Acute myeloid leukemia in a genetically related captive population of axolotls ( Ambystoma mexicanum )

Axolotls (Ambystoma mexicanum) are critically endangered paedomorphic salamanders with remarkable regenerative capacity. While nearly extinct in the wild, they are commonly maintained in captivity as companion animals, zoological display animals, and biomedical research colonies, where they serve as an animal model for genetics, developmental biology, and regenerative medicine. This report documents a case series of spontaneous myeloid leukemia in genetically related, co-housed adult axolotls in a zoological collection that resulted in 50% overall mortality over 38 months. Affected axolotls exhibited a range of signs, including generalized edema, hydrocoelom, long-term wasting, and sudden death. The most common gross lesions were splenomegaly (5/10) and hepatomegaly (4/10). Microscopically, widespread intravascular neoplasia, consisting of large round cells, was consistently observed (10/10). Neoplastic cells infiltrated and effaced the parenchyma of numerous visceral organs, particularly the spleen and liver. Cytochemical staining of neoplastic cells in blood smears showed strong positive cytoplasmic reactivity for α-naphthyl butyrate esterase and α-chloroacetate esterase and weak staining with periodic-acid Schiff. In tissues, neoplastic cells did not react with Sudan black B and did not express CD3 or ionized calcium-binding adaptor molecule 1 (IBA-1). The morphologic features of the cells and phenotyping tests supported acute myeloid leukemia. This report represents the first documentation of acute myeloid leukemia in caudates.