Peritoneal Mesothelioma in a Free-Ranging American Black Bear (Ursus americanus) Open Access

Mesotheliomas are rare neoplasms that arise from mesoderm-derived cells which line serosal surfaces, such as the pleura, peritoneum, pericardium, and tunica vaginalis (Lopez 2007). In humans, pleural and peritoneal mesotheliomas are associated with inhalation of certain types of asbestos fibers or cigarette smoking (Lopez 2007). The disease has been detected in domestic animals, especially in bovine calves, but clear association between predisposing factors and mesotheliomas in domestic animals is difficult to establish (Lopez 2007).

In July 2016, an adult, female black bear (Ursus americanus) was found emaciated and unable to rise near a stream in the mountains of central Utah, US (approximate location 40822′34N″, 111833′18″W). Due to poor condition, the bear was humanely euthanized by Utah Division of Wildlife Resources biologists and submitted for postmortem examination.

The 95-kg bear had low amounts of subcutaneous and abdominal fat stores, and several liters of yellow, cloudy fluid distended the peritoneal cavity. An irregular, multinodular, solid, white to tan mass diffusely thickened the intestinal mesentery (Fig. 1). Hundreds of 0.2- to 1.5-cm-diameter firm, white to tan, frequently coalescing nodules were on serosal or capsular surfaces of the intestine, spleen, kidneys, urinary bladder, and parietal peritoneum. Occasionally, nodules were so dense that they obscured underlying tissues.

Microscopic examination of sections of mesentery revealed numerous cuboidal to polygonal cells arranged in tubules separated by moderate amounts of vascularized collagen. The neoplastic cells lining tubules were a single layer thick. Larger tubules had multiple papillary infoldings lined by similar cells. In masses adhered to tubular viscera (intestine, uterus, and urinary bladder), masses extended through the serosa to partially replace the tunica muscularis. In the spleen, the masses extended through the capsule into sinus areas. Sections of esophagus, stomach, liver, trachea, lung, heart, kidney, urinary bladder, uterus, spleen, and brain lacked significant lesions.

Immunohistochemically, neoplastic cells were immunoreactive to antibodies directed against cytokeratin (AE1/AE3, Dako Laboratories, Santa Clara, California, USA), and vimentin (clone V9, Dako Laboratories; Fig. 2). Although not validated for use on bear tissues, control tissues from the bear stained as expected. Use of cytokeratin and vimentin markers was merited because nearly all epithelioid and most sarcomatoid mesotheliomas are immunoreactive to each (Churg 1985). Although neoplasms of other abdominal viscera, including uterine adenocarcinomas and renal tubular carcinomas (Azumi and Battifora 1987), exhibit both cytokeratin and vimentin immunoreactivity, neoplastic masses were not identified in these tissues and these neoplasms rarely mimic gross and microscopic findings seen here. Taken together, a diagnosis of peritoneal tubulopapillary mesothelioma was made.

Mesotheliomas have not been reported previously in American black bears, but have been described in captive sloth bears (Melursus ursinus; Hellmann et al. 1991), spectacled bears (Tremarctos ornatus; Wolff 1988), polar bears (Ursus maritimus; Hellmann et al. 1991), and various other species. Most cases have been documented in captive mammalian wildlife of advanced age, and reports in free-ranging wildlife are limited to koalas (Phasco- 232 JOURNAL OF WILDLIFE DISEASES, VOL. 57, NO. 1, JANUARY 2021 larctos cinereus), spotted-tailed quoll (Dasyurus maculatus), and blue shark (Prionace glauca; Borucinska et al. 2003). The detection of tumors in captive wildlife is not surprising, because they are more likely to live long, and postmortem examinations are often performed upon death. Further, captive wildlife can be exposed to predisposing factors such as building materials more frequently than their free-ranging counterparts, although as in domestic animals, associations between development of neoplasia and exposure to specific predisposing factors are difficult to ascertain.

Neoplasia in general has infrequently been reported in American black bears. In free-ranging black bears there is one report of a mediastinal teratoma in a 2.75-yr-old, female black bear from North Carolina (Munk et al. 2013). Reports in captive black bears include carcinoma of the urinary bladder and tongue, mammary neoplasia, laryngeal squamous cell carcinoma, gastric leiomyoma, lipoma, and squamous cell carcinoma (Bourne et al. 2010), fibroma (Karesh et al. 1982), cystadenoma (Storms et al. 2004), and squamous cell epithelioma (Llambes et al. 1956). A comprehensive review of tumors in bear species was conducted by Bourne et al. (2010), who reported that tumors in bears frequently involve the hepatic or gastrointestinal system, especially in Asian bears used for bile farming due to the frequent trauma and inflammation of the hepatobiliary system. Other reports of tumors in North American bear species includes disseminated pleomorphic myofibrosarcoma and medulloblastomas in captive grizzly bears (Ursus arctos horribilis; Mete et al. 2012; Mitchell et al. 2015), and pancreatic islet cell tumor and osteosarcoma in a captive polar bear (Ursus maritimus; Ponomar'kov and Khutorianskii 1995; Fortin and Benoit-Biancamano 2014). Most affected animals were of advanced age.

It is unknown what predisposing factors occurred in this free-ranging black bear from Utah, and the case is most likely singular and rare. Even so, mesotheliomas should be considered as a differential diagnosis for tumors in American black bears.