Balamuthia mandrillaris is a free-living ameba and an opportunistic agent of lethal granulomatous amebic encephalitis in humans and other mammals. Balamuthia mandrillaris is highly cytopathic but, in contrast to the related Acanthamoeba, does not feed on bacteria and seems to feed only on eukaryotic cells instead. Most likely, the cytopathogenicity of B. mandrillaris is inseparable from its infectivity and pathogenicity. To better understand the mechanisms of B. mandrillaris cytopathogenicity, an assay for measuring amebic cytolytic activity was adapted that is based on the release of a reporter enzyme by damaged target cells. The ameba is shown to lyse murine mastocytoma cells very efficiently in a time- and dose-related manner. Furthermore, experiments involving semipermeable membranes and phagocytosis inhibitors indicate that the cytolytic activity of B. mandrillaris is essentially cell contact-dependent. Standard and fluorescence light microscopy, as well as scanning and transmission electron microscopy support and extend these findings at the ultrastructural level.

Background: The free-living amoeba Balamuthia mandrillaris may cause fatal encephalitis both in immunocompromised and in -apparently -immunocompetent humans and other mammalian species. Rapid, specific, sensitive, and reliable detection requiring little pathogen-specific expertise is an absolute prerequisite for a successful therapy and a welcome tool for both experimental and epidemiological research.

Balamuthia mandrillaris is an opportunistic agent of lethal granulomatous amebic encephalitis (GAE). In mice, we have shown that intranasally instilled B. mandrillaris amebae infect the brain via the olfactory nerve pathway. In this study, we raised the question whether this ameba might also reach the brain after an oral/gastrointestinal infection. Immunocompetent (WT) and immunodeficient (RAG) mice received B. mandrillaris amebae by gavage into the esophagus. Mice of both groups became ill and some died (WT 20%, RAG 40%) within 42 days. All orally infected mice revealed B. mandrillaris amebae in the central nervous system. Outwardly intact amebae and/or specific antigen were found widely distributed in various organs and the stool. The data indicate that oral infection with B. mandrillaris leading to GAE is possible. Exit from the gastrointestinal tract and dissemination remains unresolved. Though stool cultures were negative, transmission of this highly pathogenic ameba via stool cannot be ruled out.

A flow cytometry (fluorescence-activated cell sorter)-based assay was adapted to detect and quantify antibodies to Balamuthia mandrillaris, a causative agent of fatal amoebic encephalitis (BAE), and to Acanthamoeba species. With sera from BAE patients for positive and a group of inconspicuous volunteers for negative reference, most of the 237 sera from random blood donors, patients with atypical encephalitis, atypical pneumonitis, visceral amoebiasis and toxoplasmosis and from subjects working with primates and other mammals were rated negative, 19% elevated and of these 2% high. In comparison, 23 of 25 West Africans living in rural areas revealed elevated, of these 15 high, and one very high B. mandrillaris-binding antibody titers, the latter well in the range of BAE patients. To date, none of the tested individuals have developed symptoms indicative of BAE. Criss-cross analysis with rabbit hyper immune sera against B. mandrillaris, Acanthamoeba comandoni (group 1), Acanthamoeba castellanii (group 2) and Acanthamoeba lenticulata (group 3) confirmed that cross-reactivity between B. mandrillaris and Acanthamoeba sp. is negligible while accentuating antigenic differences between the three morphological groups of Acanthamoeba.

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