The pathogens most commonly associated with biliary disease are cytomegalovirus and Cryptosporidium. Microsporidia, Mycobacterium avium complex, Candida albicans, Pneumocystis carinii, and usual enteric bacteria have been identified less frequently [1-3]. Malignancies, including lymphoma and Kaposi sarcoma, have also been associated with biliary tract disease. However, in as many as 55% of cases, a potential cause cannot be identified [1]. We report the first case of Isospora belli infection associated with biliary disease, specifically with acalculous cholecystitis.

Case Report

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The patient was a 39-year-old homosexual man with a history of injection drug use. The acquired immunodeficiency syndrome was diagnosed in 1985 on the basis of Kaposi sarcoma of the skin; esophageal candidiasis was diagnosed in 1987. He began receiving aerosolized pentamidine as prophylaxis for P. carinii pneumonia. Between 1987 and 1991, he had intermittent diarrhea that was variably associated with night sweats and weight loss. Initial stool examination in 1987 showed Endolimax nana, Entamoeba coli, and Blastocystis hominis; however, fecal samples obtained between 1988 and 1991 were repeatedly negative. Examination of duodenal biopsy specimens on two occasions showed inflammation but no enteric pathogens on hematoxylin and eosin, acid-fast, and cytomegalovirus immunohistochemical stains.

In April 1991, the patient developed right upper-quadrant pain, nausea, postprandial vomiting, and diarrhea. Physical examination showed a temperature of 38.9 °C and right upper-quadrant tenderness. Noteworthy laboratory findings included a CD4 count of 0.072 x 10⁹/L, a leukocyte count of 2.4 x 10⁹/L with 6% eosinophils, and normal alkaline phosphatase, bilirubin, and aminotransferase levels. Ultrasound of the abdomen showed a thick-walled gallbladder with normal biliary ducts; the gallbladder was not seen on radionucleotide biliary scintigraphy. Laparoscopic cholecystectomy for presumed cholecystitis showed a grossly thickened gallbladder wall. Microscopic examination showed marked chronic cholecystitis with architectural atrophy and nonspecific inflammation. No pathogen was identified on hematoxylin and eosin, Brown-Hopps, acid-fast, and Gomori Methenamine Silver stains.

Continued nausea, vomiting, abdominal pain, diarrhea, and weight loss characterized the postoperative course. The patient died 4 months later, shortly after a right hemicolectomy for a colo-colic intussusception of the midtransverse colon. Examination of bacterial cultures and the histopathologic findings of operative specimens showed no evidence of enteric pathogens. Permission for postmortem examination was not obtained.

Postmortem Review of Pathologic Findings

Two years later, during a pathologic review of gallbladders from patients with AIDS, infection with I. belli was diagnosed, first on sections previously stained with hematoxylin and eosin and then confirmed by Giemsa stain of recuts from the paraffin block. Many parasites in all stages of development with distinctive features of I. belli were noted within parasitophorous vacuoles in the cytoplasm of numerous epithelial cells [4] (Figure 1). Free merozoites were noted in areas of epithelial disruption and within the gallbladder lumen. Parasites were not identified in the lamina propria. Associated nonspecific findings that constituted chronic cholecystitis included simplification of the trabecular architecture, diffuse expansion of the lamina propria by a dense lymphoplasmacytic infiltrate and eosinophils, epithelial disarray, and prominent intra-epithelial lymphocytes. A thorough review of all of the patient's gastrointestinal biopsy specimens and the colectomy specimen failed to show additional foci of Isospora infection or infection with other enteric pathogens.

View larger version (161K): [in this window] [in a new window]   Figure 1. Isospora belli in the gallbladder. The gallbladder epithelium is in disarray. Merozoites (arrowheads) are seen longitudinally and in cross-section. A macrogamete is also present (arrow). A prominent halo surrounds the organism and represents a parasitophorous vacuole, within which all forms mature. (Hematoxylin and eosin stain; original magnification, x 250).

Discussion

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Isospora belli, a relative of Toxoplasma, Cryptosporidium, and Sarcocystis species, is a protozoan parasite of the Phylum Apicomplexa, Class Sporozoea, Subclass Coccidia, and Family Eimeriidae [5]. It was first recognized as a human pathogen in 1915 [6]. Although numerous species of Isospora are known to infect reptiles, birds, and mammals, only I. belli is known to infect humans [5].

The parasite is acquired after ingestion of a sporulated oocyst, which excysts in the small intestine, invades small intestinal epithelial cells, and undergoes asexual schizogony to form merozoites. The merozoites may undergo further asexual replication (merogony) or sexual replication (gametogony), ultimately forming oocysts that are excreted in the stool, sporulated outside the body in 24 to 48 hours, and again become infectious. Pathologic examination of the duodenum and jejunum typically shows mucosal atrophy with crypt hyperplasia and shortened or fused villi and infiltration of the lamina propria with eosinophils and other inflammatory cells [4].

Isospora belli infection usually causes a gastrointestinal illness that is characterized by loose stools or watery diarrhea and is often associated with abdominal pain, malabsorption, weight loss, and peripheral eosinophilia. Although chronic and severe illness in infants and otherwise healthy adults has been reported [4, 5, 7, 8], in normal hosts the illness is typically self-limited. However, in patients with AIDS and other immunodeficient states, the illness is chronic and may be associated with severe dehydration and debilitation [9-11].

Although isosporiasis is primarily an illness of tropical or subtropical areas, sporadic cases have developed in patients who have not left the mainland United States. Prevalence is estimated to be 15% in the AIDS population in Haiti but less than 0.2% in the U.S. AIDS population [10, 11].

Isospora organisms may be difficult to identify by stool examination because the diagnostic oocysts, seen best by a modified acid-fast stain of concentrated stool, may not be abundant during the initial phase and height of illness, when asexual replication predominates. Diagnosis may require numerous stool examinations and possibly duodenal aspirate or biopsy [9-11].

In 1987, Restrepo and colleagues [12] reported a case of disseminated isosporiasis found during an autopsy of a patient with AIDS. In addition to finding I. belli organisms in all stages of the life cycle in the small and large intestine, Restrepo and colleagues identified merozoites within histiocytes of mediastinal, periaortic, and mesenteric lymph nodes. They suggested that coinfection and ulceration of the intestine caused by cytomegalovirus may have permitted an otherwise unusual lymphohematogenous spread of the parasite.

Except for this report of lymphatic dissemination, I. belli infection was thought to be confined to the columnar cells or lamina propia of the intestinal epithelium. To our knowledge, the finding of I. belli in the gallbladder in association with cholecystitis is unique. It is notable that Isospora was never identified in the stool of our patient nor in duodenal biopsy specimens obtained 15 months before and 1 month after his cholecystectomy. Although it could be theorized that sporozoites made their way to the gallbladder without active replication in the intestinal epithelium, this seems unlikely. We suspect that the patient had localized, undetected intestinal infection with subsequent spread from a periampullary focus retrograde into the gallbladder.

Because isosporiasis of the gastrointestinal tract responds readily to therapy with trimethoprim-sulfamethoxazole, an associated case of cholecystitis might also resolve with medical treatment. Although the prevalence of this pathogen is low in the United States, and the use of trimethoprim-sulfamethoxazole as prophylaxis for P. carinii pneumonia has probably further reduced its occurrence, I. belli should be considered a part of the spectrum of potentially treatable infectious agents that can cause biliary disease in patients with AIDS.