Recent studies have shown that ultraviolet (UV) radiation reduces the infectivity of Cryptosporidium parvum oocysts and Giardia spp. cysts. The objective of this work was to further examine UV inactivation of C. parvum oocysts and G. muris cysts in the presence of suspended particulate matter. Naturally occurring particulate matter consisting of a heterogeneous mixture of biological and non-biological particles, mainly in the 5–25 μm size range, was concentrated from a lake. Different aliquots of this material were mixed with purified oocyst and cyst preparations in aqueous suspensions, and the suspensions were exposed to UV fluences of 5 and 40 mJ/cm2 from a medium-pressure mercury arc lamp using a collimated beam apparatus. Parasite inactivation was determined using mouse infectivity assays. Addition of particulate matter was correlated with statistically significant reductions in C. parvum and G. muris inactivation of 0.8 log10 and 0.4 log10, respectively, even after the fluence was adjusted for increased absorbance due to the presence of the particles. The corresponding increases in turbidity were 0.3–20 NTU for C. parvum and 7.5–20 NTU for G. muris. The magnitude of the effect was a function of the final suspension turbidity but was independent of fluence. In this study, the effect of particles on parasite inactivation at turbidity values of less than 10 NTU was small and difficult to measure, and would likely be unimportant in practice. Although the mechanism for this reduction was not determined, the results suggest that the particulate matter present in natural surface waters may interact with oocysts and cysts and thereby result in a reduction in UV inactivation over and above that attributable to simple absorbance. The effect of turbidity on inactivation measured in this work may not necessarily apply generally since the characteristics of the particles that compose turbidity may differ considerably between water sources.

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