Snake or spider bit

My mom's cat gizmo, was bitten by somethig last night , she rushed him to the vet were they said it had to be a spider or snake bite. Shouldn't they be able to tell the difference?
He's back leg was so swollen that he could walk and his fever was up to 105,
They've gotten his fever down but he still can't keep down him meds. I told her to take him to her normal vet, as she rushed him to the Animal ER in the area, i just think that they should be able to tell the differnce between and snake and sprider bite, and would antivenom work on a feline?
Miranda

They probably can't tell until the swelling goes down an bit, but I would say this cat might be better off on a 24 hour sleepover at the vet. At least till he can tolerate the meds on his own.

Good luck with him, bites of any kind are always scary.

Hmmmm ??? Maybe if it is just a small snake who was biting him that they are not able to tell ? Just a wild guess though . But spider bite's can do that to a cat . I have seen a big dog at the vet one day with his hole mouth swollen and kind of look like rotten lips to me It turn out to be a spiter bite . I know by dogs the antivenom works I seen it on the animal planet in E/Vets , but have no clue on cats .I hope "gizmo" is going to be ok .

I hope he will be to, he was my mom's mother's day gift the year i left for college, he was the new baby. I'll let everyone know as soon as i hear back from her.