Walsh & Hoyt: Free-living Amoebae

Infection of humans by these organisms is rare. Because they are distinct from other pathogenic protozoa by nature of their free-living existence, there are no known insect vectors, no human carrier states of epidemiologic importance, and little relationship of poor sanitation to the spread of infection. Free-living amoebae were first identified by Antonie van Leeuwenhoek, the inventor of the microscope, who observed them in a drop of water in the summer of 1674. Almost 300 years later, Jahnes et al. described free-living amoebae as contaminants of cultures of monkey kidney tissue and reported that the amoebae seemed to exert a cytopathic effect on the cells. One year later, Culbertson et al. demonstrated that intranasal inoculation of mice with Acanthamoeba caused a purulent meningoencephalitis with direct invasion of the CNS via the nasal mucosa. It subsequently has been shown that Acanthamoeba and at least three other free-living amoeba called Naegleria, Sappinia diploidea, and Leptomyxida are capable of producing severe meningoencephalitis in humans. Naegleria fowleri and Sappinia diploidea produce a distinct syndrome called primary amoebic meningoencephalitis (PAM), whereas Acanthamoeba species and Leptomyxida produce a condition called granulomatous amoebic encephalitis (GAE). These two conditions have distinct clinical manifestations and pathology that usually permit differentiation between them.