Is it a brownish/black waxy substance on the skin, and at the bottom of hair follicles ,really hard to wipe off? Doesn't wash off when bathing?
does not wash off when bathing.....yes kinda waxy.
It's yeast/fungal. My basset is loaded with it, they are prone to it with their body shape, not much airflow, lol. my beagle gets it too. Especially this time of year, with us down here it is always so hot and humid. I'm surprised your vet didn't catch it...but then again I've had a lot of dogs, a lot of dog issues and some vets just aren't familiar with everything I guess, I've had some make mistakes too.
Anyways, I think you said it started after him getting baths? it could be he wasn't being totally dried as it grows/spreads with moisture. And now with the summer it's perfect weather for it to grow more, especially if he swims too. A lot of these allergy's and things don't start showing until they are between 2-5 yrs of age. My boys were just fine, then just exploded when they got to about 2-3.
The keflex is just for bacteria so thats why it didn't work. My beagle was on that for his bacterial pyodermas, but also on Fluconazole for the yeast he had. There are different types of yeast/fungals, some will present with a lot of itching, redness, inflammation, just like us w/yeast infections, but no waxy stuff and others are this dark waxy stuff but not much itching. it's basically the same brownish yucky/stinky stuff they get in their ears. With my basset, he hasn't needed the fluconazole or other RX meds as he doesn't have the other severe allergies that my beagle does.
So with him I use a shampoo called "MALASEB" it used to be RX only but is now available OTC online, just google the name and many sites will come up that sell it, got mine on amazon, and also Dr. Fosters & Smith online store I think. It also comes in a spray and wipes (pledgets) for between baths. There are also other yeast/fungal shampoos available but this has worked well for me. You can get at your vets probably too but they'll charge you double or more. When I've run out I've also gone up to the drug store and bought the yeast infection kit w/ the tube of cream to use, as the shampoo has miconazole in it. But thats a pain because i have to watch he doesn't lick it. It's just for when in a pinch, waiting for the shampoo.
You can also start giving him Omega 3 fish oil supplements, very good for fungals/allergies among many other things like heart health, joints etc.. I see you are a nurse so you probably know all this stuff. But you can look up the fish oil they have for dogs at the pet catalogue sites and see with your boys weight what dose he would need. They get a much higher dose than the human dose, so even though the doggie brands like 3-V snips are more expensive, it can be easier than giving so many human capsules with them being so big. You can also buy the bottled oil itself and add to food. I also give Benedryl to both my boys too, it works synergistically with the fish oil against these fungals/allergy's. The Benedryl is very safe, it's basically 1mg per pound of doggie. My boys weigh around 70 lbs and I give them 2-3 25mg caplets 2-3 x a day. Just the regular Benedryl, no added extra stuff. If he isn't itching you may do well w/ just the shampoos and omega 3. There is also a cream rinse called "DERMAL SOOTHE" also available online, I use that too.
Both of these shampoos/rinses need to be lathered in, left on 10-15 minutes then rinsed off. So it's good to have the spray/wipes for in between. And when you bath him get a softer bristled brush to scrub the waxy stuff off and to get the shampoo better rubbed in. Thats the only way to get it off. Once you get some control of it and start using this stuff it's easier. My Basset only gets bad when I skip a bath or 2 or it's nasty hot & humid out. The gentamycin you have might be helpful after you get the existing stuff off. I use that sometimes for their ear infections.
Since you get him bathed....they might offer a bath w/ these types of shampoos, just more money probably.
But if it's bad you may want to try the fluconazole to get it under control. My beagle was on 150mg 1x a day. Same as the human stuff.
Well....I think thats it....LOL.....I just had my Beagle to the doggie dermatologist a few months ago to get tested, cost mucho bucks, but he is sooooo much better, and they are so worth it. So this info does come from a Doc and my experience with many dogs, not just me talking off the top of my head, lol......just so you know.
Sorry for such a long message, but I just want the furbabies to feel better. Sometimes they are the best medicine for US........
Take care, Teresa ; ) .......and give your boy hugs & smooches from us!