Hello, my name is***** and I have over 20 years of experience as a veterinarian. I can take over for Dr. Peter. I am sorry to hear that Duncan has a thickened, orange-red hairless area on his underbelly that he is licking and seems to be increasing in size. Cats get an allergic form of skin disease called eosinophilic granuloma complex. The lesions that we see with that disease process are hairless, pink or orange in color, very thickened oozing, ropey looking areas of skin. We generally see lesions on the skin of the inner and rear legs, abdomen, neck, chin and around the lips.
Cats also get another type of lesion in response to allergic stimulation called miliary dermatitis. We usually see crusts, scabs and hair loss along the back, and on the face, especially in the thinly haired area between the ears and eyes. You might look for evidence of miliary dermatitis on him too. Some allergic cats have both lesions, or some only have one type of reaction.
The most common reason for these lesions tend to be secondary to flea bite or food allergies, sarcoptic mites, mosquito bite hypersensitivity (usually affects the bridge of the nose and ears but can cause lesions wherever they are bitten), inhaled allergies, ringworm, and rarely autoimmune condition conditions such as pemphigus.
Since he goes outdoors ringworm, flea bite and mosquito bite hypersensitivity, inhaled or food allergies, or sarcoptic mites are possible.
If he isn't improving with the suggestions I give you I would recommend having your veterinarian examine him to look at a skin scraping for signs of sarcoptic mites, or evidence of fleas. A biopsy may be necessary to confirm allergic versus autoimmune (body attacks itself) disease. A fungal culture should be checked for completeness.
In the meantime you may wish to consider a true hypoallergenic diet such as Royal Canin Duck & Green Pea or Hills z/d to test him for food allergies. This must be the only things he eats for at least 8 to 12 weeks, no treats or flavored medication. I know that this might be tough as he goes out and may find to eat things outside.
If you choose to try to test/treat him for a food allergy I recommend a prescription diet. The trouble with "limited ingredient" or "low allergy" pet store brands is that the same machinery is used on multiple lots of food without sterilization cleaning in between. So for example even if a food says it has salmon and rice if the previous batch had beef and corn then you will get traces of those ingredients in your bag of food. Not a big deal if your cat isn't allergic but a waste of money thinking that the food was hypoallergenic and not good for your cat if those happen to be allergens for your cat. The veterinary brand true hypoallergenic foods are more expensive because it isn't cheap to thoroughly remove all traces of a previous food mixture from the machines used to process food. Generally what I recommend is trying to clear the skin and then adding one food item (chicken, beef, corn wheat etc) every month to see what they react to. Then we can find a regular food to try that doesn't have their allergens in it. I do tend to stick with Purina Pro Plan brands or Nature's Recipe pet store brands to feed long term as I find those rarely if ever have cross contamination.
You might also wish to try a combination of antihistamines and omega 3 fatty acids to help with inhaled, flea, or mosquito bite allergies. You can use either:
1) chlorpheniramine 4mg at half to one full tablet orally every 12 to 24 hours
2) Benadryl (diphenhydramine) at half of a 25mg tablet per 8 to 15 pound cat every 8 to 12 hours (1mg to 2mg per pound of body weight every 8 to 12 hours).
3) Zyrtec (Cetirizine hydrochloride) at a dose of 5 mg per cat given orally every 24 hours. (Make sure it is NOT the formulation with a decongestant (such as Zyrtec-D) because cats cannot tolerate decongestants.)
Antihistamines can cause sleepiness or hyperactivity in some cats but those effects usually wane with repeated use.
Good brand name omega 3's to try are 3V Caps or Derm Caps. I recommend a dose based upon the EPA portion (eicosapentanoic acid) of the supplement as if we do that the rest of the supplement will be properly balanced. Give him 20mg of EPA per pound of body weight per day. For example an 8 pound cat could take 160mg of EPA per day. Together antihistamines and omega-3 fatty acids work synergistically. These should help reduce the itch.
The way to avoid mosquito bites is by keeping him in at dawn and dusk when these insects are most likely out.
If he is not improving with these measures then then the diagnostics I mentioned above and possibly a skin biopsy would be the best way to get a definitive diagnosis and direct treatment.
Please let me know if you have any further questions.