Background: Free-living Naegleria fowleri and Acanthamoeba spp. amoebae are causative agent of lethal brain infection in human. Due to high fatal rate of the patients, a survey of these pathogenic amoebae should be done for prevention and control in public health.

Objective: To investigate the presence and prevalence of N. fowleri and pathogenic Acanthamoeba spp. in the Chao Phraya River and canals located close to Siriraj Hospital (Bangkok, Thailand).

Materials and Methods: One hundred eighty-nine freshwater samples collected from 21 sites were investigated between January and April 2017. Sample sediments were incubated at 37°C (regular temperature) and 42°C (tolerant temperature) for five to seven days. All samples were identified by morphologic characteristics and polymerase chain reaction.

Results: The samples from eight sites (Bangkoknoi District Office, Taling Chan Floating Market, Wang Lang, Wat Amarinthararam, Wat Khruawan, Wat Pho Riang, Wat Rakhang, Wat Taling Chan) yielded 38% (72/189) Naegleria spp. at 37°C, and 19% (36/189) Naegleria spp. at 42°C by morphological identification. The thermotolerant Naegleria spp. samples (9.5%, 18/189) from Wang Lang and Wat Rakhang were identified as N. fowleri. Forty-five of 189 (23.8%) water samples taken from the Taling Chan Floating Market, Wang Lang, Wat Khruawan, Wat Pho Riang, Wat Taling Chan collection sites were positive for pathogenic Acanthamoeba spp.

Conclusion: Our findings suggested that fresh water from the Wang Lang, Wat Rakhang were contaminated with N. fowleri, whereas Taling Chan Floating Market, Wat Khruawan, Wat Pho Riang or Wat Taling Chan were with pathogenic Acanthamoeba spp. Interestingly, we found N. fowleri and pathogenic Acanthamoeba spp. from Wang Lang site.

Keywords: Naegleria spp., Acanthamoeba spp., Chao Phraya River, Thailand