Sorry for the long 7 years wait for this. I bet by now you already know the answers, but there's always new people getting into fisting and this is the perfect opportunity to raise awareness to risk decrease in this kink.
So first things first, you have to understand that technically there is no fluid exchange in fisting and that gloves were first introduced in the 1980s during the AIDS crisis. That being said, it's not that bare fisting has 0 risks, but they're minimal compared to regular anal intercourse.
Whenever I get fisted I check the top's hands for the size of their nails, then I check for any possible cut or scratch or skinning they have on their hands, fingers and forearms. Like I said, risks are minimal but they're still there. In case of cuts, gloves are recommended for two reasons: 1- reducing the chances of having fluid exchange and 2- avoiding getting bacteria from the rectum directly on the wounds. Let's remember that the rectum is full of bacteria that is needed for the functionning of our bowels and getting them on a cut elevates considerably the risk of infections.
Although some tops claim that wearing gloves minimize the scratching of nails on the rectum's walls, most commercially available gloves are not thick enough to prevent any damage of this sort. So basically wearing gloves does not replace getting your fingernails cut. Even by wearing them it is recommended to have them trimmed as short as possible.
Now just as a small trivia, when AIDS and HIV were spreading in the early 1980s there used to be a sex club in San Francisco called The Catacombs. They were the ones who first introduced the use of gloves in this kink. The health inspectors said that their club was very clean but they still needed to provide condoms, even if they said they didn't do regular penetration there. But in order to keep having their gatherings they'd needed to provide protection. They put condoms available everywhere and members were like "what do I do with these? Put one on each finger?" and that's how they came up with the idea of wearing latex gloves and veterinary gloves for deep fisting.
After the rise in popularity of PrEP people have once again ditched gloves. But the risk of getting a bacterial infection on the top's hand is still there because technically that is not an STI.
