Life Cycle and Transmission

Infection is via the fecal-oral route and C. parvum can infect epithelial cells of the gastrointestinal and respiratory tracts, and the biliary and pancreatic ducts. Cryptosporidium is so named because the oocyst contains 4 naked sporozoites i.e. the sporocyst is either concealed or absent.  (FIGURE 1).

http://www.ksu.edu/parasitology/basicbio

http://www.biosci.ohio-state.edu/~parasite/cryptosporidium.html

Figure 1. Diagrammatic representation of C. parvum life cycle (after Fayer)

 Like that of other coccidia (see Isospora belli) the oocyst wall of C. parvum has distinct inner and outer layers with a suture at one end. (FIGURE 2A).

            Figure 2. A. Intact oocyst; B. Sporozoites emerging from oocyst

                                    Su, suture; Sp, sporozoite

The infection is acquired by the ingestion of water or food contaminated with mature oocysts passed in the feces. Once in the intestine, the trypsin-sensitive suture dissolves during excystation, thereby opening the wall through which the sporozoites leave the oocyst. (FIGURE 2B) The sporozoites attach to the enterocytes, and by means of the apical organelles—rhoptries and micronemes--entry into the enterocyte is gained (FIGURE 3).

Figure 3. A. Sporozoite by TEM      B. Sporozoite (diagrammatic)

Ec, electron dense collar; Pl, pellicle; Mn, micronemes; Rb, residual body; Rh, rhoptry; Art 1,2, apical rings; Im, inner membrane; Mt, microtubule;Gc, Golgi complex; C, conoid; Ct, cytostome; Pr, polar rings; NI, nucleolus

 In addition to these apical organelles the sporozoites also have a pellicle, electron dense granules, ribosomes, subpellicular microtubules, nucleus and apical rings, but lack mitochondria, polar rings, micropore and conoid. Within the enterocyte the trophozoite (and schizont) are located extracytoplasmically, just under the brush border

Figure 4. Scanning EM of  C. parvum               Transmission EM of C. parvum

Ec, electron dense collar; FI, fibrous layer; El, electron dense layer (From Fayer at al)

Two schizogonic cycles take place: type I schizonts produce 8 merozoites, and type II produce 4 merozoites. The latter invade fresh enterocytes and undergo gametogony to produce macro and microgametes. There is fusion of gametes, and the formation of one of two kinds of oocysts: thin-walled and thick-walled. Sporogony takes place within the oocyst and gives rise to 4 sporozoites. The thick-walled oocysts are passed out with the feces, whereas the thin-walled oocysts, which are already infective, can excyst within the lumen of the intestine, resulting in autoinfection.  (FIGURE 5).

Figure 5. TEM of oocysts. A. Thin-walled and B. Thick walled

Ow, outer wall; Lb, lipid body; Sp, sporozoite; Ec, electron dense collar; Fo, feeder, organelle; Pv, parasitophorous  vacuole; Su, suture

In this way an infection can begin anew without ingestion of oocysts. The  time after ingestion of infective oocysts to complete the life cycle and to have new oocysts excreted is 4-22 days in humans and 2-7 days for calves.