Myelolipomas in exotic pets, research, zoo, and wild animals: description of 52 cases and literature review

Myelolipomas are benign neoplastic lesions composed of adipose and hematopoietic tissue. Excluding dogs and cats, myelolipomas have been extensively described in the spleen of cheetahs (<i>Acinonyx jubatus</i>), the liver of Goeldi's monkeys (<i>Callimico goeldii</i>), and birds in the order <i>Psittaciformes</i>. To better describe the species diversity and the clinical and pathologic relevance of myelolipomas, we conducted a detailed literature review and a retrospective multi-institutional study of myelolipomas, excluding cases diagnosed in dogs and cats. A total of 52 cases from 27 different species were diagnosed or reported (65% mammals and 35% birds) from 8 institutions. Notably, 18 of 27 (67%) of the animal species diagnosed with myelolipoma in our study lacked previous documentation in the literature. In mammals, myelolipomas were diagnosed in the spleen, liver, and adrenal gland, and less commonly in lymph nodes, mesentery, broad uterine ligament, and subcutis; most often, these proliferations were incidental findings at autopsy unrelated to animal death or euthanasia. In birds, the most frequently affected location was the liver, followed by the kidney and celom, occasionally resulting in antemortem clinical disease and adverse outcomes. Myelolipomas had clinical and pathologic relevance in 9 of 52 (17%) cases including birds and mammals. Osseous metaplasia was found within the myelolipomas in 2 of 52 (4%) cases. We conclude that myelolipomas are present in more species than previously documented and most cases are probably underreported. The clinical relevance of a myelolipoma can be determined by postmortem examination, and depends on the species affected, the anatomic location, and the size of the lesion.