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Epidemiologic Notes and Reports Pseudo-outbreak of Intestinal Amebiasis -- California

In October 1983, the Los Angeles County (California) Department of Health Services was notified by a local medical laboratory of a large increase in the laboratory's diagnoses of intestinal amebiasis (Entamoeba histolytica infection). Thirty-eight cases were identified from August to October. The laboratory staff estimated that, before August, they had diagnosed approximately one E. histolytica infection per month.

A preliminary investigation failed to identify a common source of the infection. There had been no increase in the number of specimens examined, and although the laboratory served several health facilities, there was no clustering of cases in particular facilities. Finally, most patients did not belong to groups recognized to be at high risk for acquiring amebiasis (such as male homosexuals, tourists to or immigrants from developing countries, or institutionalized persons). The most common complaint of patients was gastrointestinal symptoms, and most improved after treatment with metronidazole. A review of amebiasis diagnoses from other laboratories in Los Angeles County did not reveal other instances of increased reporting.

To evaluate the accuracy of E. histolytica diagnoses, 71 slides from the 38 patients were reexamined by the University of California at Los Angeles Clinical Laboratory or the Los Angeles County Public Health Laboratories. Only four slides from two (5.3%) patients were found to contain E. histolytica. Of specimens from the 36 patients found not to have E. histolytica, 34 contained polymorphonuclear neutrophils and/or macrophages, and two contained nonpathogenic protozoa.

The laboratory reporting the increase follows approved procedures for the collection and examination of stools for protozoa. Permanent slides are prepared from fecal material preserved in polyvinyl alcohol and stained by the Gomori-trichrome method (1). One technician was responsible for reading parasitology slides and had performed that job for the preceding 4 years. The technician's supervisor reviewed all positive slides. The only change in procedure that had been recently introduced was the assignment of a different person to the preparation of the initial smears. This person prepared slides that were "less dense," and the slides were "easier to read." Reported by L Garcia, MT, University of California at Los Angeles Medical Laboratory, F Sorvillo, MPH, M Epstein, MD, K Mori, B Agee, MD, R Barnes, PhD, Los Angeles County Dept of Health Svcs, J Chin, MD, State Epidemiologist, California Dept of Health Svcs; Protozoal Diseases Br, Div of Parasitic Diseases, Center for Infectious Diseases, Laboratory Program Office, CDC.

Editorial Note

Editorial Note: This pseudo-outbreak of intestinal amebiasis serves as a reminder that identification of E. histolytica is difficult. Although E. histolytica can be confused with other intestinal protozoa, a more common problem is that leukocytes or macrophages in stool specimens are identified as E. histolytica (2). In 1981, the College of American Pathologists (CAP) conducted a proficiency survey using a stool specimen, which contained many leukocytes, from a patient with inflammatory bowel disease (3). None of 15 referee laboratories but 100 (16.7%) of 599 participating laboratories reported one or more intestinal protozoa, most commonly E. histolytica. Similarly, as shown in a report of seven suspected outbreaks of amebiasis in the United States between 1971 and 1974, three laboratories might have mistakenly diagnosed amebiasis in as many as 1,200 patients a year for 20 years (2).

A summary of proficiency surveys for parasites conducted by the CAP from 1973 to 1977 showed that E. histolytica infections are also often overlooked (4). Twenty-seven percent of participating laboratories overlooked trophozoites, and 37% overlooked cysts of E. histolytica in stool specimens.

Results of CDC's Proficiency Testing Program in Parasitology closely paralleled those reported by the CAP. In 1982, CDC conducted a parasitology proficiency testing survey using a stool specimen that contained no parasites and numerous leukocytes. None of the 17 reference or referee laboratories reported the presence of intestinal parasites; however, 74 (14.0%) of the 528 participant laboratories incorrectly reported one or more intestinal parasites, most commonly E. histolytica cysts. A summary of CDC proficiency testing surveys in parasitology from 1973-1977 also demonstrated that E. histolytica is often overlooked. Twenty-nine percent of participating laboratories overlooked E. histolytica trophozites, and 33% overlooked E. histolytica cysts in stool specimens.

To avoid errors when attempting to diagnose parasitic diseases, physicians should identify laboratories in their areas whose staffs are experienced in diagnostic parasitology and who participate in and score well on proficiency testing for parasitic diseases.


  1. Garcia LS, Ash LR. Diagnostic parasitology clinical laboratory manual. St. Louis, Missouri: CV Mosby, 1975:16-7.

  2. Krogstad DJ, Spencer HC Jr, Healy GR, Gleason NN, Sexton DJ, Herron CA. Amebiasis: epidemiologic studies in the United States, 1971-1974. Ann Intern Med 1978;88:89-97.

  3. College of American Pathologists. Special parasitology survey (critique specimen P-12), 1981.

  4. Smith JW. Identification of fecal parasites in the special parasitology survey of the College of American Pathologists. Am J Clin Pathol 1979;72:371-3.

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