Infectivity of Giardia lamblia cysts in municipal wastewater after ultraviolet treatment

Giardia lamblia is a waterborne parasite that is commonly present in treated municipal wastewater; however, the public health risk associated with the cysts released to the environment is not clear. The objectives of this thesis were to investigate the ultraviolet inactivation of G. lamblia present in municipal wastewater effluent under field conditions and the factors that might impact parasite inactivation by UV, including differences in UV susceptibility between different strains, the reactivation potential after UV exposure, and the interaction with particulate matter. In laboratory experiments, two human-derived G. lamblia isolates (WB and H3) were found to have different susceptibilities to UV inactivation as measured by the Mongolian gerbil model. UV-exposed cysts produced different levels of infection in gerbils and these different levels of infections complicated the assessment of infectivity and inactivation by UV. Trophozoites exposed to low UV doses (< 20 mJ/cm2) were found to be able to subsequently divide in in vitro culture, which suggests that the parasite is not as readily inactivated as previously believed or may be able to repair UV-induced damage. In a field study carried out at four municipal wastewater treatment plants, indigenous G. lamblia cysts present in primary effluent produced strong and moderate infections in gerbils; however, only low intensity infections were produced in gerbils inoculated with cysts obtained from secondary effluent both upstream and downstream of UV reactors. This suggests that the cysts survived the secondary treatment may be more resistant to UV irradiation and may reactivate after exposure to UV. There was no evidence that cysts survived due to association between G. lamblia cysts and particulates. Overall, the study suggests that assessment of UV inactivation of G. lamblia cysts in wastewater effluents might not be as straightforward as indicated by previous studies. It is difficult to translate low intensity infections in gerbils to the potential for these parasites to cause disease in humans. The residual cysts in the wastewater effluents and the dilution factor in receiving waters suggest that these parasites would cause a relatively low public health risk provided that the hosts are immunocompetent.