Rash on French Bulldog Spreading

Hello, I have a 1 year old French Bulldog and we've recently switched her to a harness vs. collar. We take her to the dog park and there is a kiddie pool that she swims in with her harness sometimes and it was shortly after she swam with the harness that I noticed some irritation starting under her armpit. I switched her to her collar again to avoid irritation but the rash seems to be spreading. They are raised bumps that feel like scabs. Any idea what this could be and suggestions? We are also in the process of transitioning her from puppy food (Royal Canin Puppy) to adult food (taste of the wild) but this seemed to occur before we began transitioning food. Thank you!

Hello!  I'm afraid that there are several possibilities for armpit skin disease, which can occur singly or in combination with others.  Let's explore a few of them. INFECTION / INFLAMMATION    First, you mention that the patient swims wearing a harness during a walk. Inevitably, the water will not be perfectly clean and a material strap across a moving part of the animal's skin under damp conditions, is likely to result in 'chafing.'  Particularly with less-than-sterile water involved, the area may then be prone to infection. Superficial bacterial infection or deeper bacterial infections or even yeast infection (often brought about when the normal microbes on a dogs' skin get to invade at a vulnerable point) may prove to be distinct possibilities. Even if bacteria and yeast aren't involved, a purely inflammatory lesion from the rubbing wet strap may be a possibility. ALLERGIES   Allergies can occur anywhere at any time, but the secondary effects of them can be exaggerated e.g. by having a taut strap across the skin in question. 1-3 years of age is typical for allergies to begin to show themselves and French Bulldogs often present with allergic skin disease. DEMODEX Demodex is a little mite that causes non-itchy hair-loss.  This is not necessarily associated with damp / covered / rubbing areas of skin, but can present with riased 'nodules' over the follicles, or hair loss. FLEAS - if not on a veterinary-approved flea treatment, fleas are always a possible cause of armpit pruritis (itching) So what is your next step?   To visit the vet, who may begin by doing further tests or trial treatments to try to get to the bottom of the problem,  ruling out various problems, perhaps including the possibilities mentioned above. Please do take their advice on this one, and let us know how you get on.  Best of luck.