An unusual case of invasive Blastocystis hominis infection

A 47-year-old African-American man presented with 3-week history of rectal bleeding. It had started 6 weeks previously while he was visiting Nigeria, with watery diarrhea, abdominal bloating, and pain. His symptoms had resolved without treatment within 2 weeks. Physical examination and hematological and biochemical profiles were all normal. Colonoscopy showed several large ulcers in the cecum, hepatic flexure, and transverse colon with normal surrounding mucosa ([Fig. 1]), and multiple small, shallow ulcers in the rectum ([Fig. 2]). Pathologic examination of biopsies showed exudates with necrosis, and pieces of colonic mucosa with severe acute and chronic inflammation, and focal acute cryptitis, plus multiple vacuolated and amoeboid structures ([Fig. 3]). Subsequent stool study with a special trichrome stain confirmed the diagnosis of Blastocystis hominis. He was treated with metronidazole for 10 days with symptom resolution, and no recurrence of diarrhea.

Fig. 1 A large ulcer in the cecum with fibrinopurulent exudates.

Fig. 2 Multiple small (2 – 3 mm) shallow ulcers in the rectum.

Fig. 3 Histological appearance of a biopsy taken from one of the rectal ulcers stained with periodic acid–Schiff (PAS). a magnification × 10. b Blastocystis hominis showing strong positive staining with PAS, magnification × 40.

B. hominis is an anaerobic nonpathogenic protozoan and one of the most common stool pathogens [1]. Most infected patients are asymptomatic carriers. A presumptive diagnosis of infection is made by the presence of more than five organisms identified per high power field. The parasite, which measures about 5 – 40 μm, the size of a macrophage, resides in the colon and is transmitted feco-orally [2] [3]. The shallow punched-out ulcers more typical for Entamoeba hystolitica and large ulcers of the colon have never been reported before in healthy adults [4] [5]. There is a single previously reported case of invasive B. hominis infection in a previously healthy 4-year-old child. Patients do not usually undergo a colonoscopic examination as the typical presenting symptom is a self-limiting watery diarrhea; therefore, it is possible that some of these immunocompetent patients could also have colonic ulcers. Though an unlikely cause, B. hominis is a pathogen to bear in mind when large colonic ulcers are diagnosed, especially in patients with a travel history and diarrhea.

Acknowledgments: This material is the result of work supported with resources and use of facilities at the John D. Dingell VAMC, Detroit, Michigan, USA.

Endoscopy_UCTN_Code_CCL_1AD_2AZ

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