Suspected Chronic Lymphocytic Leukemia (CLL) in Three Atlantic Bottlenose Dolphins (Tursiops truncatus)
Three adult female wild caught Atlantic bottlenose dolphins (Tursiops truncatus) showed repeated elevated absolute lymphocyte counts and abnormal lymphocyte microscopic morphology on routine blood work. Dolphin #1 (est. age 22 years old) and dolphin #2 (est. age 43 years old) were housed at the same facility, while dolphin #3 (est. age 35 years old) resided at a separate facility.
Peripheral blood leukocytes were subjected to analytical flow cytometry for the purpose of identifying perturbations in lymphocyte subpopulation numbers and apparent indications of clonality based upon variation in cell-surface density of leukocyte adhesion proteins and differentiation antigens. Additionally, lymphocyte morphology was evaluated on peripheral blood smears by a board certified clinical pathologist.
Dolphin #1 had the most dramatic lymphocyte profile that was considered consistent with classic, chronic lymphocytic leukemia (CLL). Chronic lymphocytic leukemia is an indolent, slowly progressive tumor of small, mature appearing lymphocytes. The animal was analyzed three times over a five year period with the number of circulating lymphocytes increasing dramatically during that time. Microscopically, lymphocyte morphology was that of small mature cells with a high nucleus to cytoplasmic ratio, condensed chromatin and inapparent nucleoli. Flow cytometry demonstrated ninety-three to ninety-seven percent of the lymphocytes expressed the leukocyte differentiation antigen CD2, which would be consistent with a T lymphocyte. Though variable percentages of natural killer (NK) cells are also capable of expressing this protein, the cellular morphology was not consistent with that of typical NK cells (e.g., large granular lymphocytes). The range in cell surface density of the CD and adhesion proteins was very small compared to normal controls, and thus consistent with monoclonality and CLL.
Dolphin #2 and dolphin #3 had lower relative lymphocyte counts compared to dolphin #1. However, lymphocyte numbers, morphology and monomorphic immunophenotype were most consistent with incipient CLL. Cell surface density of CD and adhesion proteins was consistent with a monomorphic elevated population of T lymphocytes (ninety-four and ninety-five percent CD+, respectively). Interestingly, T cells in dolphin #2 also expressed CD19 (a differentiation antigen associated with B lymphocytes) at a very high and monomorphic density; T cells of dolphin #1 and dolphin #3 did not.
While the T cell numbers were definitely elevated and carried a monomorphic appearance in dolphin #2 and dolphin #3, additional data will need to be collected over time to establish a pattern of continued increase in T cell number and maintenance of the apparent monoclonality. This temporal analysis, combined with a recently initiated molecular approach to unequivocally establish monoclonality by analyzing T cell receptor gene rearrangements may ultimately confirm these cases as CLL. Dolphins #2 and #3 may simply represent an earlier form of CLL compared to dolphin #1. Acquisition of bone marrow and spleen samples pre-mortem would also support a definitive diagnosis. Interestingly, all three (probable) CLLs described here are of T cell origin, which is in marked contrast to humans where CLL is almost exclusively a B cell disease. CLL in dogs and cats is also mostly a T cell tumor.
The authors wish to thank the staff of Atlantis Paradise Island, Theater of the Sea, and UC Davis, School of Veterinary Medicine for their support.