95 Tropical Biomedicine 24(1): 95–104 (2007) Reduction of Cryptosporidium and Giardia by sewage treatment processes Lim, Y.A.L., Wan Hafiz, W.I. and Nissapatorn, V. Department of Parasitology, Faculty of Medicine, University of Malaya, 50603 Kuala Lumpur, Malaysia. Email: limailian@um.edu.my or yvonne@ummc.edu.my Received on 5 April 2007; received in revised form 12 April 2007; accepted 15 April 2007 Abstract. Cryptosporidium and Giardia are two important pathogenic parasites that have caused many waterborne outbreaks which affected hundreds of thousands of people. Contamination from effluent discharged by sewage treatment plants have been implicated in previous waterborne outbreaks of Cryptosporidium and Giardia. This study evaluated the reduction of Cryptosporidium and Giardia (oo)cysts in two sewage treatment plants (STPA and STPB) in Malaysia which employed different treatment processes for a period of a year. Raw sewage influents and treated sewage effluents were concentrated by repeated centrifugation, subjected to sucrose density flotation and concentrated to a minimal volume depending upon the levels of contaminating debris. Cryptosporidium oocysts and Giardia cysts were enumerated using epifluorescence microscopy. The parasite concentrations in raw sewage were 18-8480 of Giardia cysts/litre and 1-80 of Cryptosporidium oocysts/litre. In treated sewage, the concentration of parasites ranged from 1-1462 cysts/litre and 20-80 oocysts/ litre for Giardia and Cryptosporidium respectively. Statistical analysis showed that sewage treatment process which employed extended aeration could reduce the concentration of Cryptosporidium and Giardia (oo)cysts significantly but treatment process which encompasses aerated lagoon could only reduce the concentration of Giardia cysts but not Cryptosporidium oocysts significantly. This phenomenon is of great concern in areas whereby effluent of sewage treatment plants is discharged into the upstream of rivers that are eventually used for abstraction of drinking water. Therefore, it is important that wastewater treatment authorities rethink the relevance of Cryptosporidium and Giardia contamination levels in wastewater and watersheds and to develop countermeasures in wastewater treatment plants. Further epidemiological studies on the occurrence and removal of pathogenic organisms from excreta and sewage are also recommended, in order that the public health risks can be defined and the most cost effective sewage treatment options developed. INTRODUCTION To date, there have been at least 325 water- associated outbreaks of parasitic pro- tozoan disease documented worldwide (Karanis et al., 2007). Cryptospordium and Giardia, two notorious pathogenic protozoa have been reported to account for a majority of these outbreaks (132; 40.6% and 165; 50.8%, respectively) (Karanis et al., 2007) affecting hundreds of thousands of individuals (Smith & Grimason, 2003). The evidence of potential role of sewage treatment plant con- tamination in Cryptosporidium outbreak was first highlighted in San Antonio, Texas (D’Antonio et al. , 1986). In 1993, con- taminated sewage was singled out as the culprit in the largest Milwaukee Crypto- sporidium waterborne outbreak that sickened 403,000 and killed more than 100 with weakened immune systems. Genetic evaluation data concluded that the outbreak was due to human Crypto- sporidium type coming from human fecal material and human-associated wastewater (Rose et al., 2002). Studies on various sewage treatment plants have been carried out throughout the world in developed and developing