Free-living amebas of the genera Acanthamoeba and Naegleria: an overview and basic microbiologic correlates.

Free-living amebas of the genera Acanthamoeba and Naegleria are allied in basic cell biology, ecology, and human disease-producing potential. However, several enigmas surrounding these amebas need further intellectual and scientific scrutiny. For instance, a clearer differentiation is needed of factors delineating pathogenic species--those most frequently associated with human infections--from nonpathogenic species. Further, have the pathogenic species bridged the gap in a step-wise fashion between their habitat and the human host to express their disease-producing potential? Second, what attributes of amebas account for the spectrum of disease caused by Acanthamoeba and Naegleria? In the brain, Naegleria is highly destructive of tissue in the course of a rapidly evolving, hemorrhagic primary meningoencephalitis in normal individuals. Central nervous system invasion by Acanthamoeba, however, occurs only in compromised hosts, and is a more slowly evolving subacute to chronic encephalitis. Further, the epidemiology of the infections are disparate. Naegleria is acquired from exposure to contaminated fresh water; this epidemiologic link is absent in Acanthamoeba meningoencephalitis. Naegleria invades the central nervous system via the olfactory nerve; Acanthamoeba is deposited in the central nervous system via hematogenous spread from a pulmonary or cutaneous focus. Additionally, Naegleria is apparently restricted to the brain; Acanthamoeba is not so bridled, and invades more sites in the human body. Finally, Acanthamoeba are beginning to appear opportunistically more frequently in patients infected with the human immunodeficiency virus. To avoid the same lack of effective therapeutics we still face in treating cryptosporidium infections in these patients, more emphasis must be placed on clinical trials dealing with the management of Acanthamoeba infections in patients with AIDS.