Toxoplasmosis is an infectious disease caused by the protozoan Toxoplasma gondii. The definitive hosts for this organism are captive and wild felids. A variety of mammals and birds can serve as intermediate hosts. In these intermediate hosts, infection is usually benign. One marine mammal species, the sea otter, has been well-described with fatal disease associated with Toxoplasma infection. Numerous reports of fatal disease in a variety of cetacean species suggest that this infection should be considered a significant cause of morbidity and mortality in cetaceans.
A case of fatal disseminated toxoplasmosis was diagnosed in a rough-toothed dolphin (Steno bredanensis). The animal had been cared for in rehabilitation for a period of six months. Over the course of one week, the animal's condition deteriorated and the animal died. On gross examination, the animal was in good nutritional condition with generalized mild lymph node enlargement. Histologic examination revealed a severe multifocal interstitial pneumonia, mild, multifocal interstitial nephritis, a mild multifocal myocarditis, and foci of both non-suppurative and necrotizing encephalitis. All of these lesions were associated with protozoal cysts or zoites. Immunohistochemistry confirmed the organisms as Toxoplasma gondii. Lymph nodes and tonsil demonstrated moderate lymphoid depletion and necrosis.
Fatal toxoplasmosis has been described in a Risso's dolphin (Grampus griseus), Atlantic bottlenose dolphins (Tursiops truncatus), an Indo-Pacific bottlenose dolphin (Tursiops aduncus), Indo-pacific humpbacked dolphins (Sousa chinansis), a spinner dolphin (Stenella longirostris), striped dolphins (Stenella coeruleoalba), and in beluga whales (Delphinapterus leucas). These infections have occurred both with and without signs of immunosuppression in these animals. Lesions in the affected species vary. Primary target organs include brain, lungs, liver, kidney, adrenal gland, and heart. Inflammation varies from granulomatous to acute and necrotizing. Toxoplasma cysts and /or tachyzoites can be identified in affected tissues on hematoxylin and eosin staining or via immunohistochemistry. Fetal infections have been described in a Risso's dolphin and an Indo-Pacific bottlenose dolphin. The Risso's dolphin demonstrated acute renal, pulmonary, and cardiac necrosis. The Pacific bottlenose dolphin demonstrated nonsuppurative inflammation in the heart and brain and areas of myocardial necrosis.