Micro Chapter 53
Entamoeba histolytica
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Causes amebiasis; may cause destruction of host tissue, especially in colon o Lesions start as small ulcerations of intestinal epithelium; amebas in lesions spread laterally as they encounter deeper layers of colon (flask-shaped ulcers) o May spread through portal circulation to produce abscesses in liver (less commonly lung or brain)
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Often cause few or no symptoms
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Morphologically indistinguishable from Entamoeba histolytica (avirulent strain); identified only by biochemical or nucleic acid-based techniques
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Transmitted through fecal-oral route
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2 forms: actively growing vegetative trophozoite and dormant highly resistant cyst
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Patients w/diarrhea pose minor threat of transmission because they excrete actively growing yet labile trophozoites easily destroyed by drying in environment or acid in stomach if ingested o Asymptomatic patients excrete cyst form of parasite
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 o Doesn’t require period of maturation in environment, so transmission not restricted to warm climate
Can be anal-oral or oral-genital transmission (sexual transmission)
Amebae adhere to colon via surface lectin on receptors on host cells containing digalactose residues o Attachment to cells inhibited by adding galactose, intestinal mucus
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Damage to host cell: receptor-mediated attachment to target cell via Gal-galNAc binding lectin, contactdependent killing (insertion of pore-forming proteins (ameba-pores) into host PM), ingestion of killed host cell by ameba o Pathogenic strains of amebas kill neutrophils and non-activated macrophages o Certain activated macrophages can kill amebas (host immunity) o Giving steroids (suppress cell-mediated immunity) causes disseminated infection despite high titers of antibodies o Amebae produce cysteine proteinase that digests IgA, IgG, and other proteins involved in humoral immune response
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Diagnosed by microscopic identification of trophozoites in freshly passed dysenteric stool or scrapings from colonic ulcers obtained through sigmoidoscope o Stool immunoassay for E. histolytica antigen may be more sensitive than microscopic exam and in some formats capable of distinguishing between E. histolytica and E. dispar o Isoenzyme analysis of cultured organisms or PCR required to make definitive diagnosis o Antiameba antibodies persist for years after infection, so serology can’t distinguish past from present
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Drug of choice for active amebic infection is metronidazole; penetrates well into most tissues, including brain o Amebas carry out anaerobic metabolism and convert metronidazole to active form o Less efficient at killing amebas in intestinal lumen, so diloxanide, paromomycin, or diiodohydroxyquin used for that w/metronidazole
Giardia lamblia
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Patients w/diarrhea pose minor threat of transmission because they excrete actively growing yet labile trophozoites easily destroyed by drying in environment or acid in stomach if ingested
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Causes giardiasis; zoonosis that may be acquired by ingestion of water contaminated by feces from animal or human carriers; cysts resistant to chlorine, so outbreaks in municipal water systems happen o Can spread fecal-oral
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Acquired by ingestion of cyst form; highly resistant to environment; can be found in “pure” mountain streams o Stomach acid stimulates cysts to transform into vegetative trophozoite form in duodenum o Trophozoites attach to epithelium of duodenum and jejunum using ventral sucking disc o Vegetative forms have characteristic face w/mustache flagella
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Signs of malnutrition resulting from malabsorption may occur as result of extensive, prolonged infection
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Not invasive and doesn’t produce bloody diarrhea or metastatic infection

o Host responds w/submucosal infiltrate of lymphocytes and effacement of intestinal villi o Malabsorption of fats can lead to greasy, foul-smelling stools; diarrhea associated w/unabsorbed fatty acids in lumen; deficiencies of fat-soluble vitamins; and weight loss
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Diagnosed by identification of parasites in stool or duodenal aspirates (cysts) o Cysts small, ovoid, non-motile bodies w/4 nuclei o Antigen detection assay for giardiasis available; more sensitive than single microscopic stool exam
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Treated w/metronidazole, but relapses can occur
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Tinidazole and nitazoxanide can be used to treat
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Can be prevented by boiling or filtering drinking water or treating it w/adequate amounts of iodine or chlorine
Cryptosporidium
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Causes zoonosis obtained from cattle; can spread from person to person in close quarters (i.e., day care)
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Particularly troublesome in patients w/advanced AIDS who lack immune mechanisms necessary to resolve
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Oocysts found in most surface waters in country, are highly resistant to chlorine, and infectious particles expelled in huge numbers in watery stool
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Infectious oocysts forms produced in intestine and spread to other animals; don’t invade past intestinal mucosa or disseminate to produce systemic infection o Carry out entire life cycle among microvilli of small intestine, where they multiply at apical end of epithelial cells and are released back to luminal surface o In immunocompetent individuals, life cycle takes place once or twice, resulting in single episode of diarrhea that lasts 2 weeks or less o In immunocompromised patients, life cycle of organism repeated many times and associated w/persistent and intractable watery diarrhea
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Diagnosis made by identifying acid-fast cysts in stool; can use stool antigen detection assay
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Nitazoxanide has activity against cryptosporidia in immunocompetent hosts, but doesn’t help compromised o Supportive therapy w/rehydration and antimotility agents is mainstay of immunocompromised; successful treatment w/antiretroviral therapy and restoration of cell-mediated immunity can improve
Cyclospora cayetanensis
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Protozoal parasite that produces acid-fast cysts in stools; larger than cryptosporidia
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Outbreaks have been linked to ingestion of raspberries imported from Central America
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Oocysts not infectious when excreted in human feces; parasite becomes infectious (sporulate) only after days to weeks of incubation in environmental sites w/warm temps and high humidity, so most infections acquired by ingestion of contaminated food or water
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Intracellular infection of apical intestinal epithelial cells; don’t invade beyond intestinal mucosa
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Infection causes watery diarrhea associated w/loss of appetite, bloating, cramps, nausea, vomiting, fatigue, muscle aches, and low-grade fever
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Relapses common
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Diagnosis depends on identification of large acid-fast oocysts in stool
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Treatment w/trimethoprim/sulfamethoxazole relieves symptoms and shortens course of infection
Other Intestinal Parasites
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Isospora belli – protozoan that causes transient watery diarrhea in healthy individuals; occurs more frequently in tropical areas; in U.S., occurs mostly in AIDS patients (get persistent watery diarrhea) o Diagnosis made by examination of stool for characteristic oocysts o Trimethoprim/sulfamethoxazole effective in controlling infection in immunocompromised patients
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Microsporidia – obligate intracellular parasites very small, lack mitochondria, and possess small rRNA
(prokaryotic origin) o Various species associated w/infections of GI tract, respiratory tract, urinary tract, liver, brain, & eye o Symptomatic intestinal infection associated w/Enterocytozoon bieneusi
 Causes transient diarrhea in healthy hosts but protracted watery diarrhea in AIDS patients
 Organism infects mucosal epithelial cells; can disseminate to distant sites in AIDS patients
(biliary tree and cause cholangitis)
 Diagnosis made by microscopic examination of stool or intestinal biopsy material (stained)
 Common antibacterial and antiprotozoal drugs not very effective

 Albendazole (antihelminthic drug) isolated from fungus Aspergillus fumigatus (fumagillin) used with some success
Trichomonas vaginalis
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Common inhabitant of vagina in 15% or more of women that occasionally causes vaginitis
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Trichomonas hominis less commonly found in GI tract; T. tenax found in mouth
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Infection transmitted by sexual intercourse
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Vaginitis associated w/frothy creamy discharge; most male partners of symptomatic women become infected but majority of infections in men asymptomatic o Male symptoms can include mild urethritis, epididymitis, or prostatitis
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Flagellates found in wet preps of vaginal secretions from infected women; infection of male partners safely assumed; single dose metronidazole or tinidazole treatment recommended
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In pregnant women, trichomoniasis associated w/adverse pregnancy outcomes; single-dose metronidazole safe and effective