Those excoriations are quite severe, and she certainly looks miserable, I am so glad that you are going to catch her and take her to a veterinarian, she is sorely need of care. Potential causes for the itchy rash include skin parasites (demodex, ectopic ear mites, notoedric mange), fungal skin infection (ie. ringworm, usually causes scaly hairloss and not as itchy as the pictures look), and food allergy. With the pus you describe, a secondary bacterial skin infection is likely. If there are lesions actually involving the inner ear pinnae and anus, I would also consider autoimmune conditions such as pemphigus or lupus. Pemphigus is usually crusty and can be itchy and lupus is more ulcerative and less itchy. A vasculitis/autoimmune reaction secondary to an anderlying viral infection (feline leukemia, feline immunodeficiency virus, FIP) is also a possibility, and obviously a big concern in a stray cat. The first step should be a blood test for feline leukemia and feline AIDs, and if these tests are negative, then more workup would be appropriate such as skin scrapings to look for parasites and infection, fungal (ringworm) culture, general bloodwork to evaluate internal organs, trial treatment for parasites with Revolution, a hypoallergenic diet trial for possible food allergy, +/- skin biopsies if no answer is found in preliminary testing.
Kimberly Coyner, DVM DACVD
Dermatology Clinic for Animals of Las Vegas