Brief Summary

Free-living Acanthamoeba species are commonly found in lakes, swimming pools, tap water, and heating and air conditioning units. Several Acanthamoeba species are implicated in human disease, including A. astronyxis, A. culbertsoni, A. polyphaga, A. castellanii, A. hatchetti, A. rhysodes, A. divionensis, A. lugdunensis, and A. lenticulata (as discussed by Kong [2009], however, many aspects of species-level Acanthamoeba taxonomy are not yet resolved). These are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE), an extremely serious disease of the central nervous system, in individuals with compromised immune systems, as well as amebic keratitis, a painful eye disease that can result in blindness. Although Acanthamoeba infections are infrequent, they appear to occur worldwide (Centers for Disease Control Parasites and Health website; Marciano-Cabral and Cabral 2003 and references therein; Schuster and Visvesvara 2004 and references therein)

Acanthamoeba species are among the most prevalent protozoans in the environment. They have been found, among other locations, in soil; fresh, brackish, and sea water; sewage; swimming pools; contact lens equipment; medicinal pools; dental treatment units; dialysis machines; heating, ventilating, and air conditioning systems; mammalian cell cultures; vegetables; human nostrils and throats; and human and animal brain, skin, and lung tissues. In the laboratory, they have been isolated from emergency eye wash stations, where they pose a danger to persons with a damaged cornea (Schuster and Visvesvara 2004). In contrast to Naegleria fowleri, an amoeba that causes rare but extremely dangerous infections, Acanthamoeba life cycles have only two stages, a dormant cyst stage and an actively feeding and dividing trophozoite stage (Acanthamoeba have no flagellated stage). The trophozoites replicate by mitosis (the nuclear membrane does not remain intact). Although the trophozoites are the infective stage, both cysts and trophozoites gain entry into the body through various means. Entry can occur through the eye, the nasal passages to the lower respiratory tract, or ulcerated or broken skin. When Acanthamoeba enter the eye, severe keratitis can result in otherwise healthy individuals, particularly contact lens users (Ibrahim et al. 2007). When they enter the respiratory system or through the skin, they can invade the central nervous system by hematogenous dissemination, causing granulomatous amebic encephalitis (GAE) or disseminated disease, or skin lesions in individuals with compromised immune systems. Acanthamoeba cysts and trophozoites are found in tissue. (Centers for Disease Control Parasites and Health website; Marciano-Cabral and Cabral 2003 and references therein)

Marciano-Cabral and Cabral (2003), Schuster and Visvesvara (2004), Khan (2006), and Visvesvara et al. (2007) reviewed the biology of Acanthamoeba and the role of these organisms as human pathogens. Thomas et al. (2010) reviewed the role of free-living amoebae, particularly Acanthamoeba species, in facilitating infection of humans by various other microrganisms, as well as strategies for controlling free-living amoebae in the environment (e.g., in water supplies). Khan (2008) reviewed medical aspects of Acanthamoeba infection in humans, with a focus on understanding how these parasites manage to invade the central nervous system.

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Acanthamoeba castellanii

The protozoan Acanthamoeba castellanii is an opportunistic free-living amoeba that plays a role in the ecosystem and capable of producing serious human infections. This unicellular amoeba has been used extensively to study the capture of prey by phagocytosis, to understand the molecular biology of motility, to understand molecular mechanisms of differentiation, for its ability to act as a Trojan horse of the microbial world, including viral, bacterial, protists and fungal pathogens, and because of its ability to cause serious human diseases. The factors that contribute to Acanthamoeba infections include parasite biology, genetic diversity, environmental spread and host susceptibility. The use of Acanthamoeba in the study metabolism and morphogenesis, classification, ecology and role in the ecosystem, host-pathogen interactions, virulence factors and immunological basis of pathogenesis, clinical manifestation, diagnosis, treatment, new target development and drug resistance and its interactions with other microbes in the environment, bacterial pathogenesis and evolutionary processes makes it an attractive model organism.

  • Dr. Naveed Ahmed Khan, Aga Khan University
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