1. Check out serum serotonin levels, its directly related some way or another.
2. rule out fat malabsorption
3. Helicobacter pylori infection/parasites
4. Try antifungal/antibiotic/&antiviral topical applications like gels .
5. It should be fungal infection which might need long term application of antifungals say might be more than 3months. so ask for any mild concentrated ointments for long term use.