Microsporidiosis

INTRODUCTION — Microsporidia are intracellular spore-forming organisms that are ubiquitous in the environment and can infect a wide range of vertebrate and invertebrate hosts, including insects, birds, fish, and mammals. The clinical manifestations of microsporidiosis are diverse and include intestinal, pulmonary, ocular, muscular, and renal disease. Microsporidiosis has been identified in immunosuppressed hosts (eg, transplant recipients, patients with HIV), travelers, children, and older adults [1].

MICROBIOLOGY — Approximately 220 genera and more than 1700 species of microsporidia have been described [2]; they are classified based on spore size, nucleus arrangement, mode of replication within the host cell, host-parasite relationship, and rRNA analysis. Microsporidia were previously considered protozoa; they have been reclassified as fungi [3]. Seventeen different species have been reported to infect humans, many of which were discovered as opportunistic infections in association with the AIDS epidemic (table 1).

The spores vary in size; those that infect humans are typically oval and 1 to 4 microns in diameter (figure 1). The spores are highly resistant to degradation and can survive in the environment for up to four months. Their structure is distinguished by the presence of a polar filament, which facilitates injection of the spore contents into the host cell.

The most common species causing infection in humans is Enterocytozoon bieneusi, followed by the Encephalitozoon species, particularly Encephalitozoon intestinalis. Genome sequencing has shown high genetic diversity for E. bieneusi, with approximately 500 genotypes and 11 phylogenetic groups identified in humans, pets, livestock, birds, and environmental samples [4]. Group 1 is the largest, containing over 300 genotypes, and Groups 1 and 2 have been divided into 9 and 3 subgroups, respectively. Many of the genotypes from Groups 1 and 2 have high potential for cross-species transmission and have been found in a broad range of hosts including humans [4]. However, some genotypes for example, genotype B, have thus far been found only in humans. Groups 3 to 11 contain fewer genotypes, are more host specific, and have limited public health importance [3,5]. Among E. intestinalis isolates, there have been no molecular differences observed among those originating from infected humans and infected animals, which may imply there is no transmission barrier between different host species. Other Encephalitozoon species causing human infection are Encephalitozoon cuniculi and Encephalitozoon hellem. Antigenic diversity has also been demonstrated among these isolates [6,7].

E. bieneusi and Encephalitozoon species usually infect the gastrointestinal and biliary tracts, although there have been a few case reports of isolation of E. bieneusi from the respiratory tract [8]. Enterocytozoon infections normally cause localized infection; in contrast, Encephalitozoon species have the ability to disseminate widely via macrophages, thus causing systemic infection involving the intestinal and hepatobiliary tracts, respiratory tract, sinuses, kidney, eye, and brain.

Several other species are capable of causing disseminated disease, including Trachipleistophora species (Trachipleistophora hominis and Trachipleistophora anthropopthera), Pleistophora species (Pleistophora ronneafiei and others), and Anncaliia species, also known as Brachiola species (Anncaliia vesicularum and Anncaliia algerae) [9]. Nosema spp and Vittaforma corneae most commonly cause ocular infections. Myositis associated with Pleistophora spp, Trachipleistophora spp, and A. algerae has been described in individual reports [10-12].

PATHOGENESIS — The pathogenesis of microsporidiosis is poorly understood. The minimal number of microsporidia required to cause human disease is unknown, and no toxins have been described. It is possible that a "carrier state" exists and that reactivation occurs with immunosuppression, although this has not been definitively documented.

Intestinal disease is characterized by distortion of the villus architecture, particularly within the small intestine, without a significant inflammatory response. Progressive morphological and functional abnormalities of the small intestine appear to occur as the number of organisms increases [13]. Infection may interfere with intestinal absorption and secretion. In one study of 21 patients with chronic diarrhea or wasting secondary to microsporidiosis, evidence of crypt hyperplasia and a decrease in villous absorptive surface area was demonstrated on jejunal biopsies [14].

Life cycle — Microsporidia spores typically enter the host via ingestion or inhalation. Once within the intestinal or respiratory tract, the spores are propelled into the host cell by their polar filament, enabling the infective sporoplasm (nucleus) to be injected from the spore into the host cell. Once within the host epithelial cell, proliferation occurs via merogony (binary fission) or schizogony (multiple fission) to form multinucleate plasmodial forms. This is followed by a period of maturation via a process known as sporogony, which involves thickening and development of the spore membranes, further divisions, and finally, formation of new mature spores. These spores ultimately burst out of the infected cell, and the released spores can infect nearby host cells or pass out into the environment. Thus, the spores can complete their life cycle within a single host (figure 2).

EPIDEMIOLOGY — Microsporidia exist worldwide, although an accurate assessment of their prevalence in the general population has not been established. In one European study of 576 immunocompetent adults, a seroprevalence of 5 to 8 percent was observed; the prevalence of diarrhea was not reported [15]. In one review, the prevalence of microsporidiosis ranged from 1 to 42 percent in individuals with HIV and from 0 to 24 percent in individuals without HIV [16].

Most symptomatic infections in humans occur among patients with HIV who are significantly immunocompromised (CD4 <100 cells/microL). The prevalence of infection rises with increased levels of immunosuppression, ranging from 7 to 50 percent in reports of symptomatic patients with diarrhea and/or wasting [17-22]. As an example, in a meta-analysis of 47 studies examining human gastrointestinal microsporidial infections in individuals with HIV found a pooled prevalence of 11.8 percent (95% CI: 10.1-13.4 percent), with higher prevalence seen in low-income countries [23]. In another study of 2652 patients with HIV with diarrhea in Peru, 3 percent were diagnosed with microsporidiosis; risk factors included contact with duck or chicken droppings and poor sanitation [24].

Patients with HIV may also be asymptomatic. In one study of 106 patients with HIV, 15 percent of patients had evidence of microsporidia on small bowel biopsy, regardless of intestinal symptoms [25]. However, much lower colonization rates have been observed in other reports of patients with HIV but without diarrhea (approximately 2 percent) [17,26].

Microsporidia infections have also been described in immunocompromised patients without HIV, such as organ transplant recipients and bone marrow graft recipients [27-30]. Among renal transplant recipients, it is a relatively late-occurring opportunistic infection [31]. Older adult individuals may also be at risk for microsporidiosis; in one report of 47 older adult patients with diarrhea, intestinal microsporidiosis due to E. bieneusi was identified in 17 percent of cases by polymerase chain reaction [32].

Infection can also occur in immunocompetent hosts, largely in the form of diarrheal illness [33-35]. E. intestinalis has been found in stool samples of travelers with chronic diarrhea, but a pathogenic role has not been definitively demonstrated [36]. A case of ocular microsporidiosis due to V. corneae has also been reported in a returned traveler from India [37].

It is unknown whether microsporidia infection routinely persists in a latent state or if reactivation occurs during conditions of immune compromise. It is also unknown if persistently infected individuals can transmit infection to others at risk [1].

Transmission — Transmission of microsporidiosis is not fully understood. Foodborne, waterborne, person-to-person, animal-to-person, vector-borne, and airborne transmission may all be possible [16,38-42]. Microsporidia spores have been detected in urine, feces, and respiratory secretions. Several cases of the transmission of microsporidiosis due to transplanted organs have also been described [2].

It has been suggested that microsporidial infection occurs more frequently in patients who drink unboiled water or from untreated streams and rivers, suggesting transmission via water ingestion [43,44]. Foodborne outbreaks of E. bieneusi have also been reported [38,45]. Microsporidial keratitis has been described following exposure to floodwaters [46].

Many animals shed E. bieneusi oocysts, including wild living and domestic animals [47], and zoonotic spread is also considered likely [48,49]. Nonhuman primates, pigs, and ruminants are often infected, as are other mammals and birds. E. intestinalis has been identified in humans and animals.

CLINICAL MANIFESTATIONS — The clinical manifestations of microsporidiosis vary depending upon the infecting species and the host immune status. Both asymptomatic and symptomatic infections have been described.

Immunocompetent patients — Microsporidia can cause asymptomatic infection, a self-limited diarrheal illness, or, less commonly, chronic diarrhea [50,51]. Clinical manifestations include watery, nonbloody diarrhea, nausea, diffuse abdominal pain, and fever. Diarrhea tends to be self-limited in immunocompetent patients. The correlation between detection of microsporidia in stool and gastrointestinal symptoms may be transient; microsporidia infection may cause clinical symptoms during the early stages of infection that resolve even though evidence of microsporidia may persist on diagnostic assessment [52].

In addition to diarrhea, other extraintestinal manifestations [53] include:

●Ocular infections with superficial punctate keratitis and stromal keratitis associated with eye pain have been described [54-56]. Clinical findings include oval, nonbudding microsporidial spores (which fluoresce bluish-white) and rosette-like clusters of epithelial cells. Risk factors include contact lens use, trauma, eye surgery, use of topical corticosteroids, and exposure to soil, mud, dirty water, or thermal springs [57,58]. Associated organisms include Microsporidium africanus, Microsporidium ceylonensis, Nosema ocularum, Nosema corneum (now V. corneae), and A. algerae [59-61].

●Cerebral infections due to E. cuniculi have been reportedly rarely in immunocompetent individuals. Manifestations include seizures, headache, and vomiting [62]. Donor-derived E. cuniculi infection has also been reported in organ recipients with unexplained encephalitis [63]. One case of pacemaker endocarditis due to E. cuniculi has also been reported [64]. An association between E. cuniculi and osteolysis of prosthetic hip implants has been reported in a small cohort [65].

●Myositis associated with Pleistophora spp and Brachiola algerae (reclassified as A. algerae) has been described in individual reports [10,11,66,67]. Symptoms include fever, myalgia, weakness, and muscle tenderness.

●E. cuniculi has been reported as a cause of osteolysis in hip periprosthetic tissue and can be associated with instability of the implant [65].

Patients with HIV — The incidence of microsporidiosis has declined dramatically with the widespread use of antiretroviral therapy. Most microsporidial infections occur in the setting of severe immunodeficiency [68]. In a systematic review and meta-analysis of microsporidiosis in more than 18,000 individuals with HIV, the pooled prevalence of microsporidiosis was 11.8 percent (95% CI 10.1-13.4 percent), with a lower prevalence in high-income countries and a higher prevalence among those with diarrhea [23].

E. bieneusi or E. intestinalis are most commonly associated with diarrhea, which is typically nonbloody, watery, may be continuous or intermittent, and can be associated with crampy abdominal pain. Patients may also have weight loss, wasting, nausea and vomiting, and malabsorption; fever is rare [69].

A number of other disorders associated with the individual microsporidia species have been described:

●E. bieneusi or E. intestinalis infection can lead to biliary tract involvement, causing cholangitis or acalculous cholecystitis [70]. (See "AIDS cholangiopathy".)

●E. bieneusi has been identified in nasal and bronchial secretions, although its clinical significance is uncertain [8,71-75].

●E. intestinalis disseminates widely and has been implicated in sinus, respiratory, liver, and renal disease. The renal disease is often asymptomatic, although impaired renal function has been described [68].

●E. hellem, E. cuniculi, and Trachipleistophora species usually cause disseminated infection, including bronchiolitis, pneumonitis, sinusitis, nephritis, cystitis, prostatitis, hepatitis, peritonitis, chronic keratoconjunctivitis, encephalitis, and nodular cutaneous lesions [76,77]. Some patients are asymptomatic. Intestinal infection with E. cuniculi is rare [78].

●T. hominis, a newly described species, and Pleistophora spp have been associated with myositis [12,79,80].

●V. corneae has been isolated from urine and sinonasal aspirate in a patient with AIDS [81].

●E. cuniculi has been reported to cause central nervous system infection in one patient with AIDS and multifocal hypodense lesions on brain imaging; the diagnosis was established with the detection of microsporidial spores in cerebrospinal fluid, sputum, stool, and urine [82,83]. Disseminated E. cuniculi infection has been reported in a renal transplant recipient [84]. Trachipleistophora species have also been associated with encephalitis and death [9].

In a retrospective review of 73 patients with HIV and microsporidiosis describing the natural history of infection, 55 percent of patients had persistent diarrhea after six months and 51 percent had weight loss [85]. Symptomatic disease was associated with a high HIV RNA viral load and absence of protease inhibitors from the antiretroviral regimen.

Other immunosuppressed patients — Transplant-associated microsporidiosis (both solid organ and hematopoietic stem cell transplants) and infection in patients with malignancy can occur, leading to a wide range of manifestations [86-92]. (See "Infection in the solid organ transplant recipient".)

DIAGNOSIS — The diagnosis of microsporidiosis consists of microscopic or genomic detection of the spores in stool, body fluids, or tissue specimens. The spores are typically oval and 1 to 2 microns in diameter. Endoscopic biopsy is not more sensitive than stool examination because infection can be patchy [71,93].

For microscopic diagnosis, the laboratory should be alerted to the potential diagnosis, and specific stains for microsporidia should be requested since routine examination for ova and parasites does not usually detect microsporidia spores. Fecal leukocytes and blood are usually absent since microsporidia infection is not associated with a significant inflammation. When present, these findings should raise suspicion for coinfection with another organism.

Light microscopy with a modified trichrome stain is often used for diagnosis. This technique stains microsporidia spores pink against a blue-green background; it can be used on stool, urine, mucus, or tissue specimens (figure 1) [94,95]. Other techniques include serologic assays (which detect immunoglobulin [Ig]M and IgG antimicrosporidial antibodies), tissue culture, and indirect immunofluorescence [96-102]. Fluorescent techniques including Uvitex 2B, Calcofluor White M2R, the FungiFluor Kit, and Fungiqual A are also available and have similar sensitivity and specificity to the modified trichrome stain [93,103,104]. These stains have a number of advantages compared with other methods because they can be performed rapidly, are equally effective on unfixed or formalin-fixed specimens, and can detect spores in stool, intestinal fluid, biopsy imprints, and paraffin biopsy sections [96,97]. However, species identification is not possible.

Transmission electron microscopy is also used for microscopic detection of microsporidia and traditionally has been the primary method for species identification [82]. Alternatively, classification can be based on molecular sequencing.

Polymerase chain reaction (PCR) assays are increasingly being used for diagnosis, simultaneously enabling detection, species identification, and genomic characterization of the microsporidial infection [39,105-111]. PCR-based tests have been used with both tissue biopsy specimens and body fluids, and have been shown to be able to detect 100 to 1,000 spores/mL in clinical samples [2]. In one study of intestinal microsporidiosis, PCR was 100 percent sensitive and 97.9 percent specific, trichrome staining was 100 percent specific and 64 percent sensitive, and calcofluor staining was 80 percent sensitive and 82 percent specific [112]. Indirect immunofluorescence using monoclonal antibodies to Encephalitozoon spp and to E. bieneusi are becoming increasingly available and are simplifying detection of microsporidia in clinical specimens [113-115]. A loop-mediated isothermal amplification assay has also been developed to detect E. bieneusi DNA from human fecal specimens [116].

Test characteristics — The accuracy of the above techniques depends in part upon the site and intensity of infection. One study evaluating sensitivity and specificity (with electron microscopy as the gold standard) noted the following: modified trichrome stain (sensitivity and specificity of 100 and 83 percent, respectively), calcofluor white stain (sensitivity and specificity of 100 and 77 percent, respectively), and indirect immunofluorescent stain (sensitivity and specificity of 83 and 96 percent, respectively) [102].

In another study, stool samples were tested via modified trichrome stain, PCR-restriction fragment length polymorphism, and immunofluorescence assay (IFA). Considering modified trichrome stain as the gold standard, the sensitivity and specificity of IFA were 100 and 99.4 percent, respectively. Considering PCR as the gold standard, sensitivity and specificity of IFA were 95.2 and 100 percent, respectively [117].

TREATMENT

Gastrointestinal infection — Albendazole is effective against most microsporidia species, particularly Encephalitozoon infections, but has minimal efficacy against E. bieneusi (table 2) [118-122]. The duration of therapy depends on the host immune status and whether the infection is localized or disseminated [123]. In immunocompromised patients with disseminated infection, the usual treatment is three weeks (or between two and four weeks) of albendazole (400 mg orally with fatty meal twice daily) [124,125].

Immunocompetent hosts can receive shorter courses of therapy and may even resolve their symptoms with no therapy at all. In a randomized trial including 200 immunocompetent children in Costa Rica hospitalized with subacute diarrhea due to microsporidiosis, albendazole (15 mg/kg/day divided twice daily [maximum 400 mg/dose] for seven days) was more effective than placebo in improving the clinical manifestations and decreasing the duration of illness [126].

Because albendazole does not reliably cure E. bieneusi infections, many other agents have been tried including metronidazole, azithromycin, doxycycline, sparfloxacin, quinacrine, sulfa drugs, atovaquone, furazolidone, nitazoxanide, itraconazole, octreotide, and paromomycin. None of these agents has consistently or effectively eradicated the infection; further study is needed.

Along with specific therapy for microsporidiosis, treatment of immune suppression is critically important in patients with HIV, since restoration of immunity has been associated with clinical and microbiological cure [127-129]. Clearance of microsporidial infections has even been reported with antiretroviral therapy alone [130,131]. Dietary supplementation is also helpful, particularly in patients who have had weight loss [132].

For treatment of E. bieneusi infection, systemic fumagillin (60 mg/day orally in three divided doses for 14 days) has been tried with some success [125,133,134]. In one small dose-response study of fumagillin in patients with HIV with E. bieneusi infection, the highest dose of 60 mg/day was associated with clearance of the organism in 72 percent of cases [133]. The majority of patients in the lower-dose groups had transient clearance from stools but all relapsed within one year. Fumagillin may also be active in other immunosuppressed hosts. A randomized, double-blind study in 12 patients (10 with HIV and 2 posttransplantation) documented clearance of the organism in all patients who received fumagillin and in none of patients receiving placebo; the six placebo recipients subsequently cleared the organism with open-label fumagillin [134]. In a report of 10 cases of E. bieneusi in renal transplant patients who were treated with fumagillin, all patients responded well clinically and all cleared the microsporidial spores from their stools [31]. Fumagillin was effective in a pediatric liver-kidney transplant patient with intestinal disease [135]. Fumagillin was curative for E. bieneusi microsporidiosis in two allogeneic hematopoietic stem cell transplant recipients [87].

Fumagillin can cause dose-related bone marrow toxicity. Among 166 immunosuppressed patients (mainly transplant recipients) treated with fumagillin in France, adverse events occurred in 31 percent, mainly thrombocytopenia (18 percent), neutropenia (5.4 percent), anemia (3 percent), and hepatitis (3 percent); two hemorrhagic events occurred with one death, and one case of liver failure occurred. Neurological toxicity was also seen. However, the agent had high efficacy (negative stool examinations at the end of treatment was seen in 94 percent of patients), and only three parasite relapses were documented [136]. In addition, abdominal pain, diarrhea, vomiting, and hyperlipasemia have been noted with the use of fumagillin. These adverse effects have limited the clinical use of the drug.

Other agents being studied in vivo against microsporidia include polyamines and fluoroquinolones, but clinical data are lacking [137]. Limited data suggest that nitazoxanide (1 g twice daily) for 60 days may also be effective for the treatment of E. bieneusi infections in patients with AIDS [138] and other immunosuppressive conditions [139].

Ocular infection — Ocular keratoconjunctivitis due to microsporidia are often treated with topical Fumidil B (fumagillin bicyclohexylammonium) in saline (to achieve a concentration of 70 mg/mL of fumagillin) [122,140]. In patients with HIV, lesions due to E. hellem, E. cuniculi, E. intestinalis, E. bieneusi, and V. corneae generally respond to this regimen. The concomitant use of albendazole is often warranted, particularly in patients with ocular infections who have evidence of systemic involvement. Topical voriconazole (1%) has been reported to be effective for treatment of microsporidial keratitis [141-143], and success has also been reported with topical fluoroquinolones [144,145]; further trials are needed. For patients with stromal keratitis, epithelial debridement or early surgical keratoplasty may be needed [37,146].

Itraconazole has also been used in the treatment of ocular microsporidiosis (especially superficial keratoconjunctivitis) due to Encephalitozoon spp; experience with this agent is limited [123]. In cases not responsive to medical therapy, keratoplasty and debulking by corneal scraping has been tried [147].

Pregnancy — The optimal approach to management in pregnancy is uncertain. Safety data of albendazole and nitazoxanide in pregnancy are limited. Minimizing immunosuppression is an important component of management.

PREVENTION — Specific measures important in preventing infection are not known since the sources of infection are not fully understood. Attention to personal hygiene is probably the most important factor. Boiled and/or bottled water may also be helpful for immunosuppressed patients, but the relative importance of tap water as a source of infection is still unknown. No vaccine is available.

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Opportunistic infections in adults with HIV".)

SUMMARY AND RECOMMENDATIONS

●Microbiology – Microsporidia are intracellular spore-forming organisms that are ubiquitous in the environment and can infect a wide range of vertebrate and invertebrate hosts. Seventeen different species have been reported in humans, many of which were discovered as opportunistic infections in association with the AIDS epidemic (table 1). The most common species causing infection in humans is Enterocytozoon bieneusi, followed by the Encephalitozoon species, particularly Encephalitozoon intestinalis. (See 'Microbiology' above.)

●Epidemiology – Transmission of microsporidiosis is not fully understood. Microsporidia spores typically enter the host via ingestion or inhalation. Most symptomatic infections in humans occur among patients with HIV who are significantly immunocompromised (CD4 <100 cells/microL) or other immunocompromised individuals (such as organ transplant recipients). Microsporidiosis can also occur in immunocompetent individuals. (See 'Epidemiology' above.)

●Clinical manifestations – Clinical manifestations include watery, nonbloody diarrhea, nausea, diffuse abdominal pain, and fever. Among immunocompetent and immunosuppressed individuals, other manifestations include ocular infection, cerebral infection, and myositis. In addition, a variety of other manifestations can occur in the setting of HIV infection. (See 'Clinical manifestations' above.)

●Diagnosis – The diagnosis of microsporidiosis consists of microscopic detection of the spores in stool, body fluids, or tissue specimens. Light microscopy with a modified trichrome stain demonstrates pink microsporidia spores against a blue-green background (figure 1). The laboratory should be alerted to the potential diagnosis, and specific stains for microsporidia should be requested since routine examination for ova and parasites does not usually detect microsporidia spores. Other assays have also been described. (See 'Diagnosis' above.)

●Treatment – We suggest albendazole (400 mg orally twice daily for two to four weeks) for treatment of intestinal microsporidiosis due to E. intestinalis and for treatment of disseminated microsporidiosis (table 2) (Grade 2B). The optimal approach for treatment of intestinal infection due to E. bieneusi or for treatment of ocular infection is uncertain; fumagillin may be effective though further study is needed. Reversal of immune suppression whenever possible is also important. (See 'Treatment' above.)