Reactivation of Giardia lamblia cysts after exposure to polychromatic UV light


Gwy-Am Shin, Department of Environmental and Occupational Health Sciences, University of Washington, 4225 Roosevelt Way NE, Suite 100, Seattle, WA 98105, USA. E-mail:


Aims:  In this study, we determined the ability of a promising alternative UV technology – a polychromatic emission from a medium-pressure UV (MP UV) technology – to inhibit the reactivation of UV-irradiated Giardia lamblia cysts.

Methods and Results:  A UV-collimated beam apparatus was used to expose shallow suspensions of purified G. lamblia cysts in PBS (pH 7·2) or filtered drinking water to a low dose (1 mJ cm−2) of MP UV irradiation. After UV irradiation, samples were exposed to two repair conditions (light or dark) and two temperature conditions (25°C or 37°C for 2–4 h). The inactivation of G. lamblia cysts by MP UV was very extensive, and c. 3 log10 inactivation was achieved with a dose of 1 mJ cm−2. Meanwhile, there was no apparent reactivation (neither in vivo nor in vitro) of UV-irradiated G. lamblia under the conditions tested.

Conclusion:  The results of this study indicated that, unlike the traditional low-pressure (LP) UV technology, an alternative UV technology (MP UV) could inhibit the reactivation of UV-irradiated G. lamblia cysts even when the cysts were exposed to low UV doses.

Significance and Impact of the Study:  It appears that alternative UV technology has some advantages over the traditional LP UV technology in drinking water disinfection because of their high level of inactivation against G. lamblia cysts and also effective inhibition of reactivation in UV-irradiated G. lamblia cysts.