Background The pathogenesis of feline allergic dermatitis (FAD) is unclear, with several differences from allergic dermatitis in dogs and humans. Hypothesis/objectives To survey cytokine expression levels in healthy cats and cats affected with allergic dermatitis or asthma. Animals Formalin‐fixed, paraffin‐embedded skin biopsies from 22 cats with allergic dermatitis and 21 cats without allergic dermatitis were used for cutaneous assays. Serum was obtained from 17 healthy cats, 18 cats with allergic dermatitis, and 18 cats with a presumptive diagnosis of asthma. Methods and materials Cutaneous mRNA expression was evaluated with quantitative PCR [interleukin (IL)‐31 and IL‐31 Receptor A] and RNA in situ hybridisation (ISH) [IL‐5, IL‐31, IL‐31RA, IL‐33 and Oncostatin M receptor (OSMR)‐β]. IL‐31 protein concentrations were evaluated in serum with an enzyme‐linked immunosorbent assay. Serum levels of 19 additional cytokines were evaluated using a Luminex panel. Results IL‐31, IL‐31RA, IL‐5 and IL‐33 mRNA expression were either expressed in low quantities or undetectable in most samples. By contrast, OSMR‐β expression was significantly higher in the skin of allergic versus healthy cats (P < 0.0001). Although serum IL‐31 was detected in a larger number of cats with allergic dermatitis than healthy cats, and concentrations appeared to be higher in cats with allergies, this difference was not statistically significant. Cats affected by asthma also exhibited insignificantly higher concentrations of IL‐31 in the serum. Conclusions and clinical relevance Our results suggest that feline allergic diseases may exhibit different pathomechanisms from allergic diseases affecting other species. These findings are useful in guiding further therapeutic development toward targets that may have a role in the pathogenesis of feline allergic skin disease.

Case summary A 12-year-old neutered male onychectomized Ragdoll cat presented for a 3 day history of swelling and hemorrhagic purulent discharge on the first digit of the left manus. Radiographs revealed fragments of the third phalangeal bone (P3) present in the partially amputated digits with swelling adjacent to the P3 fragment on the first digit of the left manus. Thoracic radiographs revealed no evidence of primary or metastatic neoplasia. Surgery was performed to remove all P3 fragments and the associated swelling on the diseased digit. On gross examination of the excised swelling, a mass was present at the cut edge of P3. The bone fragment and associated mass were submitted for histopathological evaluation. Osteosarcoma was diagnosed. Because neoplastic cells extended to the surgical margins, amputation of the left thoracic limb was performed. The cat recovered from surgery, and survival time at the time of writing was 8 months. Relevance and novel information To our knowledge, this is the first reported case of onychectomy-associated osteosarcoma. Trauma from partial P3 amputation during onychectomy is suspected to have played a role in osteosarcoma development in this case. Malignant transformation may be considered a potential complication of onychectomy achieved by partial P3 amputation.