A veterinarian presents with a chief complaint of thin, watery, non-bloody diarrhea for the past two weeks. He appears fatigued and mildly dehydrated. Stool examination is negative for leukocytes and culture for bacterial pathogens is negative. Acid fast stain of the stool demonstrates oocysts that are 5-7 microns in diameter. Where does the sexual phase of this organism's life cycle occur?

This is a case of cryptosporidiosis, which is acquired by fecal/oral transmission between human beings, or between animals and man. The intracellular location of these parasites is distinctive from that of Toxoplasma, Isospora, or Cyclospora, the other coccidian parasites infecting epithelial cells of animals. This organism invades the brush border of the intestine, causing blebs to form on the luminal surface of the cells, the most superficial of all the intracellular locations within this group. Balantidium coli, which does not have a sexual phase in its life cycle, attaches to the colonic mucosa of swine. Asexual stages of Giardia lamblia occur extracellularly in the small intestine of muskrats and beavers. The sexual stages of Toxoplasma gondii occur intracellularly in the intestinal mucosa of felines. Ref: Ray C.G., Ryan K.J. (2010). Chapter 50. Sporozoa. In C.G. Ray, K.J. Ryan (Eds), Sherris Medical Microbiology, 5e.