It's not clear from your post whether or not the veterinarian actually diagnosed a fungus as the cause of the chewing or only assumed this to be the case. I've had the experience of a dog chewing her feet, especially the toe nails, and found this to be caused by stress. In her case, it was being left alone for an extended period of time. Once this was addressed, the chewing stopped. Perhaps your dog has undergone some sort of out of the ordinary stress? Best of luck!