Canine peritoneal and pleural larval mesocestoidosis with sarcoma development.

A 5-year-old, spayed female Anatolian Shepherd dog adopted from Turkey presented for an ultrasound. A splenic mass, abdominal cysts, and peritoneal effusion were incidentally found. A presumptive diagnosis of peritoneal cestodosis was made. Despite antiparasitic treatment, the patient developed lethargy, hyporexia, tachypnea, dyspnea, and progressive peritoneal effusion. Due to the declining condition and poor prognosis, humane euthanasia was elected, and a necropsy was performed. The main gross findings were severe peritoneal and pleural effusion with a large amount of tan, turbid fluid, fibrin and free-floating tetrathyridia. Multiple, variably sized, cystic nodules containing tetrathyridia were found throughout the serosal surfaces, spleen, liver and lungs. Two large, firm, multilobular masses were in the spleen and liver. Histologically, the nodules consisted of parasitic granulomas with intralesional tetrathyridia, which presented calcareous corpuscles and lacked a digestive tract. The hepatic and splenic masses were characterized by interweaving streams of neoplastic spindle cells, consistent with a soft tissue sarcoma. The neoplastic cells presented strong positive immunolabeling for vimentin and smooth muscle actin indicating a mesenchymal origin with myofibroblastic phenotype. Tetrathyridia were multifocally within the neoplastic sections, suggesting a possible neoplastic transformation related to the parasitic chronic inflammatory response. Polymerase chain reaction and sequencing of the parasites identified the specimens as Mesocestoides sp. confirming the diagnosis of canine peritoneal and pleural larval mesocestoidosis. A chronic inflammatory response to Mesocestoides tetrathyridia could have played a role in oncogenesis. To our knowledge, this is the first report of Mesocestoides spp. parasitism associated with a soft tissue sarcoma in a dog.