The idiosyncrasies of itchy cats
Cats are not small dogswe've heard that before. It's never quite so true in veterinary medicine as for a dermatologist!
"Cats don't do things that dogs do," says Rudayna Ghubash, DVM, DACVD, Animal Dermatology Clinic in Marina del Rey, California. "You can't base your dermatologic differential list on what part of the body is affected by evidence of pruritus like you can with dogs." Just because a cat is itchy on its head and neck, doesn't mean that the underlying cause is a food allergy. In Dr. Ghubash's experience, head and neck pruritus are usually involved in atopy and food allergy, but not always!
So what is the answer for felines? Dr. Ghubash reiterates that because cats' physiologic response to allergenic opportunities are so different from dogs', it takes a different approach to diagnosing what's causing the clinical signs.
Physical examination pointers
Pay attention to a patterns of lesions. Are there excoriations present indicating trauma? Are there milia indicative of miliary dermatitis, commonly a sign of allergic disease? Check the lips and palates for eosinophilic granuloma complex lesions.
Dr. Ghubash says treating secondary infections you identify is paramount. It's not wrong to start with administering antibiotics alone for a week or two and if you're not making any progress in clinical signs, then use corticosteroids.
Dr. Ghubash's recommended approach to a cat presented to you for pruritus is to start with a careful and deliberate history and to examine each pruritic cat in a methodical way. She estimates that about 50% of your valuable information will come from the signalment and history, if you ask the client the right questions. For example, a cat younger than 6 months old is more likely to have an underlying parasite or dermatophyte causing its pruritus. Atopy is rare in kittens that young. Click here to download a dermatologic history form.
A complete history-taking of the cat's previous and concurrent disease is very important. For example, cats don't get pododermatitis from allergies like dogs do. If they get it, the underlying cause is more likely to be pemphigus foliaceus. Dr. Ghubash says they tend to get more of the "shake and bake" appearance of ear crusts.
Particulars on pruritic predicaments in cats
Explore dermatophytosis in your patients first because it can mimic any other dermatologic disease, pruritic or not, says Dr. Ghubash. Always recommend performing a dermatophyte test medium (DTM) culture. Keep in mind that Wood's lamp examinations are only accurate about 50% of the time.
If you've taken a biopsy, you should request periodic acid-Schiff (PAS) and Gomori's methenamine silver (GMS) stains if you're concerned about dermatophytes.
Quick tip: In young Persian cats with dermatologic ailments, dermatophytosis should be at the top of the differential diagnosis list until proven otherwise by diagnostics, the gold standard currently being dermatophyte culture, Dr. Ghubash says.
Food allergies can start at any age, so it isn't unusual to have an older cat that breaks out with pruritus due to food allergies. Dr. Ghubash cautions that, in her opinion, you should be on the lookout for this more in Siamese and Siamese cross-breeds than others.
A big tip: Consider the calendar: If a cat's pruritus is seasonal, you do not need to perform a food trial. But if the pruritus is not seasonal, you do need to perform a food trial. Dr. Ghubash adds, "Blood testing for food allergies is unreliable, inaccurate and a total waste of your client's money."
Changing the diet to a veterinary prescription diet for 12 weeks is the only accurate way to diagnose food allergies in Dr. Ghubash's opinion. The reason the over-the-counter (OTC) diets don't work for food trials is because the food companies frequently "cobatch" when they produce their foods. This means that they use the same equipment to produce different brands of specific foods. This can frequently lead to differences in allergen content in each bag on the shelf. Dr. Ghubash points out that there was a study that demonstrated that OTC venison diets had microscopic traces of other proteins in them.1 As a result, there is no OTC diet that she recommends for a food trial.
Think food allergies are off the hook if the cat improves with corticosteroids? Dr. Ghubash says think again as some cats do.
You can't use a blood allergy test to diagnose allergies or atopy. Make the diagnosis and then use the blood tests to tailor your immunotherapy.
It's never wrong to step back and do a biopsy, says Dr. Ghubash. The results might tell you that the animal is allergic. Sometimes you may need multiple samples, and remember to send your samples to a pathologist who is comfortable reading skin histopathology. The results might tell you that the animal is allergic, or that they have a condition only diagnosable via biopsy.
Allergies to parasites
If the cat goes outdoors at all, rule out flea allergies before anything else, even if you don't find any fleas or flea dirt and the cat is receiving monthly flea control. Some cats are excellent groomers and can groom any live fleas or their evidence (flea dirt) that were ever there off of their bodies before a reaction is shown. Dr. Ghubash says that, "Regardless of the history, the vast majority of itchy cats that you see are going to be flea-allergic."
You may note a pattern in these cases, with the pruritus being located mostly in the dorsal and lumbar regions. If you see that, think parasites: fleas, Cheyletiella and Notoedres species and Demodex gatoi.
A note about diagnosis: it can take months to rule out flea allergy, but Dr. Ghubash would recommend doing that before initiating food trials. "Putting them on once-a-month flea preventives and still letting them go outside is not ruling flea allergy out," she says. She recommends keeping the cat indoors, implementing environmental flea control, and using a topical flea control product every two weeks for at least three doses (selamectin-containing products are some of her favorites in cats since they also rule out Notoedres species infection).
Taking a thorough history with cats that are allergic to mosquito bites is also super important. Dr. Ghubash has seen cats with mosquito bite hypersensitivity be misdiagnosed as having neoplasia. These cats often present with dermatitis on their ears, rostral nares and paws (Figure 1).
Figure 1. A cat with mosquito bite hypersensitivity, showing lesions on the pinna and nares.
This autoimmune disorder can certainly mimic allergies in cats, says Dr. Ghubash, which is another reason why vaccination and drug administration history is important. Cats with pemphigus foliaceus tend to present with lesions on their ears, nailbeds and nipples (Figures 2-4). As a side note, Dr. Ghubash says, "Cats don't get allergic dermatitis like dogs do." If the nailbeds are involved, think pemphigus first.
Figures 2 and 3: Cats with pemphigus foliaceus lesions on their pinna.
Figure 4. A cat with pemphigus foliaceus lesions on the paws, specifically around the base of the nailbeds.
"The biggest thing in treating this condition is making sure you are identifying and adequately treating the infection," Dr. Ghubash says. "You may need six to eight weeks of antibiotic therapy to knock it out."
Eosinophilic granuloma complex
Remember that eosinophilic granuloma complex is a clinical sign of allergies. It can produce four types of allergic reactions in cats:
> Indolent ulcers
> Eosinophilic plaques
> Linear granulomas
> Eosinophilic granuloma plaques
When you see any of these lesions, be sure to check the mouth and palate of the cat you're examining. Dr. Ghubash says that she has found quite a few cats that have these other signs and then painful other lesions that were missed in those cats' mouths.
And don't forget ...
Other syndromes that can cause pruritus in cats are cutaneous lymphoma, paraneoplastic syndrome and autoimmune disease. Also, if the cat has a history of upper respiratory viral infection, you might consider viral dermatitis as a cause.
1. Raditic DM, Remillard RL, Tater KC. ELISA testing for common food antigens in four dry dog foods used in dietary elimination trials. J Anim Physiol Anim Nutr (Berl) 2011;95(1):90-97.