A microsporan Protozoa tentatively identified as Enterocytozoon bieneusi has been obtained from a single, ill, offshore specimen of the Atlantic bottlenose dolphin, Tursiops truncatus. This case constitutes the first report of any microsporan from a marine mammal. The pathogenicity of this parasite in cetaceans is unknown.

Fresh and formalin-fixed fecal specimens were stained with trichrome, acid fast and chromotrope stains. Three spores, measuring 1.0-1.5 microns, were observed in the fecal material and stained pinkish red with chromotrope stain. Additional samples were then subjected to polymerase chain reaction (PCR) assay. Results showed a positive pattern with PCR primers for the human microsporidia pathogen, Enterocytozoon bieneusi, which primarily, but not exclusively, infects immunocompromised patients. Histological sections from a necrotic area of the small intestine confirmed the presence of microsporan spores.

Within 3 weeks of stranding, the young male dolphin developed a chronic intermittent often mucoid diarrhea, marked flatulence with rare episodes of eructation, rare to occasional vomiting, brief transient abdominal pain, excess positive buoyancy, variable episodes of lethargy and activity, and generally good to excellent appetite. Fecal examination was characterized by intermittent mild inflammation and occasional intact erythrocytes. Immunological testing and histopathological studies indicated that the dolphin was immunocompromised. Clinical signs persisted over a three-month period until the dolphin's condition took a dramatic downward trend, and he had to be euthanized. A severe, chronic-active enterocolitis was diagnosed histologically post mortem. Interestingly, the dolphin's ESR remained normal (1-15mm/hr) throughout this case. Antiprotozoal drug therapy regimens included metronidazole, clioquinol, paromomycin, iodoquinol, ofloxacin, metronidazole/ofloxacin, and iodoquinol/ofloxacin. These drugs provided, at best, transient clinical improvement, but did not eliminate the organism or the condition.

Pathological alterations of the intestine were not proven to have a microsporan etiology, even though the dolphin's clinical signs corresponded to similar signs observed in AIDS patients with infections of E. bieneusi. Perhaps the spores came from an ingested, infected fish or other source. Enterocytozoon salmonis, the only other species reported in this family, occurs in Chinook salmon, primarily within mononuclear leukocytes. Further research on dolphins should establish relationships, if they exist, between infection of E. bieneusi and both health status of the host and inshore and offshore habitat of the host.