I get this too - rarely does it appear that stool is coming out.  I believe the yellowish stuff is mucous and/or pus from the tract.  Mine waxes and wanes, I don't know why.  Fistulas are so mysterious.

It could be a bacteria.  Since you say it's very narrow, it's possible that stool is too think to come through.  Good luck, these this are more than annoying.

I too have an RV fistula. When I was first diagnosed, the doctor called it 'small bowel output'. So not quite stool, but the juices that flow through your intestines. 

I've had it for 10 years now, so if you have any questions, feel free to ask me. 

C-jay

It's mucus. A fistula is in essence now part of your bowels (this is why your body won't heal it; it's considered normal bowel tissue to it, and triggers no need to attack it).  All bowel makes mucus. Hence the term "mucus fistula" when GI fistulas form. Generally yellow in color with little or no smell. It's normal. 

Thanks for that Rachel. But I have to say I am still confused as to why it is only mucous that is draining and not stool if the tract is connecting the bowel (at anastomosis) to the vagina.

Mine is 90% mucus, occasionally the color will change, and that's when I know some stool slipped through. It's probably not constant because the internal hole isn't always up against stool, you know?  Unless some stool slips by if, say, it sneaks through the sphincters?  Maybe...

Thanks. I think I need to learn the anatomy to get a better sense of it all. I will take mucous over stool any day though!