Cutaneous silica-associated (cat litter) granulomas in the paws of 13 domestic cats

Silica is a well-known stimulus of granulomatous inflammation in the lungs of humans and other animals. However, it has been poorly studied as a cause of cutaneous inflammation in domestic animals, despite the predominance of silica in traditional (clay-based) cat litter substrates. Here we characterize the clinical and pathologic findings of 13 surgical biopsies from the paws or paw pads of domestic cats submitted to 2 veterinary institutions between 2005 and 2023. Gross lesions often included chronic or intermittent paw pad swelling, ulceration and bleeding, or draining tracts, particularly in the front paws. All biopsies consisted of granulomatous to pyogranulomatous inflammation with birefringent, amorphous to granular, blue-gray, intrahistiocytic material consistent with silica. Raman spectra were collected from 3 biopsy samples at 45 different locations. Using multivariate analysis, the foreign material in the biopsies had similar Raman spectra to multiple brands of silica-based cat litter. Ten of 13 biopsies also contained evidence of plasma cell pododermatitis, suggesting that underlying paw pad disease may predispose some cats to cutaneous cat litter implantation. In these cats, subsequent granulomatous inflammation may obscure the underlying condition and exacerbate clinical disease. Routine polarization of feline paw or paw pad biopsies is recommended to aid in recognition of cat litter granulomas. Careful examination for a predisposing condition is also warranted when cat litter granulomas are identified. Recurrence is common following surgery; affected cats, and predisposed cats with paw pad disease, may benefit from the use of alternate litter substrates to prevent this lesion or its recurrence.