Introduction

Lymphosarcoma (malignant lymphoma) is defined as a lymphoid malignancy that originates from solid organs (e.g., lymph nodes, liver, and spleen) and is differentiated from lymphoid leukemia which originates in the bone¹. Among marine mammals, hematopoietic neoplasms appear to be the most common malignancy of pinnipeds.^5,6,8,9 Among cetaceans, Lymphosarcoma has been described in a killer whale (Orcinus orca),¹⁰ fin whale (Balaenoptera physalus),⁶ beluga whale (Delphinapterus leucas)³ and the Atlantic bottlenose dolphin (Tursiops truncatus).² This report describes an Atlantic bottlenose dolphin who died as the result of widely disseminated lymphosarcoma.

Case Report

This nine-year-old female dolphin had a two-year history of intermittent periods of inappetence and lethargy. In each case, the animal responded to antibiotic therapy and supportive care. Two days prior to death, the animal was anorectic and was described as being very weak and listless. Approximately 24 hours prior to death, the dolphin had a low red blood cell count (PCV 17.5%), a low white blood cell count (3.9 x 10³/mm³) comprised of 53% lymphocytes, a low hematocrit (24.4%) and an elevated erythrocyte sedimentation rate (150). Blood chemistries were within the normal range with the exception of elevations in phosphorus (7.3 mg/dl), blood urea nitrogen (93 mg/dl), and creatinine (2.2 mg/dl).

Necropsy Findings

At necropsy, the prescapular, mediastinal and mesenteric lymph nodes were reactive and hemorrhagic. Also, the mediastinal Lymph nodes were two to six times normal size. The lungs were firm and congested with multiple areas of scaring and loss of normal pulmonary parenchyma. The liver was enlarged. On histopathological examination, all lymph nodes contained neoplastic lyrnphocytes of the immunoblastic type. There were numerous foci of neoplastic cells in the lobules of the lung. The liver had numerous infiltrates of neoplastic cells located in the parenchyma of the lobules. The histological diagnosis was generalized lymphosarcoma.

Discussion

The cause of lymphosarcoma in marine mammals is unknown. Retroviruses frequently have been isolated from or associated with malignant lymphocytes of animals as diverse as fish, snakes, birds, rodents, cats, cattle and non-human primates.⁴ In the cat, approximately 70% of lymphomas are associated with feline leukemia virus viremia (FELV).¹ Epidemiologic studies in cattle indicated that the adult form of bovine lymphosarcoma, or enzootic bovine leukosis, is contagious.⁷

The most likely transmission may be insects and traumatic, surgical or other similar events that lead to blood contamination. Griner has reported on the nearly simultaneous occurrence of malignant Lymphomas in two harbor seals in one zoo exhibit which lead him to conclude that the disease might have been of viral origin.⁵

References

1.  Couto, C. G. 1992. Lymphoma in the cat and dog. In Nelson, R. W. and C. O. Couto (eds.) Essentials of Small Animal Internal Medicine. Mosby Yearbook, St. Louis, Missouri, pp. 861-870.

2.  Britt, J. O. and E. B. Howard. 1983. The hematopoietic system. In Howard, E. B. (ed.) Pathobiology of Marine Mammals, Vol. II, CRC Press Inc., Boca Raton, Florida, p. 68.

3.  DeGuise, S., A. Lagace and P. Beland. 1992. Tumors in cetaceans: Updated review with special reference to the St. Lawrence belugas. In Alexander, J. W. (ed.) Proceedings 23rd Annual IAAAM Conference, Hong Kong, pp. 115- 117.

4.  Gallo, R. C. and F. L. Meyskens. 1978. Advances in viral etiology of leukemia and Iymphoma. Semin. Hematol. 15, pp. 379-398.

5.  Griner, L. A. 1971. Malignant leukemic lymphomas in two harbor seals (Phoca vitulina geronimensis). J. Am. Vet. Res. 32, pp. 827-830.

6.  Landy, R. B. 1978. A review of neoplasia in marine mammals (pinnipedia and cetacea). In Montali, R. J. and G. Migaki (eds.) Comparative Pathology of Zoo Animals. The Symposia of the National Zoological Park, Smithsonian Institution, Washington, D. C., pp. 579-584.

7.  Miller,J.M. 993. Bovine leukemia virus. In Howard, J.L.(ed.) Current Veterinary Therapy: Food Animal Practice. W. B. Saunders Co., Philadelphia, Pennsylvania, pp. 450-451.

8.  Stedham, M. A., H. W. Casey, and M. C. Keyes. 1977. Lymphosarcoma in an infant northern fur seal (callorhinus ursinus). J. Wildlife Dis. 13, pp. 176-179.

9.  Stroud, R. K., and D. R. Stevens. 1980. Lymphosarcoma in a harbor seal (Phoca vitulina richardii). J. Wildlife Dis. 16, pp. 267-273.

10. Yonezawa, M., H. Nakamini, T. Tanaka, et.al. 1989. Hodgkin's disease in a killer whale (Orcinus orca). J. Comp. Path. 100, pp. 203-207.