RESUMO
The free-living amoebae (FLA),Naegleria fowleri and Acanthamoeba spp. are ubiquitousin nature. Some thermotolerant FLA cause fatal human infections and the most importantsources of infection are in water. To determine the occurrence of thermotolerant FLA insome natural water sources around Chiang Mai, a total of 56 surface water and bottomsoil samples were collected from 28 water sources: water-loggedfi elds, ditches alongthe Chiang Mai-Lampang Superhighway, paddyfi elds, fi sh farms and large ponds. Thesediments of samples were cultured at 37 °C and thermotolerance was tested at 42 °C, withsubcultured amoebae.Naegleria and Acanthamoeba were morphologically differentiated,based on pseudopodia. Forty of the 56 samples (71.4%) yielded amoebae at 37 °C, and16 thermotolerant positive samples (28.6%) were further identifi ed at 42 °C. The thermotolerantpositive samples were recovered from all pH ranges of 5.0-8.5 and temperatureranges of 25-35 °C. Samples from the water-loggedfi elds gave the highest positive rate(75%), followed by those from paddyfi elds (50%), ditches (25%), large ponds (16.7%)andfi sh farms (15%). The number of thermotolerant positive samples obtained from watersamples (62.5%) was higher than that from soil samples (37.5%). Among 16 thermotolerantpositive samples, 6 (37.5%) were identifi ed as Naegleria spp., 3 (18.8%) as Acanthamoebaspp., 5 (31.3%) as a mix of both species, and 2 were unidentifi ed. High prevalenceof thermotolerant amoebae in the paddyfi elds and ditches indicated a health risk for localpeople.Chiang Mai Medical Journal 2009;48(3):117-124.