Amebae

The free-living amebae (FLA) are found in both the natural environment and in man-made facilities and may occasionally be human parasites. While many FLA have been identified, the three main genera associated with human disease are Acanthamoeba spp., Balamuthia mandrillaris and Naegleria fowleri. Infection with N. fowleri is exceedingly rare and is not common in immunocompromised hosts. Both Acanthamoeba species and B. mandrillaris cause granulomatous amebic encephalitis (GAE), primarily in immunocompromised hosts, while Acanthamoeba spp. can also cause a localized infection of the eye, referred to as amebic keratitis (AK).

The life cycle of both FLAs involves two forms: an actively dividing trophozoite stage and a dormant cyst stage. Trophozoites are uninucleate, divide through binary fission, and feed on surrounding bacteria and algae through phagocytosis or pseudopod formation. If environmental conditions become challenging (e.g., desiccation, osmolarity imbalance, nutrient deprivation, etc.), trophozoites will encyst and become resistant to most antimicrobials and biocides. Cysts remain viable for years and will yield pathogenic trophozoites. Of note, the virulence of excysted trophozoites decreases with duration of encystment. While the trophozoite stage is the infectious form, infection can also be acquired through contact with cysts which will subsequently excyst under amenable conditions. Exposure to both organisms is thought to be due to inhalation of the organism into the lower respiratory tract and/or contamination of broken skin.