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By Michelle Gerhard Jasny V.M.D.
Published: March 12, 2009

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Ringo's owners called on Monday morning. This recently adopted young cat had a sore on his hind leg that he kept licking. They had been treating it at home, but it wasn't healing. They made an appointment for that afternoon, but my day was unexpectedly interrupted by an event involving my daughter's knee, a protruding nail, and a trip to the emergency room. (No, I don't suture my own children, since so many of you asked.)

Ringo was rescheduled for Tuesday, along with several other feline dermatology cases. There was Jo, the middle-aged, free-roaming cat who had been hit by a car last year and broken her jaw. Ever since then, she had a recurrent rash on her chin, and now she was also pulling out the fur on her front legs. There was King Tut, an elderly indoor cat who kept breaking out in little scabs on his head. All three were exhibiting pruritus, the technical term for itchiness. (An interesting aside for those who enjoy etymology: the word "prurient" is derived from the same root as pruritus - it just refers to a different sort of "itch.")

Ringo arrived first, a long, red strip of irritated, hairless skin extending down the back of his hind leg. "Eosinophilic granuloma," I pronounced grandly after about two seconds. My clients looked suitably impressed. "What causes that?" they naturally wanted to know. Eosinophilic granuloma complex is also known as eosinophilic plaque or "rodent ulcer," which is a holdover from the days when people thought the lesions were caused by bite wounds from rats and mice. Nowadays, we know eosinophilic granulomas are usually caused by allergies. The difficulty lies in figuring out precisely what an individual cat is allergic to.

Jo came next. She had a common condition called feline acne, manifest as blackheads, papules, and pustules on her chin. In addition, she had self-induced alopecia on her front legs. That's the fancy way of saying she was pulling out her own hair, leaving bald patches. The skin on her legs looked perfectly normal and healthy - no redness, pimples, or scabs - just no hair. The presence of short broken hairs confirmed that this was self-induced. Clients often think an animal's hair is falling out spontaneously, but if this were the case, the area would be smooth with no stubble.

"Why is she pulling out her fur?" her owner wanted to know. When I was in veterinary school, many moons ago, we were taught about a condition called psychogenic alopecia, believed to be a behavioral issue in which cats over-groomed as a result of stress, emotional upset, or psychological problems. In recent decades, however, dermatologists have agreed that most cases previously labeled "psychogenic" are probably the result of allergies. Jo isn't neurotic. She's pruritic.

King Tut was the final arrival. He had classic miliary dermatitis - another impressive-sounding name that means he had lots of tiny, scabby bumps. Some cats break out along their backs, others on the head and neck. Tut had never had skin problems until last year when he ran into a little trouble with recurrent constipation, requiring the frequent use of prescription laxatives. Not long after, his mother noted the scabs. When she stopped the laxative, Tut's skin cleared, but the constipation recurred. She then tried a different over-the-counter laxative. The scabs reappeared. Was this coincidence or an allergy to some common ingredient in both laxatives?

Differential diagnoses for any itchy kitty include external parasites such as fleas and mites, fungal infections such as ringworm, and allergies. Allergies may be to food, inhalants such as pollen, house dust, insects such as mosquitoes, or fleas. Ever wonder why one cat can have fleas and you never even notice while another cat goes crazy scratching if you are one day late with your monthly flea control? It's because some cats are actually allergic to the flea saliva.

I carefully checked each cat for evidence of fleas. Owners are always positive their pets are flea-free, until I run a flea comb down the animal's back. It's one of my favorite moments when I get to say, "See these little black specks? That's flea poop." Sometimes I even get a live hopper on my comb to convince a skeptical client. But Ringo, Jo, and Tut were all clean. Ringo was getting monthly total-body flea control and Tut was an indoor cat with low risk of exposure. Jo, however, does go out and she was not currently getting any parasite control.

"Sometimes I have seen cats like this," I said to Jo's mom. "I don't find any evidence of fleas, but when we start flea control, they get better." I explained how one flea bite could set off a whole body reaction in an allergic individual, but that there were many other possibilities for the underlying cause or her condition. We started Jo on flea control, antibiotics for the chin acne, and oral corticosteroids for the generalized pruritus.

Ringo we opted to treat empirically with injectable corticosteroids, which usually will clear up an eosinophilic granuloma, at least temporarily. His old medical records indicated that he had had this problem before, but since he was new to this family, we wanted to wait and see for ourselves. We would note if the issue was seasonal or year-round, how soon and how frequently it recurred, and then make a long-range plan.

For Tut, we opted for a simple test. We stopped the current laxative product and substituted a completely different medication. If the scabs clear spontaneously, we will make a presumptive diagnosis of an allergy to the previous laxatives.

I have suggested to all three owners that if problems continue, we try an eight-week trial of hypoallergenic prescription diet to rule out food allergy. They could also consider testing for inhalant allergies with a blood test or by referral to a veterinary dermatologist for intra-dermal tests. For all our fancy multisyllabic terminology, from linear eosinophilic granuloma to miliary dermatitis, it's not always easy to identify exactly what's making that cat so itchy.