Free-Living Amebae as Opportunistic Agents of Human Disease

Members of the free-living amebic genera Acanthamoeba, Balamuthia, and Naegleria are known to cause infections of the central nervous system (CNS) of humans and other animals. Several species of Acanthamoeba cause an insidious and chronic disease, granulomatous amebic encephalitis (GAE), principally in immunocompromised hosts including persons infected with HIV/AIDS. Additionally, Acanthamoeba spp. also causes infection of the human cornea, Acanthamoeba keratitis. B. mandrillaris, the only known species of Balamuthia, causes GAE in both immunocompromised and immunocompetent hosts. Both Acanthamoeba and B. mandrillaris also cause a disseminated disease including the lungs, skin, kidneys, and uterus. N. fowleri, on the other hand, infects immunocompetent children and young adults leading to an acute and fulminating, necrotizing primary amebic meningoencephalitis. This review describes the biology of the amebae, clinical manifestations, diagnosis including molecular identification, immunological, and epidemiological features associated with the infections caused by these amebae.