Download microbiology ch 53 [9-4

Micro Chapter 53
Entamoeba histolytica

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)
 Often cause few or no symptoms
 Morphologically indistinguishable from Entamoeba histolytica (avirulent strain); identified only by biochemical
or nucleic acid-based techniques
 Transmitted through fecal-oral route
 2 forms: actively growing vegetative trophozoite and dormant highly resistant cyst
 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
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
 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
 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
 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
 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
 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
 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
 Signs of malnutrition resulting from malabsorption may occur as result of extensive, prolonged infection
 Not invasive and doesn’t produce bloody diarrhea or metastatic infection
o
o
Host responds w/submucosal infiltrate of lymphocytes and effacement of intestinal villi
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
 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
 Treated w/metronidazole, but relapses can occur
 Tinidazole and nitazoxanide can be used to treat
 Can be prevented by boiling or filtering drinking water or treating it w/adequate amounts of iodine or chlorine
Cryptosporidium
 Causes zoonosis obtained from cattle; can spread from person to person in close quarters (i.e., day care)
 Particularly troublesome in patients w/advanced AIDS who lack immune mechanisms necessary to resolve
 Oocysts found in most surface waters in country, are highly resistant to chlorine, and infectious particles
expelled in huge numbers in watery stool
 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
 Diagnosis made by identifying acid-fast cysts in stool; can use stool antigen detection assay
 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
 Protozoal parasite that produces acid-fast cysts in stools; larger than cryptosporidia
 Outbreaks have been linked to ingestion of raspberries imported from Central America
 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
 Intracellular infection of apical intestinal epithelial cells; don’t invade beyond intestinal mucosa
 Infection causes watery diarrhea associated w/loss of appetite, bloating, cramps, nausea, vomiting, fatigue,
muscle aches, and low-grade fever
 Relapses common
 Diagnosis depends on identification of large acid-fast oocysts in stool
 Treatment w/trimethoprim/sulfamethoxazole relieves symptoms and shortens course of infection
Other Intestinal Parasites
 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
 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
 Common inhabitant of vagina in 15% or more of women that occasionally causes vaginitis
 Trichomonas hominis less commonly found in GI tract; T. tenax found in mouth
 Infection transmitted by sexual intercourse
 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
 Flagellates found in wet preps of vaginal secretions from infected women; infection of male partners safely
assumed; single dose metronidazole or tinidazole treatment recommended
 In pregnant women, trichomoniasis associated w/adverse pregnancy outcomes; single-dose metronidazole safe
and effective