1. Case Discussion
報告人：I2 劉孟達

2. Present illness
A 22-year-old male college student, who had recently returned from a surfing trip to Acapulco, Mexico, presented to the emergency department suffering from crampy abdominal pain, malaise, nausea, fever, and bloody, mucoid diarrhea. Stool specimens were collected and sent to the laboratory for routine culture for enteric bacilli and examination for ova and parasites.

3. Lab finding ： Stool cultures were negative for bacterial pathogens
A moderate number of ameboid trophozoites, measuring 20 to 30 μm, with finely granular cytoplasm, evenly distributed peripheral nuclear chromatin, and ingested red blood cells were seen in the permanent trichrome stain.

4. Q1
Based on the morphological description given, which intestinal parasite would you suspect is causing this patient's infection? Could this parasite be confused with nonpathogenic parasites? If so, which nonpathogenic parasites could it be confused with?
Ans：
1. Entamoeba histolytica
2. Entamoeba dispar , Entamoeba coli and Entamoeba hartmanni

5. Acute amebic colitis has a gradual onset presenting with a 1- to 2-week history of abdominal pain, diarrhea, and tenesmus. Stool samples, which are watery and contain blood and mucus, have little fecal material. Fever is noted in only a minority of patients. Lower quadrant abdominal tenderness may be noted.
The trophozoite of E histolytica averages 25 μm, ranging from mm. It has a clear ectoplasm and a somewhat granular endoplasm containing several vacuoles. The trophozoite has a single 3- to 5- μm nucleus with fine peripheral chromatin and a central nucleolus. Ingested RBCs may be present within the trophozoite.

6. Other Problems to be Considered:
Infectious
Shigella
Salmonella
Enteroinvasive Escherichia coli
Enterohemorrhagic Escherichia coli
Noninfectious
Inflammatory bowel disease
ischemic colitis
Diverticulitis

7. 左上：Entamoeba histolytica 左下：Salmonella 右： Shigella

8. Q2
Although no information regarding the stool consistency (formed, semisolid, or liquid) was given, what would you suspect the consistency of this patient's stool to be? Why?
Ans： liquid
Formed → 可在stool中見到cyst
semisolid →可在stool中見到cyst 及trophozoite
liquid →可在stool中見到trophozoite

9. Q3
Which nonpathogenic parasite is morphologically indistinguishable from this parasite? What characteristic can be used to differentiate pathogenic from nonpathogenic species of this parasite?
Ans：
1. Entamoeba dispar is a nonpathogenic protozoon morphologically identical to E histolytica. These 2 species of Entameba can be distinguished by the monoclonal antibodies.
2. Other morphologically distinct organisms, such as Entamoeba coli and Entamoeba hartmanni, are also nonpathogenic

10. Entamoeba hartmanni trophozoites (4-12 μm) are similar to E
Entamoeba hartmanni trophozoites (4-12 μm) are similar to E.histolytica trophozoites, present a small karyosome often eccentrical.
Entamoeba coli trophozoites measure μm and have a vescicolous nucleus with a large eccentric karyosome and an irregulary distributed peripheral chromatin. The cytoplasm is vacuolated containing bacteria and yeast.

11. Q4
Is this parasite capable of causing extraintestinal infection? Explain.
Ans：
Extraintestinal disease
* Liver abscess
* Pleuropulmonary disease
* Peritonitis
* Pericarditis
* Brain abscess
* Genitourinary disease

13. Q5 How is this parasite transmitted? Ans：
→ Humans are the only reservoir of E histolytica. Cysts passed in the feces can survive in moist environmental conditions for weeks to months. Upon ingestion of fecally contaminated food or water, the cysts travel to the small intestine, where the trophozoites are released. In 90% of patients, the trophozoites re-encyst and produce asymptomatic infection, which usually resolves spontaneously within 12 months. In the remaining10% of patients who are infected, the parasite causes symptomatic amebiasis. Under unfavorable conditions, the trophozoite reverts to the cyst form, and the life cycle is repeated.

15. Q6
What laboratory techniques are recommended to diagnose infection with this parasite?
Ans：
Enzyme immunoassay (EIA): This is the best test for making the specific diagnosis of E histolytica in the clinical setting
Indirect hemagglutination antibody (IHA) test detects antibody specific for E histolytica.
Immunodiffusion (ID) is simple to perform, making it ideal for the laboratory that has only an occasional request for amebic serology
immunoglobulin M (IgM) antibodies specific for E histolytica has been reported,

16. Q7 Should this patient be treated? How? Ans：
Asymptomatic intestinal infection may be treated with iodoquinol, paromomycin, or diloxanide furoate.
drugs for treatment of symptomatic intestinal disease and for hepatic abscess are metronidazole and tinidazole.
Surgical intervention may be necessary in
→Patients with severe colitis requiring intravenous volume replacement
→Patients with fulminant colitis that may require surgical intervention
→Patients with liver abscess of uncertain etiology or not responding to therapy
→Patients with suspected liver abscess rupture

17. Q8
Discuss the pathogenesis of amebiasis.
Ans：

18. Invasive disease begins with the adherence of E histolytica to colonic mucins, epithelial cells, and leukocytes. Adherence of the trophozoite is mediated by a galactose-inhibitable adherence lectin.
After adherence, the trophozoites of E histolytica lyse the target cells by using lectin to bind to the target cells' membranes and using the parasite's ionophorelike protein to induce a leak of ions (ie, Na+, K+, Ca+) from the target cell cytoplasm.
A number of hemolysins, encoded by plasmid (ribosomal deoxyribonucleic acid [rDNA]) and cytotoxic to the intestinal mucosal cells.
An extracellular cysteine kinase causes proteolytic destruction of the tissue, producing flask-shaped ulcers

19. Q9 Describe the life cycle of this parasite. Ans：
→ Humans are the only reservoir of E histolytica. Cysts passed in the feces can survive in moist environmental conditions for weeks to months. Upon ingestion of fecally contaminated food or water, the cysts travel to the small intestine, where the trophozoites are released. In 90% of patients, the trophozoites re-encyst and produce asymptomatic infection, which usually resolves spontaneously within 12 months. In the remaining10% of patients who are infected, the parasite causes symptomatic amebiasis. Under unfavorable conditions, the trophozoite reverts to the cyst form, and the life cycle is repeated

20. Q10
Discuss the epidemiology of this infection.
Ans：
→Approximately 10% of the world's population is infected by either E histolytica or E dispar. Amebiasis affects about 50 million persons each year, resulting in approximately 100,000 deaths, all of which are due to E histolytica. Incidence of amebiasis is higher in developing countries. Areas of high prevalence include the Indian subcontinent, southern and western Africa, the Far East, South America, and Central America

21. Q11
Discuss the prevention and control of amebiasis.
Ans：
→Educate patients about the prevention of amebiasis during travel to endemic areas. This includes avoiding drinking contaminated water and avoiding eating raw fruits and salads, which are difficult to sterilize. Bottled water may be used during such travel. Eating only cooked food or self-peeled fruits in endemic areas minimizes risk

22. Referance
1. Chen KT, Malison MD:Outbreak of Scombroid fish poisoning, in Taiwan. Am J pub Hlth 1987;77:
2. Lai SC, Chu YJ, Kno CF, et al: A survey of hyperuricemia in Tungau and Tungyueh of Ilam County. Department of Health Executive Yuan ROC. Epidemiology Bulletin 1991;7:
3. Weinstin L, Swartz MN: Pathogenic properties of invading microorganisms. In: Sodeman WA Jr, Sodeman WA, eds. Pathologic Physiology: Mechanisms of Disease. Philadelphia: WB Saunders 1974: 472p.
醫用寄生蟲學- 杜文圓 編譯 p22-p57