Abstract
Three calves were studied in stages of spontaneous cryptosporidial infection with particular reference to the relation of the cryptosporidia to the follicle-associated epithelium (FAE) over the ileal Peyer's patch (IPP). In early infection scanning electron microscopy and streptavidin immunoperoxidase staining showed marked predilection of cryptosporidia for the FAE. Cryptosporidial antigen was also found in subepithelial tissue, both in the domes over the IPP and in villi, apparently in macrophages, where the parasites seemed to be progressively degraded. The FAE showed long tightly spaced microvilli, replacing normal low folds and protrusions, particularly in late infection. Endocytosis of indian ink was restricted to the cell periphery in late infection, contrasting the normal, more even distribution of endocytosis in the FAE apical cytoplasm. Few parasites were seen in the intestinal mucosa at this stage. At convalescence the FAE was normal, but all stages of infection were characterised by elongation of microvilli in absorptive cells.