Cat with unusual skin lumps caused by heartworm infection

Abstract

Subcutaneous dirofilariosis is a well-known disease caused mainly by Dirofilaria repens and described in several mammalian species including humans, dogs, and cats. Additionally, early developing stages of the heartworm Dirofilaria immitis are rarely reported in subcutaneous localization from humans and dogs. To our knowledge, confirmed clinical evidence of this condition has not been described in the cats yet, even if the feline hosts can be affected either by the classic adult-related heartworm form or heartworm-associated respiratory disease (HARD) caused by immature stages. A 2 year old, spayed male cat was presented for three subcutaneous nodules on the head and trunk. The cat lived in Northern Italy and was regularly vaccinated and treated monthly with an antiparasitic spot on formulation containing selamectin. One of the three nodules was surgically excised and examined. Histology showed the presence of a nodular lesion in the subcutis characterized by a severe inflammatory infiltrate composed of macrophages, small lymphocytes, with fewer eosinophils, and mast cells, supported by a proliferation of mature fibroblasts (fibrosis). Inflammatory cells were multifocally surrounding sections of parasites identified as adult nematodes. Microscopic features were compatible with D. immitis, which has been molecularly confirmed (98.2% identity to D. immitis isolate OP107739). The cat tested negative for D. immitis antigenemia and the two remaining nodules disappeared spontaneously in a few months. In region where heartworm is prevalent, aberrant localization of D. immitis should be considered in the differential diagnoses of subcutaneous filarial worms in cats and dogs.