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2014-09-14 21:44:08
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  1. How many amoebas are known to parasitize humans?  What is the scientific name for each?
    • Most amoebas are free-living and nonpathogenic
    • 7 species parasitize humans
    • Entamoeba histolytica
    • Entamoeba hartmanni
    • Entamoeba coli
    • Entamoeba polecki
    • Entamoeba gingivalis
    • Endolimax nana
    • Iodamoeba butschlii
    • *NOTE- Naegleria, Acanthamoeba, and Hartmanella are free-living, but may accidentally parasitize
  2. Describe the two types of pseudopods we learned about
    • Lobopod: blunt-ended pseudopodia of amoeba
    • Filopoda: thread-like pseudopod found in some amoeba
  3. (detailed) Describe the nuclear appearance and morphology of genus Entamoeba
    • Vesicular nucleus (fluid-filled, appears hollow) that contains a small endosome (karyosome)
    • Chromatin granules arranged around periphery of nucleus
    • Cytoplasm contains many food vacuoles (important diagnostic tool)
    • Lysosomes, ER, and ribosomes are abundant (electron microscope studies)
    • Helical bodies (packaged RNA including ribosomes) may be found in some trophozoites
    • helical bodies clump to form chromatoidal bars in the cyst stage, which stain darkly with basic dyes (shape can be diagnostic tool)
  4. What is the best known species of parasitic amoeba?  What is its associated disease?  How many people does it infect?
    • Entamoeba histolytica
    • amoebic dystentery (amoebiasis)
    • infects 10% of world's population from north to south pole!
  5. When and where was E. histolytica discovered? By whom?  Describe the process
    • Discovered in 1875 in St. Petersburg Russia by D. F. Losch
    • Microscopic examination of stool from a child w/ amoebiasis showed large # of E. histolytica (w/ RBC in vacuoles)
    • A dog was infected, developed amoebiasis, and died.
    • Autopsy revealed identical ulcers (filled with amoebas) in the dog instestinal mucosa and the child's intestinal mucosa
  6. (detailed) Describe the morphology and life cycle of E. histolytica
    • trophozoite -> precyst -> cyst -> metacystic trophozoite
    • trophozite: (active form) lives in large intestine and feeds on RBC to form ulcers
    • *NOTE- this ulceration is what leads to amoebiasis
    • Multiply via binary fission within the gut
    • Stimulated to become precyst during dehyration of feces
    • Highly resistant cysts can survive in the enviornment for up to one month
    • Cysts that are ingested pass through the stomach unharmed
    • The alkaline pH of the intestines begins excystation, and the tetra-nucleated cyst becomes 8 metacystic trophozoites
    • the metacystic trophozoites travel to the large intestine where they continue to feed, grow, and multiply into adult trophozoites
  7. Describe the size, movement, and morphological characteristics of the E. histolytica trophozoite
    • 20-30um in diameter
    • actively crawl with monopod (but rarely seen in slides)
    • filopod rarely produced
    • Single nucleus w/ prominant endosome
    • ectoplasm: thin, clear outer region of cytoplasm
    • endoplasm: thick, granular inner region of cytoplasm where food vacuoles containing RBCs are found
    • multiply via binary fission in human gut
    • NO chromatoidal bars (helical bodies are present)
    • Stimulated into precyst during dehydration of feces in stool formation
  8. Describe the size, movement, and morphological characteristics of the E. histolytica precyst
    • Shrink in size as food vacuoles are extruded
    • Very little movement
    • More spherical than trophozoite
    • Very rich in glycogen (carbohydrate storage)
    • *NOTE- large glycogen vacuole may occupy most of the cytoplasm
    • Chromatoidal bars form in this stage (rounded at ends)
  9. Describe the size, movement, and morphological characteristics of the E. histolytica cyst
    • formed after the precysts secretes tough hylaine cyst wall
    • Spherical, 10-20um wide
    • 2-4 vesicular nuclei
    • infective stage
    • highly resistant to desiccation and chemicals, they can survive ~1 month in water and ~2 weeks on dry land
  10. Describe the size, movement, and morphological characteristics of the E. histolytica metacystic trophozoite
    • After the cyst is passed through the stomach unharmed (cyst wall) the alkaline medium of the intestine facilitates excystation
    • The cyst wall dissolves and the tetra-nucleated organism undergoes mitosis producing 8 small uni-nucleated metacystic trophozoites
    • These trophozoites pass into the large intestine where they feed, grow, and reproduce (similar to adult trophozoites except size)
  11. Describe the epidemiology of E. histolytica. Include distribution/prevailance, high-risk groups, major sources of infection, and potential vectors
    • Globally distributed 
    • 3rd most common cause of parasitic deaths (>100,000/yr)
    • ~500 million infected at any time. 10% symptomatic
    • Higher prevalence in areas of crowding and poor sanitation (85% of Mexicans vs 15% Americans)
    • High risk groups include homosexuals (90%+ in some groups), immunocompromised, and children
    • Mode of infection is my ingesting cysts (usually from contaminated hands/food/water)
    • Main source of infection is asymptomatic carrier or chronic patient (luminar or chronic amoebiasis)
    • Acute carriers don't typically pass infective cysts
    • Flies and roaches may be mechanical vectors of the infective cyst stage by feeding on infective feces then defecating on food
  12. What is the difference between luminar, chronic, and invasive amoebiasis?
    • *NOTE- caused by E. histolytica
    • Luminar/Chronic amoebiasis: asymptomatic carriers
    • main source of infection
    • Invasive amoebiasis: acute carriers
    • usually pass noninfectious trophozoites in diarrhea
  13. Describe the pathogenicity of E. histolytica. (symptoms and stages of the disease)
    • Trophozoites live and multiply indefinitely in the intestinal mucosa
    • Food vacuoles contain RBCs, WBCs, etc- leading to amoebic dysentary
    • can secrete proteolytic enzymes that enable access to submucosal tissue, causing vast destruction of body tissues (extrainestinal amoebiasis)
    • hepatic amoebiasis: E. histolytica trophozoites are carried to the liver by hepatic portal system 
    • stage detected by observing necroses of hepatic cells (leading to abscesses)
    • most common organ affected
    • pulmonary amoebiasis: lung being affected by E. histolytica, which can lead to secondary bacterial infections (dead tissue attracts bacteria)
    • secondary amoebiasis: brain, heart, spleen, or skin may also become affected
  14. What is the percentage of asymptomatic vs symptomatic infections for amoebiasis? Intestinal vs extraintestinal amoebiasis?
    • asymptomatic 85-95%, symptomatic 5-15%
    • intestinal 95%, extraintestinal 5%
  15. What factors affect the severity of infection for E. histolytica?
    • Host's natural or acquired resistance to the amoeba
    • Hosts's physical or emotional conditions
    • Location of infection
    • Intensity of infection
  16. Describe the specific symptoms of the two types of amoebiasis.
    • Acute amoebiasis: severe diarrhea comprised of bloody mucus (15-50 times per day)
    • cramps, vomiting, headache, general weakness develop after 1-4 weeks (incubation period)
    • fever of 100-102F
    • Chronic amoebiasis: continuous attacks of diarrhea with intervening periods of milder intestinal problems
  17. What is the most serious form of amoebiasis?  Why?
    • Hepatic amoebiasis
    • abscesses may rupture the abdominal wall or extend through the diaphragm to the lungs (fatal)
  18. Describe the process of diagnosis for E. histolytica
    • Requires microscopic fecal preparations and ID of trophozoites or cysts
    • Especially important in asymptomatic infections (cysts may be scant)
    • serological tests can be used to detect tissue invasion
    • X-Ray scans may reveal abscesses in the liver
  19. Describe the treatment of E. histolytica
    • Basic cure is difficult to obtain
    • Chemotherapy destroys trophozoites (relieves symptoms) and controls secondary infection (bacteria)
    • Metronidazole is drug of choice (minimal side effects)
    • Bed rest and bland diet recommended
  20. Describe the prevention and control of E. histolytica
    • Contaminated water/food will ALWAYS lead to transmission of amoebiasis
    • Water should be boiled before drinking (1 minutes kills parasites, bacteria, and viruses)
    • Drink only bottled or carbonated water or canned/bottled sodas.
    • NOT KILLED by Cl, I, chemical water purification tablets
    • Drink only pasteurized milk and dairy
    • Avoid ice cubes, salads, and peeled fruits
    • Food should be protected from roaches and flies (mechanical vectors)
    • Piped water should be well maintained
    • Food should be thoroughly cooked to kill parasites/bacteria/viruses
    • Raw vegetables should be washed with a strong detergent soap before eating
    • Always wash hands thoroughly with soap and warm water
    • Food handlers in endemic areas should undergo screening and be treated if necessary
    • Broad education to improve sanitation
    • Ban on using untreated human feces as fertilizer