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This is a broad group of parasites that are spore forming and are obligate intracellular organisms. There are at least 12 species belonging to 7 genera.


Of these organisms, Enterocytozoon bieneusi is the most common species affecting humans. It commonly produces a diarrheal illness.


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Intestinal microsporidiosis in human immunodeficiency virus-infected patients with chronic unexplained diarrhea: prevalence and clinical and biologic features.

Molina JM, Sarfati C, Beauvais B, Lemann M, Lesourd A, Ferchal F, Casin I, Lagrange P, Modigliani R, Derouin F, et al.

Division of Infectious Diseases, Hopital Saint-Louis, Paris, France.

J Infect Dis 1993 Jan;167(1):217-21 Abstract quote

Eighteen patients infected with human immunodeficiency virus and with chronic unexplained diarrhea were prospectively studied to investigate the prevalence and clinical and biologic features of intestinal microsporidiosis.

All patients underwent extensive evaluation for bacterial, viral, and parasitic pathogens. Enterocytozoon bieneusi was found in 9 patients (50%; 95% confidence interval, 27-73) in stools and duodenal and jejunal biopsies. In 8 patients, it was the sole pathogen found. Other pathogens were also isolated from the intestinal tracts of 4 patients, but diarrhea remained unexplained in 6. Patients with intestinal microsporidiosis had significantly lower mean Karnofsky scores (69.4 vs. 85.5, P = .009), CD4 cell counts (18.6 vs. 209.8/microL, P = .02), and D-xylose absorption tests (0.13 vs. 0.36 g/L, P < .001) than did patients without intestinal microsporidiosis.

Intestinal microsporidiosis appears to be a frequent cause of unexplained chronic diarrhea in patients with AIDS and is associated with diminished D-xylose absorption.


Examination of the prevalence and seasonal variation of intestinal microsporidiosis in the stools of persons with chronic diarrhea and human immunodeficiency virus infection.

Conteas CN, Berlin OG, Lariviere MJ, Pandhumas SS, Speck CE, Porschen R, Nakaya T.

Division of Gastroenterology, Southern California Permanente Medical Group, Los Angeles 90027, USA.

Am J Trop Med Hyg 1998 May;58(5):559-61 Abstract quote

The epidemiology of human microsporidiosis is poorly understood and environmental factors affecting transmission of the organism have not been fully elucidated. Temporal variation in the prevalence of microsporidia in the stool of patients with human immunodeficiency virus (HIV) infection and diarrhea was studied to evaluate the role of water-borne transmission. From January 1993 to December 1996, 8,439 stools from HIV-infected individuals were examined for microsporidia spores in southern California. Yearly positivity rates were 8.8% in 1993, 9.7% in 1994, 6.6% in 1995, and 2.9% in 1996. An analysis for linear trend showed a statistically significant decrease in stool positivity rates over time (chi2 = 81.9, P = 0.001).

No significant seasonal variation in the prevalence of microsporidiosis was seen over that time period. These results suggest the constant presence of microsporidia in the environment, rather than a seasonal association with recreational water use or seasonal contamination of the water supply, and a real decrease in yearly prevalence of microsporidia related diarrhea. Factors related to a progressive decrease in prevalence are subjects of future investigation.

Microsporidia and Cyclospora: epidemiology and assessment of risk from the environment.

Mota P, Rauch CA, Edberg SC.

Clinical Microbiology Laboratory, Yale-New Haven Hospital, CT, USA.

Crit Rev Microbiol 2000;26(2):69-90 Abstract quote

Two classes of parasites with an environmental stage in their lifestyle have recently emerged as significant gastrointestinal pathogens for humans.

Microsporidia represent a group that contains a number of genera related to the genus Cryptosporidium. They are generally transmitted via direct human to human contact, but can survive in water and food, and recently have been found in surface water used as drinking source water. Their most common host range is in patients with clinical AIDS. Limited work to date suggests the group is susceptible to chlorine achievable CxT (concentration x time) values and is coagulated by filtration. Cyclospora cayetanensis is a species of parasite that has caused outbreaks from contaminated food.

Its major risk is from the use of inadequately treated water used for irrigation. Cyclospora can infect normal and immunosuppressed hosts. Current information regarding the lifestyle, transmission, and control of both groups of parasites are discussed, with a health risk assessment analysis.




Prevalence of intestinal microsporidiosis in HIV-infected individuals referred for gastroenterological evaluation.

Kotler DP, Orenstein JM.

Department of Medicine, St. Luke's-Roosevelt Hospital Center, Columbia University, New York.

Am J Gastroenterol 1994 Nov;89(11):1998-2002 Abstract quote

OBJECTIVES: The reported prevalence of enteric pathogens, especially microsporidiosis, in HIV infection varies greatly. In this study, the prevalence rates for microsporidiosis and other enteric pathogens in HIV-infected individuals referred for gastrointestinal symptoms were compared.

METHODS: This prospective study included 250 HIV-infected individuals (179 with AIDS) who were referred for GI evaluation (diarrhea in 194). The prevalence rates of symptomatic intestinal disease due to microsporidiosis and other intestinal pathogens were determined by clinical evaluation, and their epidemiological, clinical, and immunological characteristics were compared.

RESULTS: Enteric pathogens were identified in 83% of 141 AIDS patients with diarrhea, 2% of 53 AIDS patients without diarrhea, and 3% of 56 non-AIDS patients. Microsporidia was the most common pathogen found (39% of AIDS patients with diarrhea). Two or more coexisting infections were found in 28% of AIDS patients with diarrhea. The prevalence rates for coexisting infections were similar to those predicted from the individual prevalence rates, with the exception of cryptosporidiosis and microsporidiosis, which were lower than predicted. Patients with microsporidiosis had severely depressed CD4 lymphocyte counts in peripheral blood. All patients with microsporidiosis, except one, had diarrhea, and D xylose malabsorption was universal in patients with microsporidiosis.

CONCLUSION: Microsporidiosis is a common cause of chronic diarrhea, malabsorption, and weight loss in AIDS patients.


GENERA Enterocytozoon




Ultrastructure, immunofluorescence, western blot, and PCR analysis of eight isolates of Encephalitozoon (Septata) intestinalis established in culture from sputum and urine samples and duodenal aspirates of five patients with AIDS.

Croppo GP, Croppo GP, Moura H, Da Silva AJ, Leitch GJ, Moss DM, Wallace S, Slemenda SB, Pieniazek NJ, Visvesvara GS.

Division of Parasitic Diseases, Centers for Disease Control and Prevention, Atlanta, Georgia 30341-3724, USA.

J Clin Microbiol 1998 May;36(5):1201-8 Abstract quote

Microsporidia are ancient, intracellular, eukaryotic protozoan parasites that form spores and that lack mitochondria.

Currently, as many as eight species included under six genera are known to infect humans, mostly patients with AIDS. Among these, Enterocytozoon bieneusi, the agent of gastrointestinal (GI) disease, is the most frequently identified microsporidian in clinical laboratories in the United States. Encephalitozoon (Septata) intestinalis, the agent that causes a disseminated infection including infection of the GI tract, is the second most frequently identified microsporidian parasite. In spite of this, not many isolates of E. intestinalis have been established in culture.

We describe here the continuous cultivation of eight isolates of E. intestinalis obtained from different samples including the urine, sputum, and duodenal aspirate or biopsy specimens from five AIDS patients originating from California, Colorado, and Georgia. The specific identification was made on the bases of ultrastructural, antigenic, and PCR analyses.


Prevalence of microsporidiosis due to Enterocytozoon bieneusi and Encephalitozoon (Septata) intestinalis among patients with AIDS-related diarrhea: determination by polymerase chain reaction to the microsporidian small-subunit rRNA gene.

Coyle CM, Wittner M, Kotler DP, Noyer C, Orenstein JM, Tanowitz HB, Weiss LM.

Department of Medicine, Albert Einstein College of Medicine, Bronx, New York 10461, USA.

Clin Infect Dis 1996 Nov;23(5):1002-6 Abstract quote

Microsporidia are emerging as opportunistic pathogens in patients with AIDS. Enterocytozoon bieneusi and Encephalitozoon (Septata) intestinalis have been implicated in enteric infections in AIDS patients with chronic diarrhea, a wasting syndrome, and malabsorption.

We used the polymerase chain reaction (PCR) and primers that amplify the conserved regions of the small-subunit rRNA (SSU-rRNA) gene of E. bieneusi and E. intestinalis in tissue specimens from HIV-infected patients with and without diarrhea to examine the association between microsporidia and diarrhea in patients with AIDS. Tissue specimens were obtained from 68 patients with AIDS and diarrhea (mean CD4 lymphocyte count, 21/mm3) and 43 AIDS patients without diarrhea (mean CD4 lymphocyte count, 60/mm3). By means of PCR with use of the SSU-rRNA primers specific for E. bieneusi and E. intestinalis, we found that 44% of patients with diarrhea were infected with microsporidia, whereas only 2.3% of the patients without diarrhea were infected with microsporidia (P < .001).

There was a clear association between the presence of microsporidia and diarrhea. In addition, the SSU-rRNA primers proved to be sensitive and specific when used in this clinical setting.

Extraction-free, filter-based template preparation for rapid and sensitive PCR detection of pathogenic parasitic protozoa.

Orlandi PA, Lampel KA.

Division Virulence Assessment, Center for Food Safety and Applied Nutrition, Food and Drug Administration, Washington, D.C. 20204, USA.

J Clin Microbiol 2000 Jun;38(6):2271-7 Abstract quote

Within the last several years, the protozoan parasites Cyclospora cayetanensis, Cryptosporidium parvum, and microsporidia have become recognized as important, rapidly emerging human pathogens in immunocompromised and immunocompetent individuals. Since the early 1990s, many of the reported outbreaks of enteric illness caused by these microorganisms have been attributed to food- and water-borne contamination.

Many inherent obstacles affect the success of current surveillance and detection methods used to monitor and control levels of contamination by these pathogens. Unlike methods that incorporate preenrichment for easier and unambiguous identification of bacterial pathogens, similar methods for the detection of parasitic protozoa either are not currently available or cannot be performed in a timely manner.

We have developed an extraction-free, filter-based protocol to prepare DNA templates for use in PCR to identify C. cayetanensis and C. parvum oocysts and microsporidia spores. This method requires only minimal preparation to partially purify and concentrate isolates prior to filter application. DNA template preparation is rapid, efficient, and reproducible. As few as 3 to 10 parasites could be detected by PCR from direct application to the filters. In studies, as few 10 to 50 Encephalitozoon intestinalis spores could be detected when seeded in a 100-microliter stool sample and 10 to 30 C. cayetanensis oocysts could be detected per 100 g of fresh raspberries.

This protocol can easily be adapted to detect parasites from a wide variety of food, clinical, and environmental samples and can be used in multiplex PCR applications.



Human microsporidiosis: Clinical, diagnostic and therapeutic aspects of an increasing infection.

van Gool T, Dankert J.

Department of Medical Microbiology, Academic Medical Center, Amsterdam, The Netherlands.

Clin Microbiol Infect 1995 Feb;1(2):75-85 Abstract quote

Human microsporidiosis is a parasitic infection due to species of four different genera: Encephalitozoon; Enterocytozoon; Nosema; and Pleistophora. Although well known as a cause of disease in animals, microsporidiosis was only occasionally reported in humans.

Recently, in human immunodeficiency virus (HIV)-infected patients, microsporidia belonging to Encephalitozoon and Enterocytozoon species have proved to be important opportunistic pathogens. Enterocytozoon bieneusi is associated with chronic intermittent diarrhea, cholangiopathy and sinusitis whereas Encephalitozoon intestinalis, Encephalitozoon hellem and Encephalitozoon cuniculi, the three Encephalitozoon species found in humans, are associated with diarrhea, rhinosinusitis, keratoconjunctivitis, nephritis and hepatitis. Diagnosis of microsporidial infections in humans was until recently an invasive, laborious procedure including electron microscopy of small intestine biopsies. However, new simple staining methods using Uvitex 2B or modified trichrome stain for feces and other body fluids have facilitated clinical diagnosis as well as drug evaluation and epidemiological studies. The application of monoclonal antibodies and molecular techniques such as the polymerase chain reaction have further improved microsporidial diagnosis.

Treatment of Entero. bieneusi has, until now, been unsuccessful whereas albendazole has proved to be an effective treatment for Encephalitozoon species infection. Identification of effective treatment for Entero. bieneusi infections and further study of the pathogenicity of these microsporidial infections in immunocompetent hosts are important future challenges.


Disseminated microsporidiosis (Encephalitozoon hellem) and acquired immunodeficiency syndrome. Autopsy evidence for respiratory acquisition.

Schwartz DA, Bryan RT, Hewan-Lowe KO, Visvesvara GS, Weber R, Cali A, Angritt P.

Department of Pathology, Emory University School of Medicine, Atlanta, Ga.

Arch Pathol Lab Med 1992 Jun;116(6):660-8 Abstract quote

Microsporidia are obligate intracellular protozoal parasites that infect a variety of cell types in a broad range of invertebrates and vertebrates. They have recently come to medical attention due to the increased frequency with which members of two microsporidian genera, Enterocytozoon and Encephalitozoon, are being diagnosed in patients with the acquired immunodeficiency syndrome (AIDS).

The majority of published reports of human microsporidiosis describe Enterocytozoon infection of small intestinal enterocytes. In addition, a growing number of AIDS patients have been identified with infection due to the two species of Encephalitozoon-Encephalitozoon cuniculi and Encephalitozoon hellem, observed in conjunctival, corneal, and, recently, sinonasal tissues. However, there are scant data regarding the systemic pathology and epidemiology of these infections. This article describes a patient with AIDS who died with systemic Encephalitozoon infection. The etiologic microsporidian was found to be E hellem by using antemortem biochemical and antigenic analyses.

A complete autopsy, the first to be reported in a patient with this infection, revealed organisms in the eyes, urinary tract, and respiratory tract. A surprising observation was the occurrence of numerous organisms within the lining epithelium of almost the entire length of the tracheobronchial tree, suggestive of respiratory acquisition. Detailed light and electron microscopic findings and the biological and diagnostic features of microsporidiosis are discussed.

Disseminated Microsporidiosis Caused by Encephalitozoon cuniculi III (Dog Type) in an Italian AIDS Patient: a Retrospective Study.

Tosoni A, Nebuloni M, Ferri A, Bonetto S, Antinori S, Scaglia M, Xiao L, Moura H, Visvesvara GS, Vago L, Costanzi G.

Pathology Unit, "L.Sacco" Hospital and Institute of Biomedical Sciences, University of Milan, Italy (AT, AF, SB, LV, GC).

Mod Pathol 2002 May;15(5):577-83 Abstract quote

We report a case of disseminated microsporidiosis in an Italian woman with AIDS. This study was done retrospectively using formalin-fixed, paraffin-embedded tissue specimens obtained at autopsy.

Microsporidia spores were found in the necrotic lesions of the liver, kidney, and adrenal gland and in ovary, brain, heart, spleen, lung, and lymph nodes. The infecting agent was identified as belonging to the genus Encephalitozoon based on transmission electron microscopy and indirect immunofluorescence. Additional molecular studies, including sequence of the rDNA internal transcribed spacer region, identified the agent as E. cuniculi, Genotype III.

We believe that this is the first report of a human case of disseminated microsporidial infection involving the ovary.


Pulmonary microsporidiosis due to Encephalitozoon hellem in a patient with AIDS.

Scaglia M, Sacchi L, Croppo GP, da Silva A, Gatti S, Corona S, Orani A, Bernuzzi AM, Pieniazek NJ, Slemenda SB, Wallace S, Visvesvara GS.

Laboratory of Clinical Parasitology, University-IRCCS S. Matteo, Pavia, Italy.

J Infect 1997 Mar;34(2):119-26 Abstract quote

The microsporidian Encephalitozoon hellem is being reported with increasing frequency in HIV-positive subjects, as an agent of disseminated microsporidiosis without involving the gastrointestinal tract.

We describe a case of pulmonary microsporidiosis in a 27-year-old Italian man with AIDS who developed fever, cough, and dyspnea. A chest X-ray showed multiple bilateral pulmonary opacities and mediastinal lymph-node enlargement. Stained smears of bronchoalveolar lavage sediment showed oval structures consistent with microsporidian spores. Viral, bacterial and fungal cultures were repeatedly negative, whereas microsporidia were successfully cultured in human and bovine fibroblast cell lines. Analysis of electron micrographs indicated that the isolate belonged to the genus Encephalitozoon. Based on further immunological, biochemical and molecular studies it was characterized as E. hellem.

Even though a temporary improvement with albendazole therapy was noticed, the patient deteriorated clinically and died of severe respiratory distress.



Light microscopic diagnosis of microsporidiosis in patients with AIDS.

Kotler DP, Giang TT, Garro ML, Orenstein JM.

Department of Medicine, St. Luke's-Roosevelt Hospital Center, College of Physicians and Surgeons, Columbia University, New York.


Am J Gastroenterol 1994 Apr;89(4):540-4 Abstract quote

OBJECTIVE: In the past, the diagnosis of chronic intestinal microsporidiosis, an important cause of diarrhea in patients with AIDS, relied upon transmission electron microscopy (TEM). In this study, the sensitivity and specificity of the light microscopic (LM) diagnosis of microsporidiosis was determined.

METHODS: Thirty-four consecutive jejunal biopsies from AIDS patients were evaluated at St. Luke's-Roosevelt and George Washington University Hospital Centers by several light microscopic stains, including hematoxylin and eosin, Gram stain, Giemsa stain, chromotrope 2R modified-trichrome stain, and Giemsa stain of mucosal touch preparations (TP). The results were compared to TEM, as the gold standard, and to estimates of parasite burden from plastic section light microscopy and TP.

RESULTS: Microsporidiosis was diagnosed by TEM in 15 cases. The diagnosis also was reached by light microscopy in most cases. The sensitivities and negative predictive values of the different techniques ranged from 57% to 88%, while the specificities and positive predictive values ranged from 94% to 100%. All stains gave concordant results in 23 of 34 cases. The parasite burden was lower by TEM (p < 0.05) and TP in cases with discordant (false-negative) results than in those with concordant results, suggesting that a false-negative diagnosis is related to a low parasite burden.

CONCLUSION: It should be possible to render the diagnosis of intestinal microsporidiosis by LM in most cases. TEM may be needed for the minority of cases with low parasite burden.


In vitro culture, ultrastructure, antigenic, and molecular characterization of Encephalitozoon cuniculi isolated from urine and sputum samples from a Spanish patient with AIDS.

del Aguila C, Moura H, Fenoy S, Navajas R, Lopez-Velez R, Li L, Xiao L, Leitch GJ, da Silva A, Pieniazek NJ, Lal AA, Visvesvara GS.

Universidad San Pablo-CEU, Madrid, Spain.

J Clin Microbiol 2001 Mar;39(3):1105-8 Abstract quote

In this report we describe the cultivation of two isolates of microsporidia, one from urine and the other from sputum samples from a Spanish AIDS patient.

We identified them as Encephalitozoon cuniculi, type strain III (the dog genotype), based on ultrastructure, antigenic characteristics, PCR, and the sequence of the ribosomal DNA internal transcribed spacer region.



Therapy for human gastrointestinal microsporidiosis.

Conteas CN, Berlin OG, Ash LR, Pruthi JS.

Southern California Permanente Medical Group, Los Angeles, USA.

Am J Trop Med Hyg 2000 Sep-Oct;63(3-4):121-7 Abstract quote

Gastrointestinal microsporidiosis is a major cause of diarrhea and wasting in persons with acquired immune deficiency syndrome (AIDS). Microsporidia demonstrate properties of both true eukaryotes and prokaryotes. The biology of microsporidia makes its elimination from the gastrointestinal tract therapeutically challenging. This organism depends greatly on the host for its energy needs and reproduction; microsporidial spores are impervious to the elements.

Microsporidial infection of the gastrointestinal tract, principally with Enterocytozoon bieneusi and Encephalitozoon intestinalis in patients with AIDS has been treated with different medical regimens with variable success. The less common pathogen, E. intestinalis, responds well to albendazole, making it excellent first-line therapy, but such is not the case for E. bieneusi.

None of the benzimidazoles has been demonstrated to be efficacious for E. bieneusi. On the other hand, E. bieneusi has shown excellent clinical therapeutic response to either direct action with fumagillin or its analogue, TNP-470, or indirectly by immune enhancement by suppression of the HIV virus with more aggressive, highly effective antiretroviral therapy. Further work is necessary to fully establish proper therapeutic protocols and manage side effects of the treatments. Other promising forms of therapy such as polyamine inhibitors and thalidomide demonstrate certain effectiveness in treatment of microsporidian in vitro (polyamine inhibitors) and in selected cases in vivo (thalidomide). Lack of either sufficiently suggestive or definitive human studies prevents the endorsement of these modes of therapy for treatment of gastrointestinal microsporidiosis at this time.

Eradication of cryptosporidia and microsporidia following successful antiretroviral therapy.

Miao YM, Awad-El-Kariem FM, Franzen C, Ellis DS, Muller A, Counihan HM, Hayes PJ, Gazzard BG.

Department of HIV Medicine, St. Stephen's Centre, Chelsea and Westminster Hospital, London, UK.

J Acquir Immune Defic Syndr 2000 Oct 1;25(2):124-9 Abstract quote

OBJECTIVES: Incidence of opportunistic protozoal infections causing diarrheal illnesses in patients with HIV has decreased since the introduction of highly active antiretroviral therapy (HAART). The objective of this study was to determine whether the parasites, cryptosporidia, and microsporidia were effectively eradicated or only suppressed following treatment.

DESIGN: Six HIV-positive patients with diarrheal symptoms caused by cryptosporidia or microsporidia were prospectively followed up with stool samples and duodenal biopsies. Samples were taken before HAART, between 1 to 3 months, and 6 months post-HAART.

METHODS: Duodenal samples were analyzed using routine histology and transmission electron microscopy. Stool samples were analyzed by both light microscopy and polymerase chain reaction (PCR) techniques.

RESULTS: Patients who responded successfully to HAART eradicated both cryptosporidial and microsporidial organisms. Symptoms improved within 1 month of therapy but complete eradication of the organisms was only observed after 6 months of treatment.

CONCLUSIONS: AIDs-related cryptosporidiosis and microsporidiosis can be cured following successful antiretroviral therapy.

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Commonly Used Terms


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