Eosinophilic granuloma complex refers to a characteristic allergic skin reaction in cats. Flea allergy is thought to be the most common cause of this type of allergic skin reaction. Allergic reactions to food components, which are allergens that are inhaled into the respiratory system (atopy), and allergic reactions to other insects such as mosquitoes also can result in eosinophilic skin lesions. In unusual cases, an allergic cause cannot be found and a hereditary disorder is suspected.

Eosinophilic plaques usually are found on the abdomen or the inner thighs. They are raised, red lesions that may be glistening in appearance or oozing serum. Eosinophilic plaque lesions are extremely itchy (pruritic) and often are surrounded by broken hairs from constant licking of the area by the cat with its barbed tongue. Other skin diseases that can produce similar lesions include bacterial or fungal infections and some types of skin cancer including mast cell tumors and cutaneous forms of lymphosarcoma.

Eosinophilic granulomas often are found on the backs of the legs, on the roof of the mouth or on the tongue, and on the lower lip causing the cat to have a pouting expression. Lesions on the back of the legs usually are raised, round, and pink or yellow in color. More than one lesion may be present and they tend to occur in a linear distribution along the leg. As with eosinophilic plaques, bacterial and fungal infections and certain types of skin cancer such as mast cell tumor and cutaneous lymphosarcoma can have a similar appearance.

Indolent ulcers, also called rodent ulcers almost always are found on the upper lip and usually are confined to one side. Occasionally, they may be found on both sides of the lip or inside of the mouth. The lesions most often are raised and ulcerated, causing a dramatic change in the appearance of the lip. Indolent ulcers may represent a precancerous lesion and, if untreated, eventually may develop into a malignant skin tumor called squamous cell carcinoma. Once again, bacterial and fungal infections and skin tumors (including mast cell tumor, cutaneous lymphosarcoma and squamous cell carcinoma) are diseases that may result in a similar appearance.

In some cats, more than one type of eosinophilic skin disease can occur simultaneously.