Sorry about the delay in responding! Thanks for the information...Skin infections (I'm assuming you meant bacterial) are almost never the original problem, there has to be another condition present in order for a bacterial infection to set in. Conditions such as allergies (this is most common; see this: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=2604), skin mites, hormonal disturbances (hypothyroidism
and others), skin cancers, auto-immune disorders, conditions such as diabetes
and others commonly predispose dogs to bacterial skin infections. It sounds like there has been a lot of work already done to screen for underlying causes... Given this information, I think the following are the most likely:1. Allergies. There is no easy way to diagnose an allergy, unfortunately but this is the most common reason for chronic skin problems in dogs; among these, flea allergies are the most common and if a dog is not on a strict flea prevention program, it must be at the top of the list. This may seem unlikely because your other pets are not itchy and because you see no fleas, but a situation like this is actually relatively common because not all pets are allergic to flea bites (i.e. some will not be itchy or lose hair) and many dogs are very good at removing the fleas from their skin through chewing (because they're itchy) at the skin resulting in very few, if any, visible adult fleas. The only way to ensure that fleas are not a problem is to treat the pet for fleas while addressing the other problems. If your dog does not have fleas but has other allergies, flea bites will make him 10 times more itchy, so all suspect allergic dogs should be on flea prevention for this reason as well. Here is a link on food allergies: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=652 and on airborne (seasonal) allergies: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=597. Please read these over, it will help you see why allergies are common and likely in this case. 2. Skin mites: this has to be ruled out by a special test called skin scraping; a skin biopsy will not always pick these up.A skin biopsy would have picked up most of the uncommon and serious diseases, such as skin cancer and most of the immune-mediated skin disorders
; if the only thing that the skin biopsy showed was a skin infection, then it's most likely that allergies are to blame... Please let me know if you have any additional questions!