Thanks again Ireneo you were on the right tract! The most common inhabitant of skin folds is a yeast called Malassezia pachydermatis. Soap and water helps but does not kill the yeast. I would add probiotics to the diet in the form of yogurt with live active cultures, and acidophilus etc. supplements to her diet. Your best bet is to see your vet of course. Your vet should have various products to combat yeast infections, including topicals and oral medications. In the meantime you can gently bathe the area with a dilute vinegar and water solution several times per day to change the ph. Yeast does not like acid. Use two tablespoons of salad vinegar: white vinegar, cidar vinegar, etc to 1/4 cup of water.
What about fungus? Perhaps candidiasis? But that would take more than soap and water to clear up. Fungus is tough, ask anyone with a foot fungus. ewww