RESUMEN
A histological study was carried out to investigate the transit of Cryptosporidium parvum oocysts through the clam Tapes decussatus. Spat of approximately 5-7 mm shell length were maintained in a tank of natural sea water contaminated with purified C. parvum oocysts. The experiment lasted 240 h and, every 24 h, five specimens were killed, placed in Bouin's fixative, and processed routinely for histological examination. Sections (3 mum) cut from the all body tissues were stained with modified Gomori's trichrome for their accurate identification; the oocysts were detected by a direct immunofluorescence procedure. Oocysts were detected in siphons, gills, stomach, digestive diverticula, and intestine. The oocysts present in the intestine were free or mixed with the intestinal contents; therefore release of these oocysts with the feces should favour dissemination of contamination. Oocysts were found in branchial mucus and within the interfilamentary spaces, which suggests the occurrence of repeated filtrations and the possibility that the retained oocysts maintain their infective capacity.