Title: Difference of Giardia lambliaisolates in response to ultraviolet radiation

Authors: Dong Li1,Stephen A. Craik1,Daniel W. Smith1, Miodrag Belosevic2

1Department of Civil & Environmental Engineering, University of Alberta

2Department of Biological Science, University of Alberta, Edmonton, Alberta T6G 2W2

Correspondence to: Dong Li, Room 3-133 NREF, Department of Civil & Environmental Engineering, University of Alberta, Edmonton, AB T6G 2W2; Telephone: (780)492-8548; E-mail:

Background

The protozoan parasites Giardia lamblia and Cryptosporidium parvum are frequently identified causes of waterborne disease outbreaks in Canada and worldwide and present a significant water supply challenge. Both parasites are consistently present at high concentrations in wastewater effluents, which are often discharged into surface water bodies which in return, as sources of drinking water supplies. Many of Giardiasis outbreaks were associated with municipal drinking water supplies using surface water source (Craun 1990), which met regulatory standards for turbidity and coliforms.Therefore, significant concerns of public health have increased over the risk from consuming such drinking water.

It was demonstrated that ultraviolet (UV) light could be used effectively to inactivate Giardia lamblia cysts (Mofidiet al. 2002), Cryptosporidium parvum oocysts (Bukhariet al. 1999; Clancyet al. 1998; Craiket al. 2000).Clancy et al.(2004)investigated the response of five strains of C. parvum to UV light and found all strains to be highly susceptible to low levels of UV light. But previous studies found that not all strains of G. lamblia cysts retained the ability to infect gerbils and the response is different between strains (Schaeferet al. 1991; Visvesvaraet al. 1988). Campbell and Wallis (2002)reported that up to 2 log inactivation was observed at a UV dose of approximately 10 mJ/cm2 using a low pressure (LP) UV lamp using human-derived WB strain G. lamblia cysts. However, more than 4 log of reduction of infectivity was achieved within a UV dose of 1 mJ/cm2 using CH3 strain G. lamblia cysts (Lindenet al. 2002). The objectives of this research were to (1) compare the kinetics of different strains of G. lamblia cysts infectivity for Mongolian gerbils and (2) determine the extent of inactivation of these G. lambliaisolates of by various dose of UV radiation.

Materials and methods

Two human isolates WB and H3 of Giardia lamblia cysts were used in this research. G. lambliacysts purified from gerbil fecal samples were exposed to UV from a low-pressure mercury arc lamp using a collimated beam apparatus as described by Craik et al.(2001).The average UV fluence was determined according to procedures described elsewhere(Bolton and Linden 2003; Craiket al. 2001). After UV exposure, the suspensions were centrifuged and serial dilution was followed to prepare the inoculum for animal infectivity assay. Gerbilswere inoculated with 0.2 mL of Milli-Q water containing different level of cysts by oral injection. Feces were collected and examined for the presence of cysts from individual gerbils daily starting on day 6 post-infection. All gerbils were terminated and examined for the presence of trophozoites in the small intestinebetween day 13 and15 post-infection. The infectivity level was determined by the presence of cysts in the feces of inoculated gerbils and the presence of trophozoites in the small intestine of gerbils (Belosevicet al. 1983).

Results and discussion

50 percent infective dose. The ID50 was calculated by the logistic dose-response model as described in Finch et al.(1993). The ID50for WB strain G. lambliais 2.38 log10, which is consistent with a previously reported ID50 of 2.39 log10 cysts (Finch et al. 1993). But the ID50 is 1.34 log10 for H3 strain G. lamblia.

Infectivity level of dose-response. It was observed that gerbils were infected at different levels by G. lamblia cysts in the same cohort animals. The infectivity level of cysts on Mongolian was classified into four categories: heavy infection, medium infection, weak infection and no infection.At same cyst-dose level, H3 strain G. lamblia is more infectious to gerbils than WB strain G. lamblia. When the inocolum is 100 cysts, about 80% of gerbils was heavily infected by H3 strain Giardia cysts; while only 6 percent of gerbils by WB strain cysts. All the gerbils were heavily infected by H3 isolate at more than 1000 cysts inoculated. Only 75 percent of gerbils is strongly infected at 10,000 cysts inoculum.

UV dose-Giardia response. Gerbils appear to be more resistant to H3 strain after UV exposure. The proportion of gerbils infected at UV 40 mJ/cm2 was 0.69 for WB strain with 25% of heavy infection. The proportion infected by H3 strain was only counted to 22% of weak infection. Heavy and medium infections, characterized by active trophozoites and/or the presence of cysts in the faeces were also observed in those gerbils inoculated with 20,000 cysts exposed to a UV dose of 0.5 mJ/cm2 and 10,000 cysts exposed to a UV dose of 1 mJ/cm2.

Keywords: Ultraviolet disinfection; Giardia lamblia; infectivity assay; strain; logistic model

References

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