Necator americanus and Ancylostoma duodenale. These hookworms infect 1 billion individuals worldwide and cause significant morbidity. Infection is initiated by larval penetration through the skin. After further development in the lungs, the larvae migrate up the trachea and are swallowed. Once in the duodenum, the larvae mature and the adult worms attach to the mucosa, suck blood, and reproduce. Hookworms are a leading cause of iron-deficiency anemia in the developing world. Detection of eggs in the stools is used for diagnosis.
Giardia lamblia. This flagellated protozoan, also referred to as Giardia duodenalis or Giardia intestinalis, is the most common pathogenic parasite in humans and is spread by fecally contaminated water or food. Infection may occur after ingestion of as few as 10 cysts and is characterized by acute or chronic malabsorptive diarrhea. Because cysts are resistant to chlorine, Giardia organisms are endemic in unfiltered public and rural water supplies. In the acidic environment of the stomach, excystation occurs and trophozoites are released. Secretory IgA and mucosal IL-6 responses are important for clearance of Giardia infections, and immunosuppressed, agammaglobulinemic, or malnourished individuals often are severely affected. Giardia evades immune clearance through continuous modification of the major surface antigen, variant surface protein, and can persist for months or years while causing intermittent symptoms. Giardia infection reduces expression of brush border enzymes, including lactase, and causes microvillous damage and apoptosis of small-intestinal epithelial cells. Giardia trophozoites are noninvasive and can be identified in duodenal biopsy specimens by their characteristic pear shape. The availability of a stool antigen assay for Giardia has simplified the diagnosis of Giardia infection.
• Entamoeba histolytica. This protozoan causes amebiasis and is spread by fecal-oral transmission. E. histolytica infects approximately 500 million people in countries such as India, Mexico, and Colombia, and causes 40 million cases of dysentery and liver abscess annually. While amebiasis affects the cecum and ascending colon most often, the sigmoid colon, rectum, and appendix can also be involved. Dysentery develops when the amebae attach to the colonic epithelium, induce apoptosis, invade crypts, and burrow laterally into the lamina propria. This recruits neutrophils, causes tissue damage, and creates a flask-shaped ulcer with a narrow neck and broad base. Parasites may penetrate splanchnic vessels and embolize to the liver to produce abscesses in about 40% of patients with amebic dysentery. Amebic liver abscesses, which can exceed 10 cm in diameter, have a scant inflammatory reaction at their margins and a shaggy fibrin lining. The abscesses persist after the acute intestinal illness has passed and may rarely reach the lung and the heart by direct extension. Amebae may also spread to the kidneys and brain via the bloodstream.
Individuals with amebiasis may present with abdominal pain, bloody diarrhea, or weight loss. Occasionally, acute necrotizing colitis and megacolon occur, and both are associated with significant mortality.
Vibrio cholerae secretesapreformedtoxinthatcausesmassivechloridesecretion.Waterfollowstheresultingosmoticgradient,leadingtosecretorydiarrhea.