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Appl Environ Microbiol, February 1998, p. 427-430, Vol. 64, No. 2
0099-2240/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Recovery of Waterborne Cryptosporidium parvum Oocysts by Freshwater Benthic Clams (Corbicula fluminea)

Thaddeus K. Graczyk,^1,2,* Ronald Fayer,³ Michael R. Cranfield,^2,4 and David Bruce Conn⁵

Department of Molecular Microbiology and Immunology, School of Hygiene and Public Health,¹ and Division of Comparative Medicine, School of Medicine,⁴ Johns Hopkins University, Baltimore, Maryland 21205; The Baltimore Zoo, Baltimore, Maryland 21217²; Immunity and Disease Prevention Laboratory, Livestock and Poultry Science Institute, Agricultural Research Service, U.S. Department of Agriculture, Beltsville, Maryland 20705³; and School of Mathematical and Natural Sciences, Berry College, Mount Berry, Georgia 30149⁵

Received 26 August 1997/Accepted 12 November 1997

Asian freshwater clams, Corbicula fluminea, exposed for 24 h to 38 liters of water contaminated with infectious Cryptosporidium parvum oocysts (1.00 × 10⁶ oocysts/liter; approximately 1.9 × 10⁵ oocysts/clam) were examined (hemolymph, gills, gastrointestinal [GI] tract, and feces) on days 1, 2, 3, 7, and 14 postexposure (PE). No oocysts were detected in the water 24 h after the contamination event. The percentage of oocyst-containing clams varied from 20 to 100%, depending on the type of tissue examined and the technique usedacid-fast stain (AFS) or immunofluorescent antibody (IFA). The oocysts were found in clam tissues and feces on days 1 through 14 PE; the oocysts extracted from the tissues on day 7 PE were infectious for neonatal BALB/c mice. Overall, the highest number of positive samples was obtained when gills and GI tracts were processed with IFA (prevalence, 97.5%). A comparison of the relative oocyst numbers indicated that overall, 58.3% of the oocysts were found in clam tissues and 41.7% were found in feces when IFA was used; when AFS was used, the values were 51.9 and 48.1%, respectively. Clam-released oocysts were always surrounded by feces; no free oocysts or oocysts disassociated from fecal matter were observed. The results indicate that these benthic freshwater clams are capable of recovery and sedimentation of waterborne C. parvum oocysts. To optimize the detection of C. parvum oocysts in C. fluminea tissue, it is recommended that gill and GI tract samples be screened with IFA (such as that in the commercially available MERIFLUOR test kit).

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^* Corresponding author. Mailing address: Johns Hopkins University, School of Hygiene and Public Health, Department of Molecular Microbiology and Immunology, 615 North Wolfe St., Baltimore, MD 21205. Phone: (410) 614-4984. Fax: (410) 955-0105. E-mail: tgraczyk{at}jhsph.edu.

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