Liz Buchanan BVSc MRCVS
Hello I wonder why this is only happening on one side? This being an elderly labrador, I wonder if she has a preferred side for lying on e.g. due to underlying arthritis or pain in the abdomen, which means that one side is either in contact with a particular surface (or not exposed to the air) more often, or hurting a lot? Sometimes, for example, pain relief for arthritis or pancreatitis can make a huge difference to skin disease.
What you are describing sounds as though there may be a yeasty skin infection (these are usually secondary to scratching: the scratching cuts the skin very lightly and then the yeast that live normally on the surface of the skin, move into the skin and cause a slightly smelly infection). I am disappointed that the shampoo has not helped; does your vet know that you are not using it? There may be alternative anti-yeast / bacterial shampoos that you can try (prescription ones have been shown to work and therefore vets prefer them; so many shampoo 'products' look very similar but are in fact never going to help fight a yeast infection). Often, anti-yeast shampoos have to be left in contact with the skin for a certain length of time before being rinsed off. Furthermore, a flea treatment may be advisable - not that your dog necessarily has a flea infestation. More specifically, most dogs that are prone to allergy tend to overreact to random fleas, so even a single flea of someone else's that they meet while out for a walk, can be enough to set a horrid reaction off - and a good quality flea treatment for 3 months can help to rule this possibility out.
in summary, there are lots of possibilities - and as always, when there are multiple possibilities, you vet might not reach the answer immediately. This case definitely merits a revisit or two. Questions such as: 'What might be causing the scratching? What will this rule out? What will you do if this doesn't work?' Might help you to extract a logical plan, so that you can understand your vets' course of approach. Best of luck and please do let us know how you get on.