Detection of Cryptosporidium oocysts and Giardia cysts in the tissues of eastern oysters (Crassostrea virginica) carrying principal oyster infectious diseases.

The potential cross-reactivity of the combined Cryptosporidium/Giardia direct immunofluorescence antibodies (IFA) of MERIFLUOR and HYDROFLUOR-COMBO tests was examined against tissues containing known developmental stages of 12 pathogens causing the principal infectious diseases in oysters. Spores of Haplosporidium nelsoni and Haplosporidium costale produced positive acid-fast stain (AFS) reactions similar in intensity to Cryptosporidium parvum oocysts. Hexamia nelsoni trophozoites produced positive IFA reactions in both IFA tests; however, the intensity of fluorescence was considerably lower and the fluorescein-staining pattern different than those of Giardia cysts. The applicability of AFS for screening oysters for Cryptosporidium oocysts is low, and positive identification of Cryptosporidium oocysts cannot be accomplished based on the AFS. Presumptive IFA identification of the Cryptosporidium oocysts or Giardia cysts in the oyster tissue should fulfill 3 criteria, i.e., bright-green fluorescence of the same intensity as C. parvum oocysts and Giardia cysts in the positive control, correct size and shape of the fluorescein-stained objects, and oocyst or cyst shell clearly visible.