Mesothelioma of the pleura was found in a male southern elephant seal, Mirounga leonina. It was a localized flat mass (60×60×8cm, 18.0kg) in the left pleural cavity filled with massive bloody effusion. The parietal pleura was thickened and nodular. Microscopically, the tumor was made of papillary projections lined with cuboidal or columnar cells with admixture of sarcomatous proliferation of spindleshaped tumor cells. Histochemical studies revealed. that the tumor cells had produced acid mucopolysaccharide digestible with hyaluronidase. There was no distant metastasis or involvement of any otherbody cavity. Histological diagnosis was localized. papillary mesothelioma. The animal died of massive hemothorax.
Mesotheliomas have occurred in human beings, cows, horses, dogs, cats, hamsters, rats, and buffalos and experimentally in squirrel monkeys, but are very rare among wild animals.
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