Hello, my name is***** and I have over 20 years of experience as a veterinarian. I am sorry to hear that Marnie Claire has suddenly developed a rash on the outside of her ears and on her nose and is itchy.
Cats get an allergic form of skin disease called eosinophilic granuloma complex. The lesions that we see with that disease process are hairless, pink, 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.
In your girl's case it sounds like her lesions are restricted to her face and ears. Facial and ear itchiness and secondary sores and scabs can be for several reasons.
The most common reasons tend to be secondary to flea bite or food allergies, a mite called Demodex gatoi, sarcoptic mites, mosquito bite hypersensitivity (usually affects the bridge of the nose and ears but can cause itchiness all over the face), inhaled allergies, ringworm, solar dermatitis which is an extreme form of sunburn (usually very light skinned cats with pink skin are affected) and rarely autoimmune condition conditions such as pemphigus.
If she has a dark haircoat then solar dermatitis is unlikely, especially because this developed so suddenly. But if she goes outdoors ringworm, flea bite and mosquito bite hypersensitivity and demodex or sarcoptic mites are possible.
Food allergy is definitely a possibility given that this seemed to develop suddenly after exposure to a new food.
If she isn't improving with the suggestions I give you I would recommend having your veterinarian examine her to look at a skin swab to look for Demodex Gatoi mites, a skin scraping for signs of sarcoptic mites, or evidence of fleas. A fungal culture should be checked for completeness.
As long as she isn't developing sudden, severe facial swelling and difficulty breathing such that she is open mouth breathing then this isn't necessarily an emergency tonight.
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 her for food allergies. This must be the only things she eats for at least 8 to 12 weeks, no treats or flavored medication. 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 ears 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. I do tend to stick with Purina Pro Plan brands or Nature's Recipe 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). Some cats find the taste of Benadryl horribly bitter and may drool excessively or even vomit in response. If that happens simply use a different antihistamine.
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 her 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.
If she will not stop rubbing or scratching her ears and nose then she may need to have any elizabethan collar (cone of shame , lampshade collar) placed so she cannot reach them.
The way to avoid mosquito bites is by keeping her in at dawn and dusk when these insects are most likely out.
If she 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.
Rarely we can see lesions like this with autoimmune disease (body attacks itself), but those lesions usually come on slowly rather than suddenly. They can be diagnosed via a biopsy.
Please let me know if you have any further questions.