Evaluation of the recovery of waterborne Giardia cysts by freshwater clams and cyst detection in clam tissue.
The Asian freshwater clam, Corbicula fluminea, inhabits environments which are contaminated with waterborne Giardia cysts. 64 tissue samples of Giardia-free clams were spiked with various numbers of Giardia duodenalis cysts within the range of 50-700 cysts. Regression analysis showed that paired numbers of spiked (x) versus recovered (y) cysts regressed significantly (P<0.01) according to the equation y = 42.57 + 1.81x (±64.3). The cyst detection threshold was 43 cysts/clam, the coefficient of determination was 77%, and the overall sensitivity of cyst detection was 42.9%. All 20 values of cyst numbers in clam tissue samples that were processed blind were located within the 95% prediction limits of the linear regression equation. The cyst retention rate of 160 clams kept in an aquarium with 38 litres of water spiked with 1.00 × 105G. duodenalis cysts was approximately 1.3 × 103 cysts/clam. No waterborne cysts were detected by membrane filtration 90 minutes after spiking the aquarium water. G. duodenalis cysts were detected in clam tissue up to 3 weeks post-exposure. Filtration of water by clams substantially depleted the aquarium water of particulate matter. The sampling programme demonstrated that the study population of 160 clams could be accurately assessed for exposure to waterborne Giardia cysts by random sampling of 86 (54%) clams. The results indicate that C. fluminea clams can be used for biological monitoring of water contamination with Giardia.