The additional problem we have with rescue or trapped cats is that we do not know their history. For example we trapped one that we found later had a kidney problem and so reacted badly to the pain meds the vet prescribed after castration. Even though i halved the dosage (as a precaution) the poor thing died and I feel terrible. I now use a new rescue vet, who tried to save him, and she won't give pain meds for that very reason and she's on speed dial for a fluid drip.We’ve had the same problem with a few fosters. One ended up spending 4 days at the vet on fluids and we nearly lost him.