ACANTHAMOEBA: The free-living amoeba that can infect humans.
Acanthamoeba is a microscopic, free-living amoeba found throughout the environment, including soil, dust, freshwater, seawater, swimming pools, air-conditioning systems, and domestic water supplies. Unlike parasites that require a host to complete their life cycle, Acanthamoeba can survive and reproduce independently in the environment. Most environmental exposure does not result in disease, but certain species and genotypes can cause serious human infections ranging from destructive corneal disease to skin infection and potentially fatal disease involving the brain and other organs.
Acanthamoeba is an environmental organism, not an obligate human parasite: The organism belongs to a group known as free-living amoebae. It normally exists independently of humans and animals, feeding on microorganisms and organic material in its environment. Human infection occurs when an environmental strain gains access to susceptible tissue or, much more rarely, establishes invasive infection and disseminates through the body. This distinction is important because detecting Acanthamoeba in the environment does not by itself indicate human disease.
The two-stage life cycle allows Acanthamoeba to survive harsh conditions: Acanthamoeba exists principally as an active trophozoite and a dormant cyst. Trophozoites are metabolically active, feed, move, and reproduce by binary fission. When environmental conditions become unfavourable, the organism can transform into a highly resistant cyst. The cyst has a protective wall that enables persistence under conditions that would destroy the more fragile trophozoite. When conditions again become favourable, cysts can exist and produce metabolically active trophozoites. This cyst-forming ability is one reason Acanthamoeba can persist in diverse environmental reservoirs.
Environmental exposure is extremely common: Acanthamoeba is widespread rather than geographically restricted. It has been detected in soil, natural water, swimming pools, household water and other environmental sources. A CDC investigation notes that Acanthamoeba has been found in a substantial proportion of sampled household water supplies, while invasive human disease remains rare. Therefore, exposure and infection are very different concepts.
Acanthamoeba can enter through the eye: The best-known human infection is Acanthamoeba keratitis, an infection of the cornea. The organism can gain access to the corneal surface and, particularly when the protective epithelial barrier has been compromised, establish infection within corneal tissue. The condition is strongly associated with contact-lens use, especially when lenses are exposed to contaminated water or handled with inadequate hygiene.
