I would reckon John the Baptist had special protection. Much like Paul had when bitten by an Asp.
The island in question, ancient Melite, is usually identified with the modern Malta, which has no poisonous snakes, but the "barbarous inhabitants" expected Paul to die. The island of Mljet in the Adriatic had the same name in ancient times and does have poisonous snakes. The Byzantine emperor Constantine Porphyrogenitus believed that Mljet was the site of St. Paul's shipwreck. But don't tell the Maltese that.