Zosyn is an antibiotic in the penicillin family and is not commonlly prescribed in the pet industry for resp issues. It is most commonly used to treat pneumonia and urinary infections in people. I dont know why they would have prescribed this for resp infections considering drugs in the penicillin family are pretty aggresive. Baytrill is usually the go to medication for snakes so if they didnt do a culture i wonder why they didnt precribe it right away. Does your vet deal with snakes alot? Id be asking questions