Eosinophilic Granuloma Complex

Eosinophilic granuloma complex (EGC) is one of the more confusing-sounding diagnoses an owner can be handed, partly because it is not one disease at all but an umbrella term for three related skin reactions that often appear in the same cat. What ties them together is the cell behind them: the eosinophil, a white blood cell that swarms to sites of allergic inflammation. When a cat's immune system over-reacts to an allergen, eosinophils pour into the skin and cause the characteristic lesions.

The first and most recognisable form is the indolent ulcer, popularly called a "rodent ulcer," which appears as a swollen, often glistening sore on the upper lip — giving the cat a lop-sided pout or "fat lip." Despite the old name it has nothing to do with rodents. The second form, the eosinophilic plaque, shows up as raised, red, intensely itchy and often weepy patches, classically on the belly or the inner thighs, where a cat can reach to lick. The third, the linear or collagenolytic granuloma, forms a firm, raised streak running down the back of a hind leg, or a swelling on the chin or lower lip. A single cat may show one, two or all three at once.

The key insight that has transformed treatment is that EGC is a symptom, not a root cause. In the great majority of cases the driver is an allergy, and the three usual suspects are the same ones behind most feline skin disease: flea-bite hypersensitivity (by far the commonest), food allergy, and environmental allergy to things like pollen or dust mites. This is why simply suppressing the lesions with steroids, while effective in the short term, tends to lead to relapse — the inflammation comes back as soon as the medication stops because the underlying allergy is still there.

Good management therefore works on two fronts. First, the lesions are calmed, usually with anti-inflammatory corticosteroids and, where a secondary bacterial infection has taken hold, antibiotics. Second — and more importantly for long-term success — the vet works methodically to uncover the trigger: rigorous flea control to rule out (and treat) flea allergy, a strict hypoallergenic food trial to test for food allergy, and investigation of environmental allergens. For cats whose allergies cannot be fully avoided, longer-term immune-modulating drugs such as ciclosporin offer steroid-sparing control. With patience and detective work, most cats with eosinophilic granuloma complex can be kept comfortable and largely lesion-free.