Submitted to: Applied and Environmental Microbiology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 23, 1997
Publication Date: N/A
Interpretive Summary: Cryptosporidium parvum is a protozoan parasite found worldwide, causing gastrointestinal illness in livestock and humans, and transmitted primarily through drinking water. Our earlier studies demonstrated that oysters and freshwater clams could remove and store infective C. parvum from artificially contaminated aquarium water in their blood and on their gills for a week or longer. The present study has demonstrated that oysters from 6 sites in rivers adjoining the Chesapeake Bay harbored C. parvum in their blood and on their gills. Furthermore, C. parvum from the gills of oysters at one site was infectious for mice. These findings indicate that fecal contamination must have been present at some time at all 6 sites and that oysters in nature can become infected with C. parvum.
Oocysts of Cryptosporidium parvum placed in artificial seawater at salinities of 10, 20, and 30 ppt at 10C and 10 ppt at 20C were infectious after 12 weeks. Those placed in seawater at 30 ppt and 20 ppt at 20C were infectious for 4 and 8 weeks, respectively. These findings suggested that oocysts could survive in saline estuarine waters long enough to be removed by filter feeders such as oysters. Thereafter, 30 Eastern oysters, Crassostrea virginica, were collected by dredging or tonging oyster bars at each of 6 sites within the drainage of the Chesapeake Bay in May/June and in August/September of 1997. Hemocytes and gill washings from all oysters were examined for the presence of Cryptosporidium oocysts and Giardia cysts by immunofluorescence microscopy utilizing a commercially available kit containing FITC-conjugated monoclonal antibodies. Giardia was not detected in hemocytes or gill washings from any of the 360 oysters. Presumptive identification of Cryptosporidium oocysts was made in either hemocytes or gill washings from oysters from all 6 sites both times that surveys were conducted. In addition, during August/September, for each of the 6 sites, hemocytes from the 30 oysters were pooled and gill washings from the oysters were pooled. Each pool was delivered by gastric intubation to a litter of neonatal mice to bioassay for oocyst infectivity. Intestinal tissue from 2 of 3 mice that received gill washings from oysters collected at a site near a large cattle farm and shoreline homes with septic tanks was positive for developmental stages of Cryptosporidium parvum. These findings demonstrate for the first time that oysters in natural waters harbor infectious C. parvum oocysts and can serve as mechanical vectors of this pathogen.