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Brief Summary

Balamuthia mandrillaris is a free-living amoeba that, like some related amoebae in the genus Acanthamoeba (which it resembles in tissue sections examined by light microscopy), can cause granulomatous amebic encephalitis (GAE), also known as balamuthia amoebic encephalitis (BAE), in individuals with compromised immune systems. Balamuthia mandrillaris has only recently been isolated from the environment and has also been isolated from autopsy specimens of infected humans and animals. It was first discovered in 1986 from the brain necropsy of a mandrill baboon (Papio sphinx) that died of a neurological disease at the San Diego Zoo Wild Animal Park, CA, U.S.A . (Siddiqui and Khan 2008).

The B. mandrillaris life cycle has only two stages, a dormant cyst stage and an actively feeding and dividing trophozoite stage (B. mandrillaris has no flagellated stage). The trophozoites replicate by mitosis (the nuclear membrane does not remain intact). Although the trophozoites are the infective stage, both cysts and trophozoites gain entry into the body through various means. Entry can occur through the nasal passages to the lower respiratory tract or via ulcerated or broken skin. 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) or disseminated disease, or skin lesions, in individuals who are immune competent as well as those with compromised immune systems. Balamuthia mandrillaris cysts and trophozoites are found in tissue. (Centers for Disease Control Parasites and Health website)

Among the many genera of free-living amoebae that exist in nature, members of only four genera have a known association with human disease: Acanthamoeba spp., Balamuthia mandrillaris, Naegleria fowleri, and Sappinia diploidea. Balamuthia mandrillaris are soil amoebae. In vitro, B. mandrillaris can feed on small amoebae (Acanthamoeba and Naegleria) and they may do so in nature. To date, B. mandrillaris is the only known species in the genus Balamuthia and it causes GAE in both humans and other animals. Balamuthiasis is similar to GAE caused by Acanthamoeba, and occurs in immunocompromised hosts, including HIV/AIDS patients and intravenous drug users; it has also been reported from immunocompetent individuals, especially young children and older individuals. The disease has a chronic, subacutephase that develops over a period of time spanning from 2 weeks to 2 years. Balamuthia mandrillaris is also known to cause a skin infection similar to that caused by Acanthamoeba. (Visvesvara et al. 2007).

Balamuthia mandrillaris infection of the skin and central nervous system has been reported with increasing frequency in recent years and this has been described as an emerging disease especially affecting people of Hispanic origin, although it remains unclear whether this is due to genetic susceptibility or occupational exposure (Matin et al. 2008; Siddiqui and Khan 2008; Bravo et al. 2011). Bravo et al. (2011) note that recognition of the skin lesion and early diagnosis can lead to successful intervention to combat this otherwise fatal infection. Although this amoeba has a worldwide distribution, its prevalence is higher in South America. Bravo et al. review diagnostic methods (including those based on molecular biology) and the different therapeutic strategies that have resulted in survival of patients. They note that a recent report dealing with organ transplant transmission of this infection has made it a subject of interest in transplant medicine.

BAE is a serious human disease with fatal consequences and a mortality rate of more than 95% even with available treatments. Unlike Acanthamoeba, Balamuthia can produce encephalitis in relatively immunocompetent individuals, almost always resulting in death. Since its identification in1986 and the first human cases, more than 100 cases of BAE have been reported and the actual number of BAE infections is likely far higher. BAE cases have been reported worldwide. The majority of BAE patients exhibit characteristic skin lesions. Siddiqui and Khan compare the epidemiology and clinical features of infection with B. mandrillaris versus Naegleria fowleri. (Siddiqui and Khan 2008)

Like other free-living amoebae, such as Acanthamoeba, Naegleria, and Hartmannella, B. mandrillaris can act as a host for intracellular survival of bacteria, including the causative agent of Legionnaires’ disease, Legionella pneumophila. The ability of amoebae to host bacteria enhances bacterial infectivity for mammalian cells, increases their transmission to susceptible hosts, and may enhance the pathogenicity of the host amoeba. (Matin et al. 2008)

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