<3-EPA Science Forum Rif >i,¦ ,-iiMBbaBburn an flu-tumnjriricnr Inactivation of Giardia muris by Low Pressure Ultraviolet Light °[ wfier: MWWare1,FWSchaeferIII1,SLHayes2,and EWRice2 J fJJf iy /ztzlf Of tUSEPA, Office of Research and Development, National Exposure Research Laboratory2 USEPA, Office of Research and Development, National Risk Management Research Laboratory ~J'fjfp) fj CJf] fj f] Giardia, a Waterborne Problem Giardia is a parasite of humans and is a cause of waterborne diarrheal disease worldwide. In the U.S., the parasites Giardia and Cryptosporidium have been associated with approximately one third of the waterborne outbreaks illness that occurred between 1990 and 2000. To assure safe drinking water, there must be adequate disinfection practices for these microorganisms. Concern over the disinfection byproducts associated with chlorination has led to increasing interest in the application of ultraviolet (UV) light for drinking water disinfection. However, the UV dose required for inactivation of G. lamblia cysts is not known. Early studies suggested that cysts may be highly resistant to UV. But these studies were done using in vitro excystation to determine viability, and work with other parasites has suggested that in vitro excystation does not correlate with animal infectivity. The goal of the research effort presented here was to develop a complete UV inactivation curve for Giardia that accurately reflects the true disinfection of the microorganism. scale bar represents 10 microns Figure 1: Intact Giardia muris cysts scale bar represents 10 microns Figure 2: Excysted Giardia muris cysts: Empty cyst walls and trophozoites Life Cycle Giardia has a two stage life cycle composed of an environmentally stable cyst, and the trophozoite. When an infectious cyst is ingested, stomach acid triggers excystation - the release of two trophozoites. The trophozoites multiply in the small intestines and cause gastrointestinal symptoms. Some trophozoites form cysts that will subsequently shed in the feces thus completing the life cycle. Graphic fom the CDC website Figure 3: Life cycle of Giardia A Two-fold Approach The biocidal potential of various UV doses was assayed using both animal infectivity and in vitro excystation. G. muris was used in this study. Although not a pathogen of humans, G. muris has been used as a conservative surrogate for G. lamblia for evaluating other disinfectants. The in vitro excystation occurred after cysts were exposed to acid, heat, and bile salts. The percent of cysts that excysted was determined by microscopy. Excysted cysts were considered viable. Animal infectivity was determined by comparing the cyst dose at which half of a cohort of mice became infected. Mice groups were exposed to various doses of cysts and infection was determined by the detection of cysts in the feces. 100000 10000 1000 100 10 1—I 1 1 1 1 1—I r 0 1.0 1.0 1.4 1.4 1.9 1.9 2.3 2.3 8 40 80 240 UV Exposure (mJ/cm2) 20 40 60 80 100 Figure 4; UV inactivation curves of 6. muris cysts by animal infectivity (highly inactivated at 2.3 mJ/cm2) and in vitro excystation (no effect until greater than 8.0 mJ/cm2) Impact of Research This data represents the only complete inactivation curve for 6. muris. This curve demonstrates that 6. muris cysts exposed to low levels of UV light are not infectious to mice. In vitro excystation indicates that 6. muris cysts are resistant to UV light,and thus in vitro excystation is not an acceptable surrogate for evaluating disinfection by UV light. This study demonstrates that low UV exposure could be an effective barrier in preventing transmission of infective Giardia cysts in drinking water. Partnering to Protect Human Health and the ^Environment ------- |