Feline eosinophilic granuloma complex

What it is

The feline eosinophilic granuloma complex (EGC) groups three types of lesion that share infiltration by eosinophils and can appear alone or together in the same cat:

• Indolent ulcer (eosinophilic ulcer, "rodent ulcer"): a well-demarcated ulcer with raised edges on the upper lip, next to the philtrum or opposite the lower canine; usually neither painful nor itchy.
• Eosinophilic plaque: raised, red, moist or eroded and intensely pruritic plaques on the ventral abdomen and medial thighs, caused partly by licking.
• Eosinophilic granuloma: firm, yellow-pink nodular or linear lesions; the linear form runs along the caudal thigh, and they also appear on the chin ("fat chin"), on the tongue and palate, and on the footpads.

It is not a final diagnosis but a cutaneous reaction pattern. In most cats the underlying cause is hypersensitivity: to flea bites, to food or to environmental allergens (feline atopic dermatitis, now called feline atopic syndrome). Mosquito bites cause a variant on the bridge of the nose and the ears. A form with genetic predisposition exists in some cat lines, and some lesions in young cats regress spontaneously. Secondary bacterial infection is common in plaques and ulcers.

Who it affects

Cats of any age, more often young and young adult cats with allergic disease. The linear granuloma and the chin form are common in young cats.

Anatomy involved

• Skin and mucosae: dermis infiltrated by eosinophils, mast cells and other inflammatory cells; in the granuloma, foci of collagen degeneration surrounded by giant cells and macrophages.
• Upper lip and mucocutaneous junction: site of the indolent ulcer.
• Ventral abdomen and medial thighs: typical site of plaques.
• Caudal thighs, chin, oral cavity and footpads: sites of the granuloma.

Clinical signs

Depending on the lesion type: lip ulcers that may disfigure the lip; moist, intensely pruritic plaques with excessive licking and alopecia; firm nodules or lines; chin swelling; or oral lesions that cause dysphagia and drooling. Other signs of allergy are often present: head and neck pruritus, self-induced alopecia or miliary dermatitis. Regional lymph nodes may be enlarged.

Diagnosis

• Clinical appearance and location of the lesions.
• Impression cytology: eosinophils, often with bacteria and neutrophils if there is secondary infection.
• Biopsy of atypical or non-responding lesions to rule out neoplasia (especially squamous cell carcinoma in lip ulcers and oral lesions) and other infections.
• Search for the underlying allergy: strict flea control on all animals in the household, an elimination diet of at least eight weeks and, if both are ruled out, a diagnosis of feline atopic syndrome (with allergy testing only to choose immunotherapy).
• Haematology: peripheral eosinophilia may be present.

Treatment

The key is to control the cause: continuous parasite control, a hypoallergenic diet when food allergy is present, and allergen immunotherapy or anti-allergic drugs in atopic syndrome. For the lesions: antibiotics when cytology shows infection (in a controlled trial, amoxicillin-clavulanate reduced plaques; in lip ulcers it reduced infection, but not their size significantly), glucocorticoids (oral prednisolone or methylprednisolone) and, as a steroid-sparing alternative, ciclosporin. Depot corticosteroids are avoided because of their adverse effects, such as diabetes. Lesions respond well in most cats, but recur if the underlying allergy is not controlled.

Key studies

• Hobi et al. (2011), a multicentre study of 502 pruritic cats. Diagnoses were flea bite hypersensitivity (29%), food hypersensitivity (12%), non-flea, non-food hypersensitivity (20%) and other pruritic diseases (24%). None of the four typical lesion patterns, eosinophilic dermatitis among them, was specific to one cause, and food and non-food allergy could not be told apart clinically. Young adult, purebred and female cats were predisposed to the non-flea, non-food form.
• ICADA diagnostic guidelines (Santoro et al., 2021): feline atopic skin syndrome is a clinical diagnosis that requires fleas and flea allergy, other parasites, infections and food allergy to be ruled out first.
• Wildermuth et al. (2012), a randomised, double-blind, placebo-controlled trial with 16 cats completing the study. All lesions had bacterial infection on cytology. After 21 days of amoxicillin-clavulanate, plaque size fell by a mean of 96.2%, which did not happen with placebo. In lip ulcers the antibiotic reduced infection, but the reduction in size (42.6%) was not significant. There were very few cats per group.
• ICADA systematic review of treatment (Mueller et al., 2021): it found good evidence of efficacy for systemic glucocorticoids and ciclosporin in feline atopic skin syndrome; limited evidence for topical glucocorticoids, oclacitinib and allergen immunotherapy; and low-to-moderate efficacy for antihistamines, fatty acids and palmitoylethanolamide.
• King et al. (2012), a randomised, double-blind, placebo-controlled trial of 100 cats with hypersensitivity dermatitis. At 6 weeks, lesions improved by more than 50% in 70% of cats given ciclosporin at 7 mg/kg daily, 47% of those given 2.5 mg/kg and 23% of those on placebo. The most common adverse effects were mild gastrointestinal signs.