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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
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Describe the two types of pseudopods we learned about
- Lobopod: blunt-ended pseudopodia of amoeba
- Filopoda: thread-like pseudopod found in some amoeba
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(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)
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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!
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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
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(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
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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
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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)
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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
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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)
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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
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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
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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
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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%
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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
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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
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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)
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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
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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
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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
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