Hello:
These types of lesions are really quite common in the cat. If this one is the type we commonly see, they are called eosinophilic plaques and are usually manifestations of allergic disease and the subsequent self-trauma the allergies induce. Since the skin is limited in the number of variations it has to respond to different types of issues, many different allergies can look the same on presentation. The most common allergies that we see include the following:
1) Flea allergic dermatitis. The animal will develop an allergy to a protein in the saliva of the flea that induces a systemic reaction causing itching, barbaring and discomfort. In most of these cases, you can find evidence of flea infestation with adults, larvae, flea dirt, etc. But in the cat, which is the penultimate groomer, absence of fleas does not rule out flea allergy. Treatment is eradication of fleas and control of the signs of allergies (usually with steroids).
2) Inhaled allergies - atopic dermatitis. Cats can develop allergies to spores of fungi, pollen from trees, shrubs or weeds, house dust or house dust mites, and any number of other things. In people this is hay fever and shows different signs. But in the cat and the dog, it generally manifests as itchy skin. Treatment is control of the allergic signs, avoidance of the allergen (if possible) and/or allergy shots to decrease the response to the allergen.
3) Food allergy. Cats can develop allergies to any of the ingredients in the diet - proteins, carbs, or additives. Intense itching can result with development of these plaques in localized regions. Diagnosis is based on disappearance of signs after changing diet to a novel source of proteins and carbs for a few weeks.
4) Parasitic allergies. Some cats can harbor intestinal parasites at low levels which do not show up on stool samples, and develop allergies to the parasites which are manifested just like any other allergies. Broad spectrum deworming done twice usually reduces signs even when we don't identify the offenders.
5) Other things can certainly be related to these and must be considered. They include: Ringworm (cultures are needed to diagnose), skin tumors (mast cells, squamous cell carcinomas, lymphomas can occur uncommonly), mites, skin infections (but that is what the convenia was directed at). But these are less common
These can be difficult to sort out and frustrating to treat. One round of treatment may not truly narrow things down. Sounds like more work needs to be done to get to the bottom of this. Talk with your vet about ruling out the things above and treating for the symptoms on a more comprehensive and prolonged basis.
Hope these thoughts help.
Dr. Coston
Cat Veterinarian
22 years of experience as companion animal veterinarian. Practice owner. Author of: Ask The Animals