Location: Small intestine, from the lower third of duodenum to 2 or 3 feet from the ileocecal valve.
Geographic Distribution: North America (Iowa), USSR (Kazakhstan).
Morphology: This species has been described by Biester (1934) and Biester and Murray (1934). The oocysts are subspherical to ellipsoidal, becoming more ellipsoidal on sporulation. The oocyst wall is often stretched by the oocysts and pinched in between them. It is smooth, composed of 2 layers, brownish yellow, and 1.5 u thick. A micropyle is absent. The unsporulated oocysts measure 20 to 24 by 18 to 21 u with a mean of 22.5 by 19.4 u. An oocyst polar granule is present. An oocyst residuum is absent. The sporocysts are ellipsoidal, 16 to 18 by 10 to 12 u with a me an of 16.4 by 11.2 u. The sporocyst wall is double, 0.7 u thick. The sporozoites are elongate. A sporocyst residuum is present. A Stieda body is absent. The sporulation time is 4 days.
Life Cycle: According to Biester and Murray (1934), I. suis invades the epithelial cells of the intestine. Many of these invaded cells migrate to a subepithelial position, but often both the host cells and the parasite appeared to undergo retrogressive changes and to be desquamated. The prepatent period after experimental infection is 6 to 8 days, and oocysts continue to be eliminated for about 8 days after a single infective feeding.
Pathogenesis: According to Biester and Murray (1934), I. suis causes a catarrhal enteritis. The epithelium of the crypts is destroyed except near the intestinal lumen. The substantia propria of the tips of the villi is destroyed, leaving a reticular honeycomb without cells or nuclei. Interstitial inflammation with marked eosinophilic infiltration is present, but there is no gross hemorrhage. Diarrhea began about the 6th day af after experimental infection, continued for 3 or 4 days, and was followed by constipation. I. suis infections are apparently not fatal, but they may retard growth and produce unthriftiness.
Cross-Transmission Studies: Biester and Murray (1934) and Biester (1934) reported that attempts to transmit I. suis to guinea pigs, rats, and dogs were unsuccessful.