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Free Living Amebic Infections
[Acanthamoeba spp.] [Balamuthia mandrillaris] [Naegleria fowleri] [Sappinia spp.]
Naegleria fowleri and Acanthamoeba spp., are commonly found in lakes, swimming pools, tap water, and heating and air conditioning units. While only one species of Naegleria, N. fowleri, is known to infect humans, several species of Acanthamoeba, including A. culbertsoni, A. polyphaga, A. castellanii, A. astronyxis, A. hatchetti, A. rhysodes, A. divionensis, A. lugdunensis, and A. lenticulata are implicated in human disease. An additional agent of human disease, Balamuthia mandrillaris, is a related free-living ameba that is morphologically similar to Acanthamoeba in tissue sections in light microscopy. Sappinia is a genus of free-living amebae rarely isolated from humans; cysts and trophs have been found in the feces of many animals, including mammals and reptiles.
Free-living amebae belonging to the genera Acanthamoeba, Balamuthia, Naegleria and Sappinia are important causes of disease in humans and animals. Naegleria fowleri produces an acute, and usually lethal, central nervous system (CNS) disease called primary amebic meningoencephalitis (PAM). Acanthamoeba spp. and Balamuthia mandrillaris are opportunistic free-living amebae capable of causing granulomatous amebic encephalitis (GAE) in individuals with compromised immune systems. Sappinia pedata has been implicated in a case of amebic encephalitis.
Acanthamoeba spp. have been found 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. Unlike N. fowleri, Acanthamoeba has only two stages, cysts and trophozoites , in its life cycle. No flagellated stage exists as part of the life cycle. The trophozoites replicate by mitosis (nuclear membrane does not remain intact) . The trophozoites are the infective forms, although 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 spp. enters the eye it can cause severe keratitis in otherwise healthy individuals, particularly contact lens users . When it enters the respiratory system or through the skin, it 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 spp. cysts and trophozoites are found in tissue.
Balamuthia mandrillaris has been isolated from the environment and has also been isolated from autopsy specimens of infected humans and animals. B. mandrillaris has only two stages, cysts and trophozoites , in its life cycle. No flagellated stage exists as part of the life cycle. The trophozoites replicate by mitosis (nuclear membrane does not remain intact) . The trophozoites are the infective forms, although both cysts and trophozoites gain entry into the body through various means. Entry can occur through the nasal passages to the lower respiratory tract , or ulcerated or broken skin . When B. mandrillaris enters the respiratory system or through the skin, it can invade the central nervous system by hematogenous dissemination causing granulomatous amebic encephalitis (GAE) or disseminated disease , or skin lesions in individuals who are immune competent as well as those with compromised immune systems. B. mandrillaris cysts and trophozoites are found in tissue.
Naegleria fowleri has three stages in its life cycle: cysts , trophozoites , and flagellated forms . The trophozoites replicate by promitosis (nuclear membrane remains intact) . N. fowleri is found in fresh water, soil, thermal discharges of power plants, geothermal wells, and poorly-chlorinated swimming pools. Trophozoites can turn into temporary non-feeding flagellated forms which usually revert back to the trophozoite stage. Trophozoites infect humans or animals by penetrating the nasal mucosa and migrating to the brain via the olfactory nerves causing primary amebic meningoencephalitis (PAM). Naegleria fowleri trophozoites are found in cerebrospinal fluid (CSF) and tissue, while flagellated forms are occasionally found in CSF. Cysts are not seen in brain tissue.
Acute primary amebic meningoencephalitis (PAM) is caused by Naegleria fowleri. It presents with severe headache and other meningeal signs, fever, vomiting, and focal neurologic deficits, and progresses rapidly (<10 days) and frequently to coma and death. Acanthamoeba spp. causes mostly subacute or chronic granulomatous amebic encephalitis (GAE), with a clinical picture of headaches, altered mental status, and focal neurologic deficit, which progresses over several weeks to death. In addition, Acanthamoeba spp. can cause granulomatous skin lesions and, more seriously, keratitis and corneal ulcers following corneal trauma or in association with contact lens use. Non-contact lens users and contact lens users with safe lens care practices can become infected. However, poor contact lens hygiene and exposure to contaminated water may increase the risk among contact lens users.