OK, well as far as I can see, just the presence of the testicles are is unlikely to directly cause the diarrhoea. The only link I can think of would be if testicles have developed tumours, and spread to the gut, or I guess if they were VERY large, they might start to put pressure on the gut and cause diarrhoea. I'm glad you have had him castrated. Did your vet send the testicles to the lab for analysis? Did they think they might be cancerous?
To be fair to your vet, it would be impossible to feel the testicles unless they were very enlarged, and if most vets were to see an older dog without external testicles, then they would assume that the dog has already been castrated. It is a little unusual for both testicles to be retained - more commonly only 1 is. I am suprised that noone discussed this with you when he was a puppy. It is one of the first things I look for. Did you have him from a pup?
So how come he was castrated today? Is the diarrhoea ongoing or has it been throughout his life?
Nick
OK, well in which case I'm sure they would have sent the teticles too. You will then know whether these have caused problems. If (worst case scenario) a tumour had spread from the testicles, then you would know as these cells would start appearing in other organs. If he has had diarrhoea from the beginning, this is an unlikely scenario as the tumours tend to take several years to develop. I definitely suspect another cause, and hopefully these biopsies will help.
I'm afraid i cant really give you an explanation why someone did not mention the retained testicles when he was a pup, but I hope and think that it is unlikely to be the current problem.
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If the diarrhoea has become worse in the last couple of months, then yes that could be the case. As above, if tumours spread off elsewhere to form 'metatstases', these metastases contain the same cells as the tissue they come from, so the lab report should be pretty conclusive if this is the case.
Regards
Dog Veterinarian
Small animal vet in UK for 8 years, Cambridge University. Special interest in diagnostic imaging.