Answer: Acanthamoeba species. The arrow points to 4 classic cysts with 2 walls each - a wrinkled outer wall (exocyst) and inner star-shaped or polygonal wall (endocyst). The trophozoites contain a single nucleus with a large karyosome (arrow head). Acanthamoeba spp. and Balamuthia mandrillaris can both show cysts and trophozoites in human infection, but the cysts of B. mandrillaris have a smooth, thick outer wall, unlike the wrinkled or stellate cyst walls of Acanthamoeba. Both are free-living amebae that can rarely cause chronic granulomatous encephalitis in immunocompromised patients.
A closely related free-living ameba, Naegleria fowleri, can also infect the human central nervous system, but typically causes a rapidly progressive meningoencephalitis. The typical host is a previously healthy young adult or child. In human disease, only trophozoites are seen. The cysts are not seen in human infection. Naegleria differs from the other two amebae in that it also has a flagellated form in the free-living state. This can be observed in amebic culture by adding tap water. Within hours, the trophozoites convert to flagellates and start swimming!
Thank you for all of your responses to this case. I can tell that I have a group of excellent parasitologists!