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Free Living Amebic Infections

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

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. 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 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 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.

Acanthamoeba spp. cysts.

 

The cysts of Acanthamoeba spp. are typically 10-25 µm in diameter. The cysts have two walls: a wrinkled fibrous outer wall (exocyst) and an inner wall (endocyst) that may be hexagonal, spherical, star-shaped or polygonal. Cysts contain only one nucleus with a large karyosome. Cysts may be found in the brain, eyes, skin, lungs and other organs.

Figure A: Cysts of Acanthamoeba spp. in culture.

Figure B: Cysts of Acanthamoeba spp. in culture.

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

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

Figure E: Cysts of Acanthamoeba sp. (green arrows) in tissue, stained with H&E.

Acanthamoeba spp. trophozoites.

 

Trophozoites of Acanthamoeba spp. are pleomorphic and measure approximately 15-45 µm. They often produce many spine-like processes called acanthapodia. Trophozoites contain a large nucleus with a large, centrally-located karyosome but no peripheral chromatin. There is no flagellated trophozoite stage in Acanthamoeba spp.

Figure A: Trophozoite of Acanthamoeba sp. from culture. Notice the slender, spine-like acanthapodia.

Figure B: Trophozoites of Acanthamoeba sp. from culture. Notice the slender, spine-like acanthapodia.

Figure C: Trophozoite of Acanthamoeba sp. in tissue, stained with hematoxylin and eosin (H&E).

Figure D: Trophozoites of Acanthamoeba sp. in a corneal scraping, stained with H&E.

Balamuthia mandrillaris cysts.

 

The cysts of Balamuthia mandrillaris are typically 10-25 µm in diameter. They cysts have two walls: a wrinkled fibrous outer wall (exocyst) and an inner wall (endocyst) that may be hexagonal, spherical, star-shaped or polygonal. Cysts contain only one nucleus with a large karyosome. Cysts may be found in the brain, eyes, skin, lungs and other organs.

Figure A: Cysts of B. mandrillaris.

Figure B: Close-up of one of the cysts in Figure A.

Figure C: Cyst of B. mandrillaris.

Figure D: Cyst of B. mandrillaris in brain tissue, stained with hematoxylin and eosin (H&E).

Figure E: Cyst of B. mandrillaris in brain tissue, stained with hematoxylin and eosin (H&E).

Figure F: Cyst of B. mandrillaris in brain tissue, stained with H&E. Image courtesy of the University of Kentucky Hospital, Lexington, Kentucky.

Figure G: Cyst of B. mandrillaris in brain tissue, stained with H&E. Image courtesy of the University of Kentucky Hospital, Lexington, Kentucky.

Figure H: Cysts of B. mandrillaris in brain tissue, stained with H&E. Image courtesy of Cook Children’s Hospital, Fort Worth, Texas.

Figure I: Cyst of B. mandrillaris in brain tissue, stained with H&E. Image courtesy of Cook Children’s Hospital, Fort Worth, Texas.

Balamuthia mandrillaris trophozoites.

 

Trophozoites of Balamuthia mandrillaris are pleomorphic and measure approximately 15-60 µm. They often produce long, slender pseudopodia. Trophozoites contain a large nucleus with a large, centrally-located karyosome but no peripheral chromatin. There is no flagellated trophozoite stage as in Naegleria spp.

Figure A: Trophozoite of B. mandrillaris in culture.

Figure B: Trophozoite of B. mandrillaris in culture.

Figure C: Trophozoite of B. mandrillaris in culture.

Figure D: Trophozoite of B. mandrillaris in culture.

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

Figure G: A single trophozoite (black arrow) of B. mandrillaris in brain tissue, stained with H&E.

Figure F: A single trophozoite (green arrow) of B. mandrillaris in brain tissue, stained with H&E.

Naegleria fowleri cysts.

 

Naegleria fowleri does not form cysts in human tissue. Cysts in the environment and culture are spherical, 7-15 µm in diameter and have a smooth, single-layered wall. Cysts have a single nucleus.

Figure A: Cyst of N. fowleri in culture.

Naegleria fowleri trophozoites.

 

There are two forms of trophozoites in Naegleria fowleri: ameboid and ameboflagellate, only the former of which is found in humans. The ameboid trophozoites measure 10-35 µm but when rounded are usually 10-15 µm in diameter. In culture, trophozoites may get over 40 µm. The cytoplasm is granular and contains many vacuoles. The single nucleus is large and has a large, dense karyosome and lacks peripheral chromatin.

Figure A: Trophozoite of N. fowleri in culture.

Figure B: Trophozoites of N. fowleri in culture.

Figure C: Ameboflagellate trophozoite of N. fowleri.

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

Figure E: Trophozoite of N. fowleri in CSF, stained with trichrome. Image courtesy of the Texas State Health Department.

Sappinia spp. cysts and trophozoites.

 

Sappinia is a genus of free-living amebae rarely isolated from human specimens. The genus is found worldwide and has been isolated in the feces of many animals, including mammals and reptiles. Cysts and trophozoites both possess two nuclei.

Figure A: Cyst of Sappinia sp. in culture, viewed under differential interference contrast (DIC) microscopy.

Figure B: Cysts of Sappinia sp. in culture, viewed under differential interference contrast (DIC) microscopy.

Figure C: Trophozoite of Sappinia sp. in culture, viewed under DIC microscopy.

Figure D: Trophozoite of Sappinia sp. in culture, viewed under DIC microscopy.

Figure E: Trophozoite of Sappinia sp. viewed under DIC microscopy.

Figure F: Four trophozoites (yellow arrows) of S. pedata in brain tissue, stained with hematoxylin and eosin (H&E). In three of the amebae, the two nuclei can easily be seen.

Indirect immunofluorescence (IIF) assay for free-living amebic infections.

 

Indirect Immunofluorescence (IIF) assay for the detection of free-living amebic infections.

Figure A: IIF of Acanthamoeba sp. viewed under UV microscopy. This image was taken at 400x magnification.

Figure B: IIF of Acanthamoeba sp. viewed under UV microscopy. This image was taken at 1000x oil magnification.

Figure C: IIF of Balamuthia mandrillaris in brain tissue, viewed under UV microscopy.

Figure D: IIF of Naegleria fowleri in brain tissue, viewed under UV microscopy. This image was taken at 200x magnification.

Figure E: IIF of Naegleria fowleri in brain tissue, viewed under UV microscopy. This image was taken at 1000x oil magnification.

Gross pathology images in free-living amebic infections.

 

Gross tissue and pathology specimens from free-living amebic infections.

Figure A: Gross specimen of brain tissue from a patient who died of granulomatous amebic encephalitis (GAE) caused by Balamuthia mandrillaris. The autopsy specimen revealed extensive necrotizing (mixed inflammatory, occasional giant cells, vasculitic) granulomatous encephalitis with a subependymal necroinflammatory process. Image courtesy of Cook Children’s Hospital, Fort Worth, Texas.

Figure B: Gross specimen of brain tissue from a patient who died of granulomatous amebic encephalitis (GAE) caused by Balamuthia mandrillaris. The autopsy specimen revealed extensive necrotizing (mixed inflammatory, occasional giant cells, vasculitic) granulomatous encephalitis with a subependymal necroinflammatory process. Image courtesy of Cook Children’s Hospital, Fort Worth, Texas.

Diagnostic Findings

In Naegleria infections, the diagnosis can be made by microscopic examination of cerebrospinal fluid (CSF). A wet mount may detect motile trophozoites, and a Giemsa-stained smear will show trophozoites with typical morphology. In Acanthamoeba infections, the diagnosis can be made from microscopic examination of stained smears of biopsy specimens (brain tissue, skin, cornea) or of corneal scrapings, which may detect trophozoites and cysts. Confocal microscopy or cultivation of the causal organism, and its identification by direct immunofluorescent antibody, may also prove useful. An increasing number of PCR-based techniques (conventional and real-time PCR) have been described for detection and identification of free-living amebic infections in the clinical samples listed above. Such techniques may be available in selected reference diagnostic laboratories.

Real-Time PCR

A real-time PCR was developed at CDC for identification of Acanthamoeba spp., Naegleria fowleri, and Balamuthia mandrillaris in clinical samples.1  This assay uses distinct primers and TaqMan probes for the simultaneous identification of these three parasites.

More on: TaqMan real-time PCR

References:
  1. Qvarnstrom Y, Visvesvara GS, Sriram R, da Silva AJ. Multiplex real-time PCR assay for simultaneous detection of Acanthamoeba spp., Balamuthia mandrillaris, and Naegleria fowleri. J Clin Microbiol 2006;44(10):3589-3595.

Treatment Information

Treatment information for acanthamoeba can be found at: https://www.cdc.gov/parasites/acanthamoeba/health_professionals/index.html

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

  • Page last reviewed: December 9, 2017
  • Page last updated: December 9, 2017
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