing wound. Many animals respond to this through licking the wound or nibbling at it, often using antimicrobials in their saliva to help the process.

The 'dirty caveman' concept is one that's primarily in your head, I'm afraid. Animal immune systems (like our own) learn as they develop through the early years of life. We are unfortunately quite clean animals, and have immune systems that are less educated than if we happened to have a more varied interaction with our pathological environment as we grow up. Lesson - feed your kids dirt!

Athon

 

[Boo]

Just as an FYI, the sensation of itching is produced by the same nerves that send pain signals. OItching is actually the lowest level of pain indication, ergo a subtle reminder that an area is injured and needs to be protected.



Boo

 

[CapelDodger]

Basically, yeah, as he said. The majority of pathogens we are exposed to today are relatively recent problems in our evolutionary history. For the most part, we have evolved alongside parasites such as ticks, worms, fleas, fungi and filiarials. Bacterial infections are 'uncommon' when compared with these parasites.

When you say "we" I don't think you mean Africans. They're the folk who've had an arms-race with parasites for millions of years. An unintended consequence for humans who had to leave Africa was a sharp drop in their parasite-load. Population soared. Animals were domesticated. And their parasites became our new diseases.

Publicity notwithstanding, bird-flu is a lot scarier than Aids. Aids is old-school, influenza is in the modern arms-race.