I would try to collect a sample (I know it's a bit yuk) and ask the vet to send it away for analysis.  The vet will give you a sample bottle to put it in, it may be that there is some tyoe of organism in it which the usual antibiotics haven't dealt with.  Don't want to scare you but I had a thoroughbred who when he was only nine developed a runny nose and cough, we thought  he was reacting to pollen so we brought him in and gave him soaked hay and did all the usuall things, a couple of weeks later it seemed to clear up.  A couple of weeks after that he had a gutteral pouch haemorrhage and bled to death in front of me in a matter of minutes.  Sorry for the horror story but I just want to prevent anything like that happening to a horse again.  You really need to get it checked further, maybe endoscopy or bronchoscopy or something.  They think it was a fungus rather than a bacteria which my horse had, and it eroded it's way through the gutteral pouch and into the artery.  Your vet really should be running some investigations after this length of time with no improvement.  Hope he gets better soon x

Hi - thanks for advice.  Horse STILL has runny nose, 10 months later...!  Vet has seen him on a number of occasions and passed him fit and healthy.  Really is baffling.  I've tried damping hay, soaking hay, haylage, vasaline around his nostrils to stop dust etc.  He's on cardboard bedding (so very little dust), large well vented stable.  I've tried echincea, liquid breathe easy redemy, naf pink powder ... changed from chaff to nuts.  He's not been out 24/7 as livery restrictions (although a couple of nights he has been haven't made any difference).  I am really at a loss - as he doesn't cough, wheeze etc and vet says he's ok I should be ok with it ... BUT I'm not, it really niggles at the back of my mind.  Have tried  a few different bits to see if that would make a difference, but it hasn't ... just don't know what to do next.  It is unsightly at shows etc, and we have been pulled up a couple of times at PC events (although I carry letter from Vet know saying he's fit).  Does anyone have any more ideas! Would really appreciate your thoughts.  Thanks.

Just ensure that he stays warm and dry. His immunity has probably been affected due to moving homes. Keep an eye on him and just keep him warm and dry - the weather hasn't exactly been brilliant since October.

Call the vet if it suddenly gets worst. There are plenty of feed supplements on the market that may help with his immunity. Shoudn't do any harm to give one a go for a month or two, I give my new mare Naf's general supplement which has really improved her coat and made her (slightly) happier in general. (shes a 2yr old ex-racehorse thats very temperamental).

google natural herb supplements for horses and see what you can find.