E. dispar is the new species name for what had been called “noninvasive” or “nonpathogenic” E. histolytica [1–4]. (Earlier reports that E. histolytica and E. dispar could “convert” in culture [5] were an artifact of culture contamination [6].) E. dispar, Entamoeba moshkovskii, Entamoeba coli, Entamoeba hartmanni, and Endolimax nana are nonpathogenic parasites of the human intestine. There has been no correlation established between infection with these organisms and gastrointestinal symptoms, and antiamebic treatment is not warranted. Dientamoeba fragilis and Entamoeba polecki have been occasionally implicated as a cause of diarrhea, and Entamoeba gingivalis is associated with periodontal disease [7].

E. histolytica is a pseudopod-forming nonflagellated protozoan parasite. The E. histolytica life cycle consists of an infective cyst and an invasive trophozoite form. The quadrinucleate cyst is resistant to gastric acidity and desiccation and can survive in a moist environment for several weeks. Only the trophozoite form invades human tissue. Some interesting differences between the biochemical pathways of E. histolytica and those of higher eukaryotes include its lack of glutathione, its use of pyrophosphate instead of ATP at several steps in glycolysis [8], a unique alcohol-aldehyde reductase [9], and its inability to synthesize purine nucleotides de novo. Molecular phylogenetic analysis of eukaryotic organisms based on sequence comparisons of small subunit rRNA has placed Entamoeba on the lowermost branches of the eukaryotic tree, closest to Dictyostelium. However, E. histolytica shares with higher branching eukaryotes mitochondrial genes that are contained within an apparently biochemically inert remnant organelle [1011]. Control of gene expression is fundamentally different from that of other eukaryotes, with a novel conserved sequence (GAAC element) in the RNA polymerase II promoter that specifies the rate and site of mRNA transcription [12]. The genome size of the parasite has recently been estimated to be slightly <20 megabases [13], and the sequencing of the entire genome is under way.


Diagnostic Criteria

Because E. histolytica is identical in appearance to the nonpathogenic parasite Entamoeba dispar, amebic colitis is best diagnosed by detection of E. histolytica in stool.





Entamoeba Histolytica Infection




Infection is acquired by ingestion of food or water containing the cyst form of the parasite Entamoeba histolytica, which is the cause of amebic colitis and liver abscess. The trophozite invades the intestinal epithelium and causes disease by destroying host tissues.

Amebic colitis commonly has a subacute onset with weight loss, and is characterized by diarrhea that usually contains occult or gross blood. Amebic liver abscess is 10 times more common in men than women. A history of alcohol abuse is common, and patients usually present with several weeks of fever and right upper quadrant pain. Most patients with liver abscess do not have concurrent dystentery.



Amebiasis in developed countries is most common in immigrants and travelers returning from the third world, and less common in men who have sex with men and immunosuppressed individuals.


Standard treatment with metronidazole plus a luminal agent cures most patients with invasive amebiasis, and drug resistance has yet to be encountered.


Complications of Injecting Drug Use

  • Local problems—Abscess (Figures 240-2 
    Image not available.

    A 32-year-old woman with type 1 diabetes developed large abscesses all over her body secondary to injection of cocaine and heroin. Her back shows the large scars remaining after the healing of these abscesses. (Courtesy of ­Richard P. Usatine, MD.)

    and 240-3; Abscess), cellulitis, septic thrombophlebitis, local induration, necrotizing fasciitis, gas gangrene, pyomyositis, mycotic aneurysm, compartmental syndromes, and foreign bodies (e.g., broken needle parts) in local areas.2
    • IDUs are at higher risk of getting methicillin-resistant Staphylococcus aureus(MRSA) skin infections that the patient may think are spider bites (Figure 240-4).
    • Some IDUs give up trying to inject into their veins and put the cocaine directly into the skin. This causes local skin necrosis that produces round atrophic scars (Figure 240-5).
  • IDUs are at risk for contracting systemic infections, including HIV and hepatitis B or hepatitis C.
    • Injecting drug users are at risk of endocarditis, osteomyelitis (Figures 240-6and 240-7), and an abscess of the epidural region. These infections can lead to long hospitalizations for intravenous antibiotics. The endocarditis that occurs in IDUs involves the right-sided heart valves (see Chapter 50, Bacterial Endocarditis).2 They are also at risk of septic emboli to the lungs, group A β-hemolytic streptococcal septicemia, septic arthritis, and candidal and other fungal infections.


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A 75-year-old triathlete complains of gradually worsening vision over the past year. It seems to be involving near and far vision. The patient has never required corrective lenses and has no significant medical history other than diet-controlled hypertension. He takes no regular medications. Physical examination is normal except for bilateral visual acuity of 20/100. There are no focal visual field defects and no redness of the eyes or eyelids. Which of the following is the most likely diagnosis?

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Age-related macular degeneration is a major cause of painless, gradual bilateral central visual loss. It occurs as nonexudative (dry) or exudative (wet) forms. Recent genetic data have shown an association with the alternative complement pathway gene for complement factor H. The mechanism link for that association is unknown. The nonexudative form is associated with retinal drusen that leads to retinal atrophy. Treatment with vitamin C, vitamin E, beta-carotene, and zinc may retard the visual loss. Exudative macular degeneration, which is less common, is caused by neovascular proliferation and leakage of choroidal blood vessels. Acute visual loss may occur because of bleeding. Exudative macular degeneration may be treated with intraocular injection of a vascular endothelial growth factor antagonist (bevacizumab or ranibizumab). Blepharitis is inflammation of the eyelids usually related to acne rosacea, seborrheic dermatitis, or staphylococcal infection. Diabetic retinopathy, now a leading cause of blindness in the United States, causes gradual bilateral visual loss in patients with long-standing diabetes. Retinal detachment is usually unilateral and causes visual loss and an afferent pupillary defect.


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A 42-year-old African-American man has been diagnosed with hypertension for the past 10 years and treated with medication. One morning, he is found unresponsive by his wife. He is taken to the emergency department and pronounced dead by the physician. An autopsy revealed cardiac hypertrophy and a narrowing of the aorta just distal to the ligamentum arteriosum, with dilation of the intercostal artery's ostia. How could the death have possibly been prevented?



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