FYI...ear mites are a relatively uncommon occurence in dogs..and Frontline and Heartguard will not get rid of ear mites...for ear mites you can use Revolution, ivermectin in the ears, orally, or parentally...but like i said, ear mites are usually not a problem in dogs...
however, did the vet ever say what the infection was? yeast? bacteria and if so, what??
when do these infections occur?? do you notice any pattern to it?? you mentioned she was allergic to beef...anything else you know of??
i will swab an ear, look under the microscope and determine treament...i will do another check in 10 days and have often found new bugs growing while the primary infection has come under control...and then if i still am not getting anywhere, i will sedate the pet and look with a video otoscope and get a sample from the middle ear, and medicate at the same time...ears can be a very frustrating thing for owner, pet and vet.
sounds to me that a good ear work-up has not been done...and i think the dermagen is like pissin in the wind...sorta useless in the ear in my opinion...the only reason i can see as to why it works is the steroid in it...but for yeast or bacteria, it is worthless.
and what is costing you 165$???
oh, i am actually a veterinarian!