SURVIVAL OF INFECTIOUS CRYPTOSPORIDIUM-PARVUM OOCYSTS IN SEAWATER ANDEASTERN OYSTERS (CRASSOSTREA-VIRGINICA) IN THE CHESAPEAKE BAY

Oocysts of Cryptosporidium parvum placed in artificial seawater at sal inities of 10, 20, and 30 ppt at 10 degrees C and at 10 ppt at 20 degr ees C were infectious after 12 weeks, Those placed in seawater at 20 p pt and 30 ppt at 20 degrees C were infectious for 8 and 4 weeks, respe ctively, These findings suggested that oocysts could survive in estuar ine waters long enough to be removed by filter feeders such as oysters , Thereafter, 30 Eastern oysters, Crassostrea virginica, were collecte d,vith a dredge or with hand tongs at each of six sites within Marylan d tributaries of the Chesapeake Bay in May and June and in August and September of 1997, Hemocytes and gill washings from all oysters were e xamined for the presence of Cryptosporidium oocysts and Giardia cysts by immunofluorescence microscopy utilizing a commercially available ki t containing fluorescein isothiocyanate-conjugated monoclonal antibodi es, Giardia was not detected by this method from any of the 360 oyster s examined, Presumptive identification of Cryptosporidium oocysts was made in either hemocytes or gill washings of oysters from all six site s both times that surveys were conducted, In addition, during August a nd September, for each of the six 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 p roduce a bioassay for oocyst infectivity, Intestinal tissue from two o f three mice that received gill washings from oysters collected at a s ite near a large cattle farm and shoreline homes with septic tanks was positive for developmental stages of C. parvum, These findings demons trate for the first time that oysters in natural waters harbor infecti ous C. parvum oocysts and can serve as mechanical vectors of this path ogen.