"Yes, he's chewing/biting/licking at the base of his tail "
I would put big money on fleas. And I'd give a hundred to one odds .
Yes, there may be something else happening, too, like environmental allergies, but you are describing fleas to a "T."
Did you see this thread? (Same poster who started the thread you initially posted on.)
http://leerburg.com/webboard/thread.php?topic_id=33382&page=7
QUOTE (from me, in that thread):
OK, found the flea bath hint in a post from me on another thread:
"A hint about the bath .... fleas tend to be on the body rather than the skull. So at the beginning of the flea-removal bath, you want to apply the shampoo and lather around the neck first, in a wide band completely lathered well into the fur. This helps a lot, IME, to prevent the body fleas from migrating to the face and ears, which are not easy areas to shampoo. (Plus you don't want to introduce moisture to a healthy ear.) Then get the lather all over the body below that band. Then have the dog sit for 5 to 10 minutes lathered up. Tiny high-value treats are a help here, and so are such tricks as "shake paw."
Rinse very very thoroughly. IME, baths are not the drying event that many think (I learned this from the excellent book by Shawn Messonnier
http://leerburg.com/971.htm ), but not getting all the shampoo out can indeed by very drying, as can using a shampoo made for humans (very different pH)."
(Ideally, this would be done just before applying the Advantix. If you applied it a few days ago, though, you can still do this bath.)
A dog who is allergic to flea saliva (and this can happen at any age, although usually starting over age 1, commonly at age 3 or so) typically gets worse rather than better. (Flea saliva contains dozens of antigens, from amino acids to polypeptides and many more.)
Any allergy, including flea hypersensitivity, can cause an overgrowth in either normal yeast or normal staph (both present on healthy dogs), or both.
In addition, a dog with flea allergy can suffer from recurring tapeworms (from biting at and swallowing the fleas).
Fleas don't reproduce on the dog, which is why the environment needs to be de-flead while the dog is being bathed. The larvae are not on the dog; they are in his bed, in the carpet, in area rugs, etc. In fact, this is where most of the fleas will be, too. The dog is removing them from his body as fast and as furiously as he can, so even though he is continually being bitten, the biggest population is usually in his living area.
On a dog who is not allergic, the topical is kind of a bait-and-poison. That is, the flea jumps on the dog, bites, and is killed.
When the dog is allergic, though, this isn't good enough. The dog is damaging his protective shield, his skin, and this is where malassezia and/or staph gain a big toe-hold. END