Thymic Hematoma Found in a Wild Eurasian Otter (Lutra lutra) Open Access

Thymic hematoma, with or without subsequent hemorrhage, is poorly understood and uncommon in animals, with few cases having been reported in dogs only (Klopfer et al. 1980; Klopfer et al. 1985; van der Linde-Sipman and van Dijk 1987; Coolman et al. 1994; Liggett et al. 2002; Stallwood et al. 2019). In some cases, the causes were identified as being the result of vehicular trauma, dissecting aortic aneurysm, or rodenticide intoxication (van der Linde-Sipman and van Dijk 1987; Liggett et al. 2002); most cases were determined to be idiopathic or of spontaneous occurrence in young animals (Klopfer et al. 1980; Klopfer et al. 1985; van der Linde-Sipman and van Dijk 1987; Coolman et al. 1994; Stallwood et al. 2019).

The Eurasian otter (Lutra lutra) is a semiaquatic mammal belonging to the family Mustelidae; it is classified as “near-threatened” by the International Union for Conservation of Nature, despite being widely distributed from East Asia to Western Europe, and the Korean government has designated it as a “natural monument.” There is little literature regarding spontaneous diseases in this species. A Eurasian otter was found dead with a thymic hematoma without any previous trauma. Herein, we describe the pathologic findings of a thymic hematoma in a Eurasian otter.

On 22 May 2019, a Eurasian otter was found dead on a sidewalk next to the local driveway, 300–400 m away from its main habitat, the Nam-gang (Nam River) in the city of Jinju, Gyeongsangnam-do, Korea. The animal was submitted to the Department of Veterinary Pathology at Gyeongsang National University for postmortem examination. The otter was male, weighed 9.5 kg, and had no external abnormalities or obvious signs of trauma (Fig. 1A, B). However, we found hemorrhagic fluid and blood clots in the thorax and a large hemorrhagic mass in the thymus (Fig. 1C). The mass encompassed the aortic and pulmonary arteries, thoracic esophagus, and trachea and was tightly adhered to the arterial external walls (Fig. 1D). There was no evidence of rupture or tearing of arterial internal walls. The lungs had multifocal hemorrhages and edema, and bloody, foamy fluid was found in the trachea. The mucosal surface of the thoracic esophagus was congested. The heart was grossly normal, although four to five filarioid nematodes were found in the right atrium. There were no significant lesions in other organs. The thymic mass was fixed in 10% neutral-buffered formalin, processed routinely, sectioned at 4 µm, and stained with H&E.

Microscopically, massive hemorrhage was observed within the thymic capsule, and the interlobular septa were markedly widened because of hemorrhage (Fig. 2A). Although the tissue was affected by postmortem changes, the thymic cortex and medulla were distinguishable (Fig. 2B). Hassall's corpuscles, a unique structure of the thymus, were found in the medulla (Fig. 2C). We assumed that the otter was subadult because thymic involution, featuring infiltration of adipose tissue and loss of lymphocytes, was not seen. There was a large hemorrhage between the tunica media and adventitia of an elastic artery with focal damage of tunica adventitia, which was, in part, surrounded by hematoma (Fig. 2D). Damage to the external arterial wall was considered the result of compression by the hematoma because there was no wall injury internally. We concluded that the hemorrhage originated from the thymus and that the thymic hematoma was the cause of death.

In dogs, most cases of thymic hematoma are considered idiopathic, and affected individuals are usually under 2 yr old (Klopfer et al. 1980, 1985; van der Linde-Sipman and van Dijk 1987; Coolman et al. 1994; Stallwood et al. 2019). Idiopathic thymic hemorrhage has been proposed to occur by rupture of weakened blood vessels in the involuting thymus of young dogs, with various contributing factors, such as repeated neck trauma by the leash and collar, excitement, or exertion (van der Linde-Sipman and van Dijk 1987; Coolman et al. 1994). Anticoagulant rodenticide intoxication has also been suggested as a cause of thymic hematoma. In cases of anticoagulant rodenticide intoxication, clinical signs of abnormal hemostasis, such as gingival bleeding, epistaxis, hemoptysis, hematochezia, melena, and hematuria and muscle or subcutaneous hemorrhage, have been observed in most dogs (Sheafor and Couto 1999). In our case, the otter had no evidence of generalized bleeding diathesis. Interestingly, dirofilaria were found in the heart. Dirofilariasis is not uncommon in otters in heartworm-endemic areas (Snyder et al. 1989; Jung et al. 2018). Dirofilaria may weaken and tear arterial walls, causing hemothorax (Miller et al. 2007), but in this otter, there was no evidence of dirofilaria-associated abnormalities, such as arterial rupture or aneurysm.

Thymic hematoma is a rare condition in animals and has previously been reported only in dogs. Moreover, there is little literature regarding diseases in the Eurasian otter.