I have had severe gastro trouble since returning from Venezuela 18 days ago, including stomach pains, diarrhoea and wind, and I've lost over a stone. Sadly, my local hospital doesn't seem to be able to identify what the bug is. Is there a specialist centre that I can visit?

Martin

Dr Jules Eden replies: From your symptoms, it sounds like you could have giardia. This is caused by a bug that makes your stool pale, your wind violent and smelly and your weight drop. The interesting thing is that it can be hard to diagnose as it is not always found in stool cultures, despite being in the bowel.

However, it is easily treated with an antibiotic called metronidazole. Some doctors suggest that you take this antibiotic and see if your symptoms resolve. Other doctors go the whole hog, carrying out a colonoscopy - basically a long tube pushed up your bottom - and taking samples direct from the bowel. I know which one I'd prefer.

There are many good clinics in London, too many to list here. However, one that I particularly recommend is the Fleet Street Travel Clinic (020-7353 5678).

· If you have any questions you want to ask Dr Jules Eden, email travel.doctor@theguardian.com, or write to The Flying Doctor, The Guardian, 119 Farringdon Road, London EC1R 3ER.