Experimental cryptosporidiosis in fetal lambs

Fetal lambs were infected in utero with purified sporulated oocysts of Cryptosporidium parvum in order to study pathogenesis and host cellular response to the enteropathogen. Ileal loops (IL) of fetuses, 124-130 days of gestation, were inoculated with 1-4 x 10⁶ oocysts usually via cannulae in the abdominal wall of the ewe. Oocysts, both free and phagocytosed, were observed in the IL content as early as day 1 post-inoculation (PI). The percentage of oocysts phagocytosed by the host's polymorphonuclear neutrophils (PMN's) and mononuclear cells remained high up to day 13, the last day of examination. Numerous parasites were observed at days 6,7, and 12 PI in the microvilli of the ileum with hypercellularity of the lamina propria, which consisted of a mixed infiltration of PMN's, mononuclear cells, including lymphoid cells, and a few eosinophils. Cytolysis and extrusion of epithelial cells, often heavily parasitized by various stages of the parasite, as well as inflammatory cells, were prominent in luminal contents. Germinal centers were prominent in mesenteric lymph nodes draining the infected loops by day 12 PI. Depletion of lymphoid cells was already present in Peyer's patches by day 4 PI.