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Balamuthia mandrillaris is another cosmopolitan amoeba capable of causing fatal encephalitis (Balamuthia amoeba encephalitis). Its species name derives from its discovery as the cause of a fatal brain infection of a mandrill baboon at San Diego Wild Animal Park in California in 1986. Although considered free‐living, particularly in association with soil, there are many more reports of it causing infections than of its recovery from the environment. There are two life cycle stages – the trophozoite and the cyst stage: which of these is/are the infective stage is uncertain (Figure 3.4). Many amoebae feed on bacteria but while B. mandrillaris ingests them, they do not appear to sustain growth. By contrast, at least in cultures, B. mandrillaris grows well when fed other species of amoebae or human tissue culture cells. Unlike N. fowleri, B. mandrillaris tends to cause a chronic disease in humans that may last up to 2 years – although with a similar almost invariable (>98%) fatal outcome. The mode of entry is uncertain but, in several case reports, it appears to have been through puncture wounds in the skin from which it then spread via the blood stream. There are also case reports of infections through organ transplant and nasal lavage using (probably) infected water. Balamuthia mandrillaris will infect tissues other than the brain, including the kidneys, pancreas, and the skin. It probably gains access to the brain via the choroid plexus (Jayasekara et al. 2004). Once established in the brain, the amoebae cause a granulomatous reaction and the site of the infection becomes surrounded by macrophages. The pathology is therefore called granulomatous amoebic encephalitis.

Figure 3.4 Life cycle of Balamuthia mandrillaris. This species exists either as a free‐ living organism in ponds, lakes, and soil or as a parasite. Both the trophozoite (T) and the cyst stage (C) are probably infectious and enter via skin wounds or through the lining of the nose. It damages various organs including the skin, kidneys, lungs, adrenal glands, and pancreas. Invasion of the central nervous system probably occurs via the choroid plexus and results in potentially fatal granulomatous encephalitis. Drawings not to scale.

Balamuthia mandrillaris is a zoonotic parasite and natural (and often fatal) infections occur in many species of wild and domestic animals (Visvesvara et al. 2007). In common with other free‐living amoebae, B. mandrillaris often acts as host for various bacteria, including Legionella pneumophila (Shadrach et al. 2005). However, its importance as a transport host for microbial infections is uncertain.