Bioaccumulation and elimination of Cryptosporidium parvum oocysts in experimentally exposed Eastern oysters (Crassostrea virginica) held in static tank aquaria

• Bioaccumulation of oocysts in oysters is exponential in the first 24 h.
• Contamination in oysters persists after 6 days of static tank depuration.
• 33–34% of inoculated oocysts were recovered in oyster fecal material from chronic exposures.

A variety of human enteropathogens, including viruses, bacteria, and parasites, have been shown to bioaccumulate in suspension-feeding bivalve shellfish. Cryptosporidium parvum is a zoonotic protozoan parasite that has been detected in many shellfish species within both fecally contaminated and clean oyster growing areas across the globe. For this study, C. parvum oocysts (1000 and 10,000) were spiked into 10 L of water in static tank systems housing Crassostrea virginica. Oysters were either held in the contaminated aquaria for 7 days of exposure or were exposed for 24 h and subsequently placed in a clean static tank system for the remainder of the trial. Individual oysters, fecal material, and tank water were analyzed for oocysts up to 7 days post-exposure via direct immunofluorescence. Oysters held under chronic exposure conditions gradually accumulated oocysts (1.5 or 34.4 oocysts/oyster/day for low or high dose exposure groups, respectively) between days 1 and 7, with an exponential uptake in oocysts observed within the first 24 h post-exposure (mean uptake of 29.6 or 241.9 oocysts/oyster, respectively). Oysters that were transferred to clean water after 24 h were capable of slowly depurating oocysts, following a linear trend. During chronic exposure trials 48–49% of the total spiked inoculum was recovered from oyster tissue, whereas 4.8–5.9% and 38–40% was recovered from tank water and from fecal material at day 7, respectively. In acute exposure trials, 30–31% of the total tank inoculum was found in oysters, suggesting that chronically exposed oysters were likely re-filtering some oocysts. Examinations of oyster fecal material from acute exposures revealed that 72–82% of oocysts recovered were already excreted at the time of oyster transfer (day 1), with only 18–28% being excreted during the static depuration phase. These data support that although most C. parvum oocysts are removed by C. virginica oysters within 24 h, elimination after this point occurs slowly. Additionally, chronic exposures demonstrate that wild or cultured oysters in saline environments that are frequently exposed to sources of Cryptosporidium may be unable to eliminate the parasites at a rate that balances initial uptake.