Science Questions

Why are cold sores so painful? Lyell Wallince

Chris - Cold sores are the herpes virus - this is herpes simplex.  There are two types of herpes simplex: type 1 and type 2.  Cold sores traditionally are caused by type 1 herpes.  This is a virus that gets into your body, usually by the age of 3 most of us have picked it up by kissing a parent.

In the first manifestation it goes into cells in the mouth and throat and infects those cells to amplify the virus many times over and increasing the infectious dose.  The first presentation is you get a sore throat and high temperature; your glands come up around your throat.  It then appears to go away for a long time, some people never see it again.

What actually has happened when you had a sore throat was the virus was also infecting sensory nerve endings that supply your mouth and throat.  These nerve endings then transmit the virus back to the spinal cord or to what’s called the dorsal ganglion which is where the cell body for those nerves lives; adjacent to your spinal cord.  In the case of the head and neck it goes up to the trigeminal ganglion which is underneath your brain.  This is where the virus hangs out just as a small circular piece of DNA for the rest of your life.  If you go to the post mortem room and you study people you’ve died you can find evidence of the virus living in 80% of the population’s nervous system.

Periodically and in response to poorly defined stimuli (these can include menstruation, they can include sunburn and tissue trauma – if you get a cigarette burn this can sometimes make it happen) some signal goes back up the nerve, says to the virus, “you’re threatened, you need to come out.”  It reactivates and the DNA turns on again, makes fresh virus particles inside the cell.  They come back down the nerve cell to a patch of skin that nerve cells supplies, the virus comes out of the nerve, onto the skin, raising the skin cells producing an infectious lesion.  That’s a cold sore and the point of this is the virus uses the cold sore to infect another person - when you kiss someone you’re infectious.  That’s how the virus gets around but the rest of it is hiding inside your nervous system.

When it reactivates in that way it can damage the nerve it’s in and those nerves very often are pain nerve fibres.  They get stimulated by the activation of the virus and that is excruciatingly painful and it can persist for a very long time.

December 2008