infectious diseases
Amebiasis
Diagnosis of Amebiasis
Intestinal infection: Microscopic examination, enzyme immunoassay of stool, molecular tests for parasite DNA in stool, and/or serologic testing Extraintestinal infection: Imaging and serologic testing or a therapeutic trial with an amebicide Nondysenteric amebiasis may be misdiagnosed as Irritable Bowel Syndrome (IBS) , Amebic dysentery may be confused with Shigellosis , Hepatic amebiasis and amebic abscess must be differentiated from other hepatic infections and tumors. Patients with amebic liver abscess often present with right upper quadrant pain and fever. Amebic liver abscess is more common in men and younger adults exposed to endemic areas, whereas pyogenic liver abscess is more common in older patients. Also, symptoms of Echinococcosis are unusual until the cyst grows to 10 cm in diameter, and Diagnosis of amebiasis is supported by finding amebic trophozoites, cysts, or both in stool or tissues; however, pathogenicE. histolyticaare morphologically indistinguishable from nonpathogenicE. dispar, as well asE. moshkovskiiandE. bangladeshi, which are of uncertain pathogenicity. Immunoassays that detectE. histolyticaantigens in stool are sensitive and specific and are done to confirm the diagnosis. Specific DNA detection assays forE. histolyticausing polymerase chain reaction are available at diagnostic reference laboratories and have very high sensitivity and specificity. Serologic tests are positive in About 95% of patients with an amebic liver abscess > 70% of those with active intestinal infection 10% of asymptomatic carriers Enzyme immunoassay (EIA) is the most widely used serologic test. Antibody titers can confirmE. histolyticainfection but may persist for months or years, making it impossible to differentiate acute from past infection in residents from areas with a high prevalence of infection. Thus, serologic tests are helpful when previous infection is considered less likely (eg, in travelers to endemic areas). Microscopic identification of intestinal amebas may require examination of 3 to 6 stool specimens and concentration methods (see table). Antibiotics, antacids, antidiarrheals, enemas, and intestinal radiocontrast agents can interfere with recovery of the parasite and should not be given until the stool has been examined.E. histolyticais indistinguishable morphologically fromE. dispar,E. moshkovskii, andE. bangladeshibut can be distinguished from a number of nonpathogenic amebas microscopically, includingE. coli,E. hartmanni,E. polecki,Endolimax nana, andIodamoeba bütschlii. Molecular analysis using polymerase chain reaction-based assays and enzyme immunoassay for fecal antigens are more sensitive and differentiateE. histolyticafrom other amebas. In symptomatic patients, sigmoidoscopy or colonoscopy may show nonspecific inflammatory changes or characteristic flask-shaped mucosal lesions, which should be aspirated, and the aspirate should be examined for trophozoites and tested for specificE. histolyticaantigen or DNA. Biopsy specimens from rectosigmoid lesions may also show trophozoites. Amebic extraintestinal infection is more difficult to diagnose. Stool examination is usually negative, and recovery of trophozoites from aspirated pus is uncommon. If a liver abscess is suspected, ultrasonography, CT, or MRI should be done. They have similar sensitivity; however, no technique can differentiate amebic from pyogenic abscess with certainty. Needle aspiration is reserved for the following: Those likely to be due to fungi or pyogenic bacteria Those in which rupture seems imminent Those that respond poorly to drug therapy Abscesses contain thick, semifluid material ranging from yellow to chocolate-brown. A needle biopsy may show necrotic tissue, but motile amebas are difficult to find in abscess material, and amebic cysts are not present. A therapeutic trial of an amebicide is often the most helpful diagnostic tool for an amebic liver abscess. Microscopic examination of stool is usually negative in patients with extraintestinal amebiasis. 