ORIGINAL PAPER Thaddeus K. Graczyk á Ronald Fayer á David Bruce Conn Earl J. Lewis Evaluation of the recovery of waterborne Giardia cysts by freshwater clams and cyst detection in clam tissue Received: 27 May 1998 / Accepted: 7 July 1998 Abstract The Asian freshwater clam, Corbicula ¯uminea, inhabits environments recognized to be contaminated with waterborne Giardia cysts. Sixty-four 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 sig- ni®cantly (P < 0.01) according to the equation y  42.57 + 1.81x (64.3). The cyst detection threshold was 43 cysts/clam, the coecient 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 equa- tion. The cyst retention rate of 160 clams kept in an aquarium with 38 l of water spiked with 1.00 ´ 10 5 G. duodenalis cysts was approximately 1.3 ´ 10 3 cysts/clam. No waterborne cysts were detected by the membrane ®ltration method 90 min 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 its partic- ulate matter. The sampling program demonstrated that the population of 160 clams examined during the study could be accurately assessed for exposure to waterborne Giardia cysts by random sampling of 86 (54%) clams. The results indicate that C. ¯uminea clams can be used for biological monitoring of contamination with Giardia. Introduction Giardia is the most frequently identi®ed etiologic agent of water and foodborne outbreaks of intestinal illnesses worldwide (Wolfe 1992). The infectious stage of the pathogen, the cyst, is transmitted by the fecal-oral route, water-associated contamination, and via recreational and drinking waters (Wolfe 1992). Although controversy exists on the subject of possible zoonotic reservoirs for waterborne cysts, aquatic and semi-aquatic mammals, e.g., beavers and muskrats (Monzingo and Hibler 1987; Erlandsen et al. 1990), small rodents (Wallis et al. 1984; Pacha et al. 1987), and domestic ruminants (Olson et al. 1995) have been implicated as possible sources of cysts infectious for humans. The Asian freshwater clam, Corbicula ¯uminea, has been used extensively for biomonitoring water and sed- iments for contamination with agricultural and indus- trial-derived pollutants (Tatem 1986; Leland and Scudder 1990; Kira et al. 1996). C. ¯uminea is an ecient seston feeder able to ®lter up to 2.50 l of water per hour (McMahon 1991). C. ¯uminea is able to recover from the water the cystic stages (oocysts) of other human water- borne pathogens, Cryptosporidium parvum (Graczyk et al. 1998a) and Cyclospora cayetanensis (Graczyk et al. 1998b). Additionally, hemocytes of C. ¯uminea are capable of rapid internalization in vitro of infectious Giardia duodenalis cysts (Graczyk et al. 1997a). How- ever, it is not known if intact clams can ®lter Giardia cysts from water and, if so, what are the sensitivity and Parasitol Res (1999) 85: 30±34 Ó Springer-Verlag 1999 T.K. Graczyk (&) Department of Molecular Microbiology and Immunology, School of Hygiene and Public Health, Johns Hopkins University, 615 North Wolfe Street, Baltimore, MD 21205, USA e-mail: tgraczyk@jhsph.edu, Fax: +1-410-9550105 R. Fayer Immunity and Disease Prevention Laboratory, Livestock and Poultry Sciences Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, MD 20705, USA D.B. Conn School of Mathematical and Natural Sciences, Berry College, Mount Berry, GA 30149, USA E.J. Lewis Cooperative Oxford Laboratory, National Marine Fisheries Service, National Oceanic and Atmospheric Administration, U.S. Department of Commerce, Oxford, MD 21654, USA