Pemphigus foliaceous in a cat: Cytologic - histologic correlation

Felipe Reggeti, Josepha DeLay, Linda Senyshen

Animal Health Laboratory, University of Guelph, Guelph, ON (Reggeti, DeLay); Cat Hospital of Mississauga, Missisauga, ON (Senyshen).

AHL Newsletter 2025;29(1):22.

A 6-year-old male neutered Ragdoll cat presented to the referring veterinarian with a 2- to 3-week history of progressive erythematous crusting skin lesions affecting the right ear, left axilla, left upper forelimb and front digits (Fig. 1). The cat was housed indoors, and there was no history or evidence of trauma. Clinical differential diagnoses included infectious agents, allergic reactions and immune-mediated disease. Initial treatment consisted of antibiotic injections (Covenia®), local antiseptics (chlorhexidine) and a topical  anti-fungal/antibacterial/anti-inflammatory preparation (Otizole®).

Figure 1. Ulcerative, crusting skin lesions. Right ear.

Several tests were performed to investigate the etiology of this process. PCR for fungal organisms, examination for ectoparasites (Demodex spp., Sarcoptes spp.) and culture for dermatophytes (ringworm) were all negative. E. coli was isolated from an ear swab. Impression smears from the cutaneous lesions on the right ear and paws were submitted to the Animal Health Laboratory (AHL) for cytological interpretation. These samples consisted of poorly preserved squamous epithelial cells intermixed with numerous neutrophils against a densely proteinaceous background with abundant debris (Fig. 2A). Intracellular bacteria or fungal organisms were not identified. The epithelial cells included a few individual hyperchromatic cells with discrete rounded cytoplasmic borders and small pyknotic nuclei. These were compatible with “acantholytic keratinocytes”, raising concerns for immune-mediated disease, specifically pemphigus foliaceous (Fig. 2B). Histopathology was recommended for confirmation.

Figure 2. Impression smears of cutaneous lesions on front digits. 2A. A cluster of keratinocytes (left) and neutrophilic infiltrates (asterisk). 2B. Acantholytic keratinocytes (arrow) amongst poorly preserved squamous epithelial cells and debris. Wright’s stain, 60x.

Multiple 6 mm cutaneous punch biopsies from the right pinna and left axilla were submitted to the AHL for histopathologic examination. Histologically, the epidermis was moderately hyperplastic, with multiple variably sized subcorneal and intraspinous pustules present. Within each pustule, there were several acantholytic keratinocytes and non-degenerate neutrophils (Figs. 3A, 3B). These histological findings confirmed the presumptive clinical and cytological diagnosis of pemphigus foliaceous.

Figure 3. Histology of punch biopsies from cutaneous lesions. 3A. Intraepidermal pustule with acantholytic keratinocytes (asterisk) and neutrophils. H&E stain, 40X. 3B. Serous exudate with neutrophils, acantholytic keratinocytes and cellular debris. H&E stain, 20X.

Pemphigus complex is a cutaneous/mucocutaneous immune-mediated disorder characterized by blisters, pustules, erosions and ulcers of the skin, caused by decreased adhesion of epidermal keratinocytes (acantholysis). Pemphigus foliaceous specifically affects haired skin. It is associated with production of autoantibodies targeting desmoglein-1, a protein present in desmosomes, potentially disrupting integrity of adhesion molecules.

The characteristic distribution of cutaneous lesions in this patient (head and paws), and the cytological and histological evidence of acantholysis confirm the diagnosis of pemphigus foliaceous. Cytological examination of impression smears from newly-formed lesions can be an easy, non-invasive and inexpensive screening test, keeping in mind that samples may contain mostly debris and/or the lesions become secondarily infected, making the cytological interpretation challenging. Histology of multiple skin biopsies from acute lesions may provide the key architectural features of the lesions required for confirmation of this diagnosis.

Reference

1. Mauldin EA, Peters-Kennedy J. Integumentary system. In: Jubb, Kennedy, and Palmer’s Pathology of Domestic Animals, 6th ed. Maxie MG, ed. Elsevier, 2016;vol 3:600.