I'm familiar with Bene-Bac (for reptiles & amphibians) or NutriBac (for reptiles & amphibians), but if you search there may be a few others too. That's what I'd try- a probiotic- and don't dust it on a damp rodent- instead, pry open the dead rodents mouth & put the powder in the oral cavity, then push the mouth closed. This way the snake won't know & possibly balk at eating a rodent that smells a bit different. (probably not an issue with a boa, but...) If you dust it on, most of it falls off anyway by the time the rodent gets swallowed.