Evaluation of the recovery of waterborne Giardia cysts by freshwater clamsand cyst detection in clam tissue

The Asian freshwater clam, Corbicula fluminea, inhabits environments recogn ized to be contaminated with waterborne Giardia cysts. Sixty-four tissue sa mples of Giardia-free clams were spiked with various numbers of Giardia duo denalis cysts within the range of 50-700 cysts. Regression analysis showed that paired numbers of spiked (x) versus recovered (y) cysts regressed sign ificantly (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 deter mination 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 bl ind 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 1 of water spiked with 1.00 x 10(5) G. duodenalis cysts was approximately 1 .3 x 10(3) cysts/clam. No waterborne cysts were detected by the membrane fi ltration method 90 min after spiking the aquarium water. G. duodenalis cyst s were detected in clam tissue up to 3 weeks post-exposure. Filtration of w ater by clams substantially depleted the aquarium water of its particulate matter. The sampling program demonstrated that the population of 160 clams examined during the study could be accurately assessed for exposure to wate rborne Giardia cysts by random sampling of 86 (54%) clams. The results indi cate that C. fluminea clams can be used for biological monitoring of contam ination with Giardia.