A terminally ill HIV infected patient develops focal neurologic signs, dementia, and coma. Amoebic parasites are demonstrated in CSF. Which of the following organisms is most likely to be the causative agent?

Two types of free-living amoeba can infect the brain and meninges: Naegleria fowleri and Acanthamoeba species. The former affects healthy adolescent or adult divers, while the latter causes infection in patients with immunosuppression because of diabetes, alcoholism, cancer, or HIV infection. The brain infection characteristically has a prominent perivascular character, which causes a multifocal hemorrhagic necrotizing meningoencephalitis. Skin ulcers, nasal infection, or pneumonia may also be present. It is thought that the organisms may release a toxin causing host tissue necrosis. Systemic antifungal agents (e.g., amphotericin) have some activity against this organism, but most cases are fatal. Entamoeba histolytica causes amoebic dysentery and liver abscess. Giardia lamblia is a flagellate, rather than an amoeba, and causes diarrhea. Naegleria fowleri is an amoebic cause of meningoencephalitis in previously healthy swimmers and divers. Ref: Ray C.G., Ryan K.J. (2010). Chapter 51. Rhizopods. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.