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DPDx

DPDx is an education resource designed for health professionals and laboratory scientists. For an overview including prevention and control visit www.cdc.gov/parasites/.

Free Living Amebic Infections

[Acanthamoeba spp.] [Balamuthia mandrillaris] [Naegleria fowleri] [Sappinia spp.]

Cyst of Acanthamoeba sp. from brain tissue, stained with hematoxylin and eosin (H&E).

Cyst of Acanthamoeba sp. from brain tissue, stained with hematoxylin and eosin (H&E).


Several trophozoites of B. mandrillaris in brain tissue, stained with hematoxylin and eosin (H&E).

Several trophozoites of B. mandrillaris in brain tissue, stained with hematoxylin and eosin (H&E).


Trophozoite of N. fowleri in CSF, stained with hematoxylin and eosin (H&E).

Trophozoite of N. fowleri in CSF, stained with hematoxylin and eosin (H&E).

Causal Agents

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.

Life Cycles

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.

Life cycle of Acanthamoeba spp.

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 The number 1 and trophozoites The number 2, 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 number 3. The trophozoites are the infective forms, although both cysts and trophozoites gain entry into the body The number 4 through various means. Entry can occur through the eye The number 5, the nasal passages to the lower respiratory tract The number 6, or ulcerated or broken skin The number 7. When Acanthamoeba spp. enters the eye it can cause severe keratitis in otherwise healthy individuals, particularly contact lens users The number 8. 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) The number 9 or disseminated disease The number 10, or skin lesions The number 11 in individuals with compromised immune systems. Acanthamoeba spp. cysts and trophozoites are found in tissue.

Balamuthia mandrillaris

Life cycle of Balamuthia mandrillaris

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 The number 1 and trophozoites The number 2, 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 number 3. The trophozoites are the infective forms, although both cysts and trophozoites gain entry into the body The number 4 through various means. Entry can occur through the nasal passages to the lower respiratory tract The number 5, or ulcerated or broken skin The number 6. 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) The number 7 or disseminated disease The number 8, or skin lesions The number 9 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

Life cycle of Naegleria fowleri

Naegleria fowleri has three stages in its life cycle: cysts The number 1, trophozoites The number 2, and flagellated forms The number 3. The trophozoites replicate by promitosis (nuclear membrane remains intact) The number 4. 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 The number 5 and migrating to the brain The number 6 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.

Geographic Distribution

While infrequent, infections appear to occur worldwide.

Clinical Presentation

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.

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  • Page last reviewed November 29, 2013
  • Page last updated November 29, 2013
  • Content source: Global Health - Division of Parasitic Diseases and Malaria
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