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Amphizoic Amoebae: Pathogenic Free-living Protozoa; Review of the Literature and Review of Cases in Thailand


Chutatip Siripanth, MSc (Trop Med)*

* Department of Protozoology, Faculty of Tropical Medicine, Mahidol University


A large variety of species of free-living amoebae (FLA) caused an indefinite form of these protozoa. Non-fixed form, as indicated by amoeboid movement and possed the bacteria to survive in nature. Two species of pathogenic FLA: Naegleria fowleri and Acanthamoeba spp. were identified as the causative agents of Primary Amoebic Meningoencephalitis (PAM) and Granulomatous Amoebic Encephalitis (GAE) respectively. They were suggested to amphizoic protozoa, capable of living as parasites or as free-living and they were also considered to be distributed worldwide. These amoebae were detected in lakes, rivers and ponds. The first case of meningoencephalitis was observed in 1961 by Fowler. Many cases were reported later on and the pathogenicity was tested by nasal inoculation of mice. In fact, quite a number of FLA were isolated but only a few species were pathogenic to humans. The three typical features which allow recognition of Naegleria spp. flagellate stage, round cyst and promitotic trophozoite. This promitosis distinguishes the Naegleria genus from Acanthamoeba spp. The disease caused by PAM usually occurs with acute onset, whereas chronic for GAE. The GAE cases mentioned are mostly in debilitated patients, chronic alcoholics or patients under treatment with immunosuppressive methods. About 6 cases of PAM were reported in Thailand during 1982-1997. Four cases of GAE were reported in 1994 and two isolated cases of Acanthamoeba from keratitis patients were reported in 2000. Finally one case of PAM and one case of GAE were reported in 2001. The surveys of FLA were set up to study the distribution of these pathogenic amoebae and determine the prevalence of amoebae in aquatic habitats of human environments. About 40% were identified as Acanthamoeba spp., 30% were Naegleria spp., 20% were Hartmanella and 10% were Vahlkampfia. Only 10% of Naegleria spp. belonged to Naegleria fowleri.

Keyword : Naegleria fowleri, Lobopodia, Acanthamoeba spp., Acanthopodia, Pathogenic free-living amoebae (FLA), Primary amoebic meningoencephalitis (PAM), Granulomatous amoebic encephalitis (GAE)

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