Man With Persistent Diarrhea and Occasional Bloody Stools

A thin 26-year-old man has hadintermittent diarrhea with abdominalpain, nausea, vomiting, and occasionalepisodes of hematochezia for8 months. He also complains of weaknessand fatigue and has lost 4.5 kg(10 lb) in the past year. The patientis homosexual and admits to havinghad unprotected sexual intercourse.He denies any significant travelhistory.Results of a screening test forHIV are negative. A stool culture isnegative, and a wet mount of a stoolsample to detect ova and parasites isalso negative.Colonoscopy reveals focal erythematousulcerations that extend fromthe cecum to the rectum (Figure 1).A biopsy specimen from an ulcershows active colitis with overlying fibrinopurulentexudates. Multiple ovalprotozoa (measuring up to 40 m)with voluminous, vacuolated cytoplasm,centrally placed nuclei, andingested red blood cells are presentamong the exudates (Figure 2). Basedon the morphologic appearance ofthese organisms, uncomplicated amebiccolitis is diagnosed.Metronidazole, 750 mg tid for10 days, is prescribed. The patienttolerates the treatment well, and hissymptoms do not recur.AMEBIC INFECTION:AN OVERVIEWThe protozoan parasite Entamoebahistolytica, the causative organismof amebic colitis, is responsible forsignificant morbidity and mortality worldwide. After malaria, amebiasisis the second leading cause of deathfrom protozoan disease.¹ It is commonin developing countries and isusually acquired through ingestion ofcontaminated food or water. In theUnited States, amebiasis is most commonin immigrants and travelers toareas where the disease is endemic.¹Amebiasis may also be contractedthrough anal intercourse; this waspresumably the mode of transmissionin our patient.Amebiasis can affect all agegroups. Most affected persons areasymptomatic and may harbor andpass the cyst form of the parasite intheir stools for months. Symptomaticpatients complain of diarrhea,abdominal pain, and weight loss.Two morphologic forms occurin the life cycle of E histolytica: thecyst and the trophozoite. The cystmeasures 10 to 20 m, contains4 nuclei, and has a thick wall.² Cystsare infectious when passed in thestool and can survive for monthsin the environment. When ingested,the cysts pass into the ileocecum,where they release trophozoites.The trophozoite measures 10 to 60m and contains a vesicular nucleuswith peripheral clumped chromatinand a central karyosome.² Trophozoitesinvade the colonic mucosaand eventually create flask-shapedulcers.DIAGNOSIS ANDTREATMENTStool examination for E histolyticacysts is used to diagnose amebiasis;however, this test is often negative, asin this case, because of the intermittentnature of cyst excretion.³ TheWorld Health Organization recommendsE histolytica-specific tests inthe diagnosis of intestinal amebiasis.¹Specific stool antigen detection testkits are available for this purpose.³Colonoscopy is helpful whenstool antigen test results are negative.¹ Biopsy specimens should betaken from the edge of coloniculcers because of the higher yieldof organisms in this area. Serologictests may be useful in diagnosingamebiasis; however, these tests maynot be able to distinguish betweenrecent and past infection.²Amebiasis frequently involvesthe right colon. However, in severecases, the entire colon may be involved.The 2 important complicationsof amebiasis are:

• The formation of an ameboma, atumorlike lesion in the colon thatgrossly mimics carcinoma.
• Hepatic abscess, which occurs infewer than 5% of patients, most ofwhom are adolescent males.¹ Hepaticabscesses have a necrotic, hemorrhagic,reddish brown content,which is classically likened to "anchovypaste."

Intestinal amebiasis is treatedwith metronidazole. Surgical drainageof amebic abscesses is oftennecessary.

¹

KEY POINTS FORYOUR PRACTICE

Anal intercourse can be a modeof transmission of amebic colitis.Consider this diagnosis in homosexualmen who have chronic diarrheaand who have engaged in unprotectedanal intercourse.

References:

REFERENCES:

1.

Petri WA Jr, Singh U. Diagnosis and managementof amebiasis.

Clin Infect Dis.

1999;29:1117-1125.

2.

Phaller MA. Parasitology. In: McClatchey KD,ed.

Clinical Laboratory Medicine.

2nd ed. Philadelphia:Lippincott Williams & Wilkins; 2001:1308-1312.

3.

Yau YC, Crandall I, Kain KC. Development ofmonoclonal antibodies which specifically recognizeEntamoeba histolytica in preserved stool samples.

J Clin Microbiol.

2001;39:716-719.