Maybe a read of this style forum's thread from styleforums.net
Seems this guy was in a similar position..
"I recently purchased a leather belt, and when I took it out of the suitcase upon arrival last night, I noticed it had a strong "fishy smell" which I hadn't noticed before. Any explanation to this, and more importantly, will it dissipate? Should I do anything? It was a cheap belt, but it fits (a miracle) and I like it."
"FISH BELT! I've had this before, and it isn't fun. I don't know why some belts smell like fish, but it's very strong. It does dissipate a little, but perhaps you could speed it along by polishing the belt."
"Here's the answer from a friend of mine

:
Fabienne, my guess is that the smell comes from the quality of the tanning process. The tannery uses chemicals, including salt, to remove hair, etc. and then extracts of tree bark (tannin) are used to strengthen and color the leather. If this process isn't done well, the leather still maintains some of its flesh-like characteristics and can slowly rot, just like flesh (yuk!) It may also be that the chemicals were not well removed at the end of the process. I suppose that you could try baking it in a low temperature oven (say 200F) a few hours to kill any microbes, (or drive off any remaining volatile chemicals) and then give it a good coat of mink or neatsfoot oil.
I think that your friend's explanation is most unlikely, and a fishy smell is not characteristic of rotting leather/flesh. Rawhide is untanned leather, and it does not produce these smells.
Oil tanning was the traditional technique to produce chamois and other soft leathers, but I suspect that the odour has been produced by one of the oils used in finishing the leather after it was dyed. I think that putting the belt in an oven would be rather drastic. I would wash it with a good quality leather soap, and use a beeswax leather finish to disguise the smell. Many people find the smell of neatsfoot oil unpleasant."
In hindsight, I don't think this is down to an STI.