My sisters poor 13yr old long hair Chiwawa can not seam to get rid of fleas. He has chewed himself to where he is losing hair and raw. He has been to the vet and have tried several perscription strength things and it is not working. She's tried Revolution, frontline plus, dips, baths every other night, bombing the house several times. She has 3 cats and hamspters, none of them have fleas. This has been an ongoing thing for years with the Chiwawa even with the previous owner. She is at wits and and feels so bad that she is even thinking about putting him down...Help...