A study was carried out on rainbow trout Oncorhynchus mykiss fed with a commercial feed contaminated with bovine-isolated Cryptosporidium parvum oocysts whose viability and infectivity were previously tested by inoculation of oocysts to neonatal Swiss CD-1 mice. Histological examination of hematoxylin-eosin-stained gastrointestinal sections from control and C. parvum-exposed fish revealed no life-cycle stages of Cryptosporidium in any part of the apical border of the digestive tract. However, sections of the stomach and pyloric region from exposed fish displayed large numbers of 5-7-microm spherical structures located deep within the epithelial tissue. Under conditions of host stress, the number of these structures increased remarkably. An immunofluorescence antibody test with IgG and IgM anti-cryptosporidial antibodies revealed fluorescence reactivity in these structures. Simultaneously, wild trout were analyzed in order to detect natural cryptosporidial infections; Cryptosporidium oocyst-like bodies were found in the intestinal content of 10% of the specimens.
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