Balamuthia mandrillaris

Balamuthia mandrillaris is a free-living leptomyxid amoeba which is known to cause disease in humans, especially the deadly neurological condition known as primary amoebic meningoencephalitis. Balamuthia has not been definitively isolated in nature, but it is believed to be distributed throughout the temperate regions of the world. This is supported somewhat by the presence of antibodies to Balamuthia present in healthy individuals.

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Morphology

Balamuthia mandrillaris is a free-living, heterotrophic amoeba, consisting of a standard complement of organelles surrounded by a three-layered cell wall, and with an abnormally large, vesicular nucleus. On average, a Balamuthia trophozoite is approximately 30-120 µm across. The cysts fall approximately in this range as well. ^[1]

Life Cycle

Balamuthia's life cycle consists of a cystic stage and a trophozoite stage, both of which are infectious, and both of which can be identified as inclusions into the brain tissue on microscopic examination of brain biopsies performed on infected individuals.

Pathology

Balamuthia mandrillaris enters the body through the lower respiratory tract or through open wounds. Upon introduction, the amoebas may form a skin lesion, or migrate to the brain. Once in the brain, Balamuthia causes a condition known as granulomatous amoebic encephalitis (GAE), which is usually fatal. The symptoms of infection by Balamuthia are unclear, as a very few patients have presented with the disease thus far. Balamuthia-induced GAE can cause focal paralysis, seizures, and brainstem symptoms such as facial paralysis, difficulty swallowing, and double vision.

Balamuthia is also known to cause a variety of non-neurological symptoms, and often causes skin lesions, through which the amoeba may enter the bloodstream and migrate to the brain. Many patients experiencing this particular syndrome report a skin lesion (sometimes similar to those caused by Staphylococcus aureus or other bacteria), which does not respond well to dermatologic treatment. The lesion is usually localised and very slow to heal, if at all. In some presentations, the lesion may be mistaken for certain forms of skin cancer. Balamuthia lesions on the face may also lead to amebic keratitis, and usually results in facial swelling.

Balamuthia encephalitis is an extremely deadly disease, and as of 2003, only two recoveries had been reported^[2], both with lasting brain damage.

Culturing and Identification

Balamuthia is most easily identifiable in a brain biopsy performed on an individual suffering from Balamuthia meningoencephalitis. The amoeba cannot be cultured on an agar plate coated with gram-negative bacteria because unlike most amoeba, Balamuthia mandrillaris does not feed on bacteria. Instead the amoeba must be cultured on primate hepatic cells or human brain microvascular endothelial cells, or HBMECs, the cells that constitute the blood-brain barrier. ^[3]

Treatment

Balamuthia infection has only been successfully treated in two cases. Both were treated with a cocktail of antibiotics and antiparasitics, although it is unclear if any or all of these medications played a part in treatment. Both victims suffered permanent neurological deficits as a result of their infection.

Other Pathogenic Amoebas

• Acanthamoeba spp.
• Naegleria fowleri

Related Amoebas

• Gephyramoeba spp.

Sources

1. ^ http://mic.sgmjournals.org/cgi/content/full/150/9/2837
2. ^ http://www.bms.ed.ac.uk/research/others/smaciver/balamuthia.htm
3. ^ Martinez, A. Julio, Visvesvara, Govinda S. (2001) “Balamuthia mandrillaris infection.” Journal of Medical Microbiology 50, 205-207