Gastric Amebiasis Due to Entamoeba histolytica in a Dama Wallaby (Macropus eugenii)

A 1.5-year-old captive female Dama wallaby (Macropus eugenii) died after a 3-month period of progressive weight loss, anorexia, bloat, and diarrhea. Histopathologic examination revealed numerous Ent- amoeba histolytica trophozoites within the gastric mucosa and, less frequently, gastric submucosa and submu- cosal vessels. Immunofluorescent antibody testing confirmed the identity of the trophozoites as E. histolytica. The trophozoites were associated with mild glandular epithelial necrosis, mucosal erosions, and lymphoplas- macytic inflammation. E. histolytica most commonly causes necrotizing and ulcerative colitis in humans and captive nonhuman primates, and it causes necrotizing and ulcerative gastritis in nonhuman primates with sac- culated stomachs adapted for leaf fermentation. Rare cases of gastric amebiasis also have been been reported in captive macropods, which also have complex sacculated stomachs. To our knowledge, this is the first report confirming E. histolytica as the cause of gastric amebiasis in a wallaby. The zoonotic potential of this infection in macropods is uncertain. 1,5,15 The outcome of infection can vary tremendously from an unapparent carrier state to severe clinical manifes- tations, depending on the virulence of the strain of E. his- tolytica, host species, host nutritional status, host immune status, host intestinal microflora, and the presence of con- current intestinal pathogens. 14,15 The life cycle is direct, and the typical source of infection is drinking water or food con- taminated with infectious cysts. Excystation occurs in the colon, and the excysted amebae develop into trophozoites that usually reside in the colonic lumen but can invade the mucosa. The trophozoites also can invade mural blood ves- sels and lymphatics in the colon and spread to other organs, most commonly the liver. The invasive trophozoites are of- ten associated with extensive necrosis in the colon and liver that is likely due to a combination of factors, including stim- ulation of inflammatory mediator production by host cells, neutrophil infiltration of affected tissues, cysteine protease production by the trophozoites, and induction of host cell apoptosis by the trophozoites. 14 Although E. histolytica most frequently affects humans and nonhuman primates, it also can cause necrotizing and ulcerative colitis in dogs and cats. 3,7,13,18 The source of infection for both captive nonhu- man primates and companion animals is thought to be cysts shed from a human carrier, and in at least one case an in- fected dog was observed to drink water routinely from a toilet bowl. 18 Infected nonhuman primates can shed infec- tious cysts and transmit the disease to humans, although in- fected dogs rarely shed infectious cysts and are therefore of