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The first known isolate of this amoeba was from a Mandrill Baboon (Papio sphinx) at San Diego Wild Animal Park (Visvesvara et al, 1990).  Balamuthia mandrillaris (presently the only described species  in the genus) was soon recognised as causing encephalitis in humans  (Anzil et al, 1991). Since then more than 80-90 incidents of granulomatous amoebic encephalitis (GAE) caused by Balamuthia have been reported (Schuster et al, 2001; Schuster et al, 2003), with only two cases that have survived (Deetz et al, 2003) to date. There is of course a strong suspicion that infection and probably all amoebal infections are grossly underestimated due to the general similarity of the symptoms caused by other agents (bacterial, viral) and the general lack of awareness of amoebal pathogenicity.   

Cell Biology
Trophozoites of Balamuthia are characterised by a peculiar ribbon-like endoplasmic reticulum. The nucleolus is very dense and it seems split in two, a feature that usefully distinguish this amoeba from Acanthamoeba especially when encountered in histological section. The nucleus appears to posses peculiar cytoplasmic vesicles (
Martinez et al, 2001).  The amoeba produces a  cyst, but no flagellate stage has been described.  The cyst bears a superficial resemblance to Acanthamoeba being about 15 mm (microns) in diameter but E.M. reveals that it has a triple wall (Schuster et al, 2003), this however often looks like a double wall by light microscope, again like Acanthamoeba.  The trophozoite is around 15-60 mm (microns) in diameter and may be bi-nucleate (Visvesvara et al, 1993).  Unlike most amoebae the nuclear envelope breaks down during mitosis.  The cells extend a broad, flat lamellipodia but this sometimes has sub-pseudopodia extending from it (Visvesvara et al, 1993).  The amoeba is described as "walking" by crab-like movement by arm like trunk and pseudopods and at ~0.15 mm/sec (Schuster et al, 2003), locomotion is slow compared to other amoeba. Most of the body of the amoeba appears to be held off the surface producing bright phase rings when viewed with phase-contrast microscopy.

This genus has been described as being a Leptomyxid amoeba (
Visvesvara et al, 1990) probably as the arm-like pseudopods give it this appearance.  This label stuck for a while but it is clear that this is not valid. According to SSUrDNA analysis (Sogin et al, 1996), is related to Endolimax nana, and more distantly to the Entamoebidae. However more recent analysis with other amoebal groups showed that Balamuthia mandrillaris groups with Acanthamoeba castellanii (Amaral Zettler et al, 2000; Peglar et al, 2003).  Balamuthia mandrillaris shows little genetic variation between different isolates tested (Booten et al, 2003a). 

Distribution in nature
Although it is often described as being a free-living amoeba (Bakardjiev et al, 2003; Rideout et al, 1997) only recently has the amoeba been isolated from the environment (Schuster et al, 2003).  Prior to this study the amoeba had only been detected in the bodies of infected human or animal patients  (Schuster, 2002).  Other amoebae  that cause human disease such as Naegleria and Acanthamoeba that are to be found in the general environment.  The sporadic appearance of Balamuthia in patients does seem to indicate that the amoeba is generally present in the human environment, as does the fact that human populations carry antibodies against the amoeba (Huang et al, 1999) indicating perhaps that people come across Balamuthia antigens naturally.  It is also possible that these antibodies are actually produced against other amoeba, and merely cross react with Balamuthia.  Antibodies raised against Balamuthia however, did not react to Acanthamoeba or Naegleria (other amoeba with a potential for human pathogenesis) (Huang et al, 1999), this is surprising if the recent study showing a kinship between Acanthamoeba and Balamuthia is correct (Amaral Zettler et al, 2000; Peglar et al, 2003).  Perhaps diverse members of the genus Acanthamoeba should be tested with the anti-sera.  Balamuthia has been detected pathologically from all over the world but from warmer countries in particular.  No underlying source of infection is suspected yet such as water sport is for Naegleria fowleri infection.



There are in the region of 100 published cases of amoebic encephalitis caused by Balamuthia (
Deetz et al, 2003).  Balamuthia presents with a wide range of symptoms, and too few cases have been studied to give any clear picture.  If the patient seeks medical attention early in the course, skin lesions are frequently described especially it seems in young patients (Taratuto et al, 1991; Reed et al, 1997; Galarza et al, 2002), although these have been reported in adults (Deol et al, 2000). Many patients present with seizures or focal paralysis resulting from infection already present in the brain.  In other cases Balamuthia produced symptoms that mimicked a brain stem glioma (Lowichik et al, 1995).  Very low CSF glucose levels have been recorded in one case at least (Katz et al, 2000).  Many animals have also been found to have become infected with Balamuthia including,  horse (Kinde et al, 1998), gorilla and other primates (Rideout et al, 1997).

Diagnosis and Identification
Amoeba in general tend to be easily identified either as trophozoites because of the large centrally located nucleolus or their cysts in tissue sections.  Often the nucleolus appears to be almost split in two in Balamuthia (
Martinez & Visvesvara 1991). Large numbers of amoeba are also seen in the CSF with Naegleria and Acanthamoeba although not with Balamuthia (Martinez & Visvesvara 2001).  Anti-sera have been used to test for the presence of Balamuthia (Huang et al, 1999; Deol et al, 2000; Shirabe et al, 2002) and more modern (and reliable) techniques such as specific PCR probes have also been developed (Booton et al, 2003b).  Contrast enhanced computed tomography (CT) scans is very useful in diagnosis of encephalitis however changes in the patterns result during the course of the disease exist which should be expected (Hsu et al, 2003).

Treatment is uncertain as would be expected for such a comparatively rarely encountered pathogen. To date only two patients are known to have survived infection with confirmed Balamuthia (Bakardjiev et al, 2003). A patient is reported to have been treated successfully a patient diagnosed with Balamuthia encephalitis (at Santa Cruz Medical Clinic under the care of Dr T. Deetz) with the drugs:-  Clarithromycin 500 mg 3 times daily, flucanozole 400 mg daily, sulphadiazine 1.58g every 6 hours and 5-fluorocytosine 1.5g every 6 hours. It was noted however that the patient was left with neurological deficits (Martinez & Visvesvara 2001).  The full details of two cases in which patient survived Balamuthia encephalitis have now been published, both received a cocktail of antibiotics (Deetz et al, 2003).  However, some of the antibiotics generally used to treat this disease (5-fluorocytosine, flucanozole and sulphadiazine) were found to have no effect on Balamuthia, in vitro (Schuster et al, 2003).

Isolation & Cultivation
The amoeba has only been isolated once from the environment (
Schuster et al, 2003).  The Balamuthia amoeba appeared long after other amoeba present.  Unlike most amoeba of this size, Balamuthia does not seem to feed on bacteria but feeds on other amoeba including Acanthamoeba and Naegleria (Schuster et al, 2003). An axenic media has been formulated for Balamuthia (Schuster & Visvesvara, 1996).


Described species:-

Balamuthia mandrillaris (Visvesvara et al, 1993), 



Amaral Zettler, L.A., Nerad, T.A., O'Kelly, C.J., Peglar, M.T., Gillevet, P.M., Silberman, J.D. & Sogin, M.L. (2000). A molecular reassessment of the Leptomyxid amoebae. Protist 151, 275-282.

Anzil, A. P., Rao, C., Wrzolek, M. A., Visvesvara, G. S., Sher, J. H. & Kozlowski, P. B. (1991) Amebic meningoencephalitis in a patient with AIDS caused by a newly recognised opportunistic pathogen: Leptomyxid ameba. Arch Pathol. Lab.Med. 115, 21-25.

Bakardjiev, A., Glaser, C., Schuster, F. L. & Visvesvara, G. S. (2002) Three-year-old girl with fever and coma. Pediatr. Infect. Dis. 21, 75.

Bakardjiev, A., Azimi, P. H., Ashouri, N., Ascher, D. P., Janner, D., Schuster, F. L., Visvesvara, G. S. & Glaser, C. (2003) Amebic encephalitis caused by Balamuthia mandrillaris: reports of four cases. Pediatr.Infect.Dis.J. 22, 447-452.

Booton, G.C., Carmichael, J. R., Visvesvara, G. S., Byers, T. J. & Fuerst, P. A. (2003a) Genotyping of Balamuthia mandrillaris based on nuclear 18S and mitochondrial 16S rRNA genes. Am.J.Trop.Med.Hyg. 68(1), 65-69.

Booton, G. C., Carmichael, J. R., Visvesvara, G. S., Byers, T. J. & Fuerst, P. A. (2003b) Identification of Balamuthia mandrillaris by PCR Assay Using the Mitochondrial 16S rRNA Gene as a Target. J. Clin. Microbiol. 41, 453-455.

Deetz TR, Sawyer MH, Billman G, Schuster FL, Visvesvara GS. 2003. Successful treatment of Balamuthia amoebic encephalitis: presentation of 2 cases. Clin. Infect. Dis. 37:13004-13012.

Deol, I., Robledo, L., Meza, A., Visvesvara, G. S. & Andrews, R. J. (2000) Encephalitis due to a free-living amoeba (Balamuthia mandrillaris): Case report with literature review. Surgical Neurology. 53, 611-616.

Galarza, M., Cuccia, V., Sosa , F. P. & Monges, J. A. (2002) Pediatric granulomatous cerebral amebiasis: a delayed diagnosis. Pediatr Neurol. 26, 153-156.

Hsu, H. L., Chen, C. J., Huang, C. C., Jung, S. M. & Chen, R. S. (2003) The evolution of contrast enhancement in granulomatous amebic encephalitis., Eur.J.Radiol.Extra. 45, 47-50.

Huang, Z.-H., Ferrante, A. & Carter, RF. (1999). Serum antibodies to Balamuthia mandrillaris, a free-living amoeba recently demonstrated to cause Granulatous Amoebic Encephalitis. J.Infect.Disease 179: 1305-1308.

Katz, J. D., Ropper, A. H., Adelman, L., Worthington, M. & Wade, P. (2000) A case of Balamuthia mandrillaris meningoencephalitis. Archives of Neurology. 57, 1210-1212.

Kinde, H., Visvesvara, G. S., Barr, B. C., Nordhausen, R. W. & Chiu, P. H. (1998) Amebic meningoencephalitis caused by Balamuthia mandrillaris (leptomyxid ameba) in a horse. J Vet Diagn Invest. 10, 378-381.

Kodet, R., Nohynkova, E., Tichy, M., Soukup, J. & Visvesvara, G. S. (1998) Amebic encephalitis caused by Balamuthia mandrillaris in a Czech child: description of the first case from Europe. Pathol Res Pract. 194, 423-429.

Lowichik, A., Rollons, N., Delgado, R., Visvesvera, G. S. & Burns, D. K. (1995) Lepomyxid amebic meningoencephalitis mimicking brain stem glioma. Amer.J.Neur.Radiol. 16, 926-929.

Martinez, A.J., Schuster, F.L. & Visvesara, G.S. (2001) Balamuthia mandrillaris; Its pathogenic potential. J.Euk.Microbiol. 6S-9S.

Martinez, A. J. & Visvesvara, G. S. (1997) Laboratory diagnosis of pathogenic free-living amoebas: Naegleria, Acanthamoeba, and LeptomyxidClin.Lab.Med. 11, 861-872.

Martinez, A. J. & Visvesvara, G. S. (2001) Balamuthia mandrillaris infection. J Med Microbiol. 50, 205-207.

Reed, R., Cooke-Yarborough, C.M., Jaquiery, A., Grimwood, K., Kemp, A.S., Su, J.C. & Forsyth, R.L. (1997)  Notable cases: fatal granulomatus amoebic encephalitis caused by Balamuthia mandrillaris. Med.J.Aust. 167: 82-84.

Rideout, B. A., Gardiner, C. H., Stalis, I. H., Zuba, J. R., Hadfield, T. & Visvesvara, G. S. (1997) Fatal infections with Balamuthia madrillaris (a free-living amoeba) in gorillas and other old world primates. Vet. Pathol. 34, 15-22.

Schuster, F. L. & Visvesvara, G. S. (1996) Axenic growth and drug sensitivity studies of Balamuthia mandrillaris, an agent of amebic meningoencephalitis in human and other animals. J.Clin. Microbiol. 34, 385-388.

Schuster, F. L. & Visversvara, G. S. (1998) Efficacy of novel antimicrobials against clinical isolates of opportunistic amebas. J.Euk.Microbiol. 45, 612-618.

Schuster, F.L., Glaser, C., Gilliam, S., & Visvesvara, G.S. (2001) Survey of sera from encephalitis patients for Balamuthia mandrillaris antibody. J.Euk.Microbiol.

Schuster, F. L. (2002) Cultivation of pathogenic and opportunistic free-living amebas. Clin.Microbiol.Rev. 15, 342-354.

Schuster, F. L., Dunnebacke, T. H., Booton, G. C., Yagi, S., Kohlmeier, C. K., Glaser, C., Vugia, D., Bakardjiev, A., Azimi, P., Maddux-Gonzalez, M., Martinez, A. J. & Visvesvara, G. S. (2003) Environmental Isolation of Balamuthia mandrillaris Associated with a Case of Amebic Encephalitis. J. Clin. Microbiol. 41, 3175-3180.

Shirabe, T., Monobe, Y. & Visvesvara, G. S. (2002) An autopsy case of amebic meningoencephalitis. The first Japanese case caused by Balamuthia mandrillarisNeuropathology. 22, 213-217.

Sogin, M. L., Silberman, J.D, Hinkle, G. & Morrison, H.G. (1996). "Problems with molecular diversity in the eukarya.".Society of General Microbiology Symposium: Evolution of microbial Life ed.Roberts, D.M., Sharp, P., Alderson, G. & Collins, M.A. Cambridge University Press. pp167-184.

Taratuto, A. L., Monges, J., Acefe, J. C., Meli, F., Paredes, A. & Martinez, A. J. (1991) Leptomyxid ameba encephalitis: report of the first case in Argentina. Trans. R. Soc.Trop.Med.Hyg. 85, 77.

Visvesvara, G. S., Martinez, A., Schuster, F. L., Leitch, G. J., Wallace, S. V., Sawyer, T. K. & Anderson, M. (1990) Leptomyxid ameba, a new agent of amebic meningoencephalitis in humans and animals. J.Clin. Microbiol. 28, 2750-2756.

Visvesara, G.S., Schuster, F.L. & Martinez, A.J. (1993). Balamuthia mandrillaris, N.G., N.Sp., agent of amebic meningoencephalitis in humans and other animals.  J.Euk.Microbiol. 40:504-514.

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