Psoriasiform-Lichenoid-like Dermatosis in a Bichon Frise Dog Without Prior Systemic Cyclosporine Exposure

Psoriasiform-lichenoid dermatitis is an uncommon clinicopathologic reaction pattern in dogs, classically described in young English Springer Spaniels. This report describes a psoriasiform-lichenoid inflammatory reaction pattern compatible with psoriasiform-lichenoid-like dermatosis in a 10-month-old spayed female Bichon Frise with multifocal crusted and verrucous lesions involving the pinnae, head, and genital region. The lesions developed at approximately 4 months of age. Cytology revealed coccoid bacteria, and dermatophyte PCR was negative. Histopathologic examination of hematoxylin and eosin (H&E)-stained sections showed papillary epidermal hyperplasia, irregular acanthosis with rete peg formation, orthokeratotic hyperkeratosis with focal parakeratosis and serocellular crusts, a superficial band-like lymphoplasmacytic infiltrate, and a focal intraepidermal eosinophilic pustule, consistent with a psoriasiform-lichenoid reaction pattern. Aerobic culture yielded Staphylococcus pseudintermedius susceptible to cefazolin and cefalotin; anaerobic bacterial and fungal cultures were negative. Cephalexin was administered initially for 3 days postoperatively and subsequently for an additional 3 weeks. Marked clinical improvement occurred after 2 weeks, with no recurrence during 6 months of follow-up after discontinuation of cephalexin. This case expands the recognized clinicopathologic spectrum of psoriasiform-lichenoid reaction patterns by documenting an affected Bichon Frise without systemic cyclosporine exposure. The isolation of Staphylococcus pseudintermedius and the clinical response to cephalexin support a possible, but unproven, contribution of bacterial involvement.