Pleural mesothelioma in a California sea lion (Zalophus californianus)

Our plain-English summary of Pleural mesothelioma in a California sea lion (Zalophus californianus), by Y. Takami, M. Tanaka, M. Morita, et al (The Journal of Veterinary Medical Science, 2023).

The study

• Authors: Y. Takami, M. Tanaka, M. Morita, T. Maruno, N. Anai, T. Sudo, C. Kezuka, T. Izawa, J. Yamate, M. Kuwamura
• Journal: The Journal of Veterinary Medical Science
• Published: 2023;85(9):1030-1033
• Type of article: Case report
• Read it: PubMed · Publisher · Free full text

Abstract

A 25-year-old female California sea lion (Zalophus californianus) reared in an aquarium died following a history of anorexia, lethargy, abnormal protrusion of the skin, and oral respiration. At necropsy, multiple yellowish-white nodules with diameters of 0.1-0.5 cm were disseminated in the thoracic cavity and lungs. Histopathologically, the nodules were continuous with normal mesothelium and were characterized by the proliferation of spindle-shaped to polygonal neoplastic cells with prominent atypia. The neoplastic cells exhibited diffuse, strong staining for vimentin and partial, weak to moderate staining for cytokeratin AE1/AE3. Based on these findings, the lesions were diagnosed as pleural mesothelioma. This study reports the first case of pleural mesothelioma in California sea lion.

In plain English

This article was a case report about one animal: a 25-year-old female California sea lion kept in an aquarium in Japan. The animal lost its appetite and became lethargic. It developed a swelling on the right side of its chest that ruptured and discharged pus, began breathing through its mouth, and died about one month after its appetite loss began. At necropsy, many yellowish-white nodules 0.1 to 0.5 cm across were spread over the chest cavity and lungs. Under the microscope, the nodules grew out of the lining of the chest (the pleura). They were made of mostly spindle-shaped tumor cells that stained strongly for vimentin and partly for cytokeratin. The authors diagnosed sarcomatoid pleural mesothelioma and reported it as the first case of pleural mesothelioma in a California sea lion. The animal also had other tumors, including a cancer of unknown origin that had spread to a right axillary lymph node. The article did not mention asbestos. The authors said it was unclear whether factors linked to tumors in sea lions played a role. Because tuberculosis was suspected at first, only major organs were sampled.

We wrote this plain-English summary from the full article with the help of AI, and a second AI model checked each sentence against the article. It is not the authors' wording; the abstract and the article are their own account.

What the study did and found

• Design: Veterinary case report (single animal, necropsy with histopathology, immunohistochemistry and electron microscopy)
• Who was studied: One 25-year-old female California sea lion (Zalophus californianus) reared in an aquarium. The authors were affiliated with aquariums in Kochi and Hyogo, Japan, and the necropsy was done at Osaka Metropolitan University. The year of the case is not stated.
• Exposure: No asbestos or other fiber exposure is mentioned. The authors listed factors associated with tumors in California sea lions generally: environmental contaminants such as polychlorinated biphenyl, Otarine herpesvirus-1 infection, and genetic characteristics. They said the involvement of these factors in this mesothelioma was unclear.
• Main results:
  • The nodules in the mediastinum (the central chest) were continuous with the thickened pleural mesothelium, the lining of the chest.
  • Similar tumor nodules in the lungs were unencapsulated and had infiltrated the lung tissue.
  • In the chest nodules, mitotic figures (dividing cells) were frequent: 33 mitoses per 10 high-power fields (2.37 square millimeters).
  • The tumor cells in the mediastinal nodules showed diffuse, strong staining for vimentin, and approximately 20 to 70% of the tumor cells showed weak to moderate staining for cytokeratin AE1/AE3.
  • Electron microscopy showed slender projections resembling microvilli on the tumor cell surfaces.
  • Because the tumor cells were predominantly spindle-shaped, the mesothelioma was classified as the sarcomatoid type.
  • The authors found no primary lesion of another tumor that could form such disseminated nodules in the chest, within the limits of what they examined at necropsy.
  • Other tumors were found at necropsy. The enlarged right axillary lymph node contained metastasis of a carcinoma of unknown origin, the right kidney mass was a renal adenoma, and the right gluteal skin nodule was a sebaceous adenoma.
• The authors' conclusion: The authors concluded that this was the first case of pleural mesothelioma in a California sea lion. They said that when disseminated nodules are found in the chest cavity and lungs of California sea lions, mesothelioma should be considered as a differential diagnosis alongside tuberculosis.
• Limitations the authors state:
  • Because tuberculosis was initially suspected, only major organs were collected, without extensive opening of the abdomen or chest, to avoid infection risk to those performing the necropsy.
  • Because the samples collected were limited, the microscopic changes in the skin mass in the right pectoral region were unknown.
  • The WT-1 antibody, a marker suggested for mesothelioma, did not stain either tumor or normal mesothelial cells, which the authors took to mean it does not cross-react with California sea lion tissues. It therefore could not support the diagnosis.

Topics

The National Library of Medicine indexes this study under subjects our site covers: Mesothelioma.

Medical terms in this study, explained in our Lexicon: Pleural Mesothelioma, Autopsy, Mesothelium.

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