Protistology 5 (4), 243–255 (2008) Protistology

Overview of and microsporidiosis 1

Elizabeth S. Didier 1 and Louis M. Weiss 2

1 Division of Microbiology, Tulane National Primate Research Center, Covington, LA, U.S.A. 2 Departments of Medicine and Pathology, Albert Einstein College of Medicine, Bronx, NY, U.S.A.

Summary

Microsporidia infections occur in virtually all invertebrate and vertebrate hosts, including mammals. In humans these small single-celled eukaryotic organisms have been recognized as emerging and opportunistic associated with a wide range of clinical syndromes in persons with HIV/AIDS, travelers, children, organ transplant recipients, and the elderly. The most common microsporidia infecting humans are and members of the Encephalitozoonidae. These infections are often overlooked due to the small size of the infectious agents. Albendazole is effective for treating infections by Encephalitozoon spp., but no effective drug has been identified for treating E. bieneusi infections. Furthermore, it is dif- ficult to study E. bieneusi because tissue culture and small animal models that simulate human infections are lacking. There is still debate about whether microsporidian infections remain persistent in asymptomatic immune-competent individuals, reactivate during conditions of immune-compromise, or are transmitted to other people under risk circumstances such as pregnancy or organ donation. Reliable serological diagnostic methods are needed to supple- ment PCR or histochemistry when shedding may be sporadic. The microsporidia have also generated much interest because of their reduced and compact genomes, and comparative molecular and phylogenetic studies continue to support a relationship between the microspo- ridia and fungi.

Key words: Microsporidia, Enterocytozoon, Encephalitozoon, , emerg- ing infection, therapeutics, diagnostic testing, genomics, proteomics

Introduction species have been identified as infectious agents of humans (Table 1). Enterocytozoon bieneusi and the The phylum Microsporidia comprises nearly 1200 Encephalitozoon spp. are the most prevalent micro- species of single-celled, obligate intracellular, eukary- sporidia identified in humans (Weber et al., 2000; otic parasites that infect animals of virtually all ani- Didier, 2005; Didier and Weiss, 2006). Molecular mal phyla, particularly fish and insects (Canning et epidemiology studies are generating a broader un- al., 1986; Larsson, 2005; Vossbrinck and Debrunner- derstanding about the wide demographic, geo- Vossbrinck, 2005). Since they were discovered to be graphic, zoonotic, and environmental range of the a cause of persistent and systemic disease microsporidia that infect humans, and the detection in persons with AIDS, the interest of the biomedi- of human-infecting microsporidia in water sources cal community in these organisms has grown tre- led to their inclusion into the NIH Category B list mendously (Desportes et al., 1985). Currently, 14 of biodefense pathogens and the EPA microbial con-

1 Materials presented on the V European Congress of Protistology (July 23–27, 2007, St. Petersburg, Russia).

© 2008 by Russia, Protistology 244 • Elizabeth S. Didier and Louis M. Weiss

Table 1. Species of microsporidia infecting humans

Microsporidia species Sites of infection a Anncaliia (syns. Nosema and Brachiola) algerae Eye, muscle Anncaliia (syns. Nosema and Brachiola) connori Systemic Anncaliia (syns. Nosema-like and Brachiola) vesicularum Muscle a Encephalitozoon (syn. Nosema) cuniculi Systemic, eye, respiratory tract, urinary tract, liver, peritoneum, brain a Encephalitozoon hellem Eye, respiratory tract, urinary tract, systemic a Encephalitozoon (syn. Septata) intestinalis Intestine, biliary tract, respiratory tract, bone, skin, systemic Enterocytozoon bieneusi Intestine, biliary tract, respiratory tract Microsporidium africanum (syn. Nosema sp.) Eye Microsporidium ceylonensis (syn. Nosema sp.) Eye Nosema ocularum Eye Pleistophora ronneafiei (syn. Pleistophora sp.) Muscle a Trachipleistophora anthropopthera Systemic, eye a Trachipleistophora hominis Muscle, eye a Vittaforma corneae (syn. Nosema corneum) Eye, urinary tract a - Species that can be grown in long-term culture. taminant candidates list of concern for waterborne 2007). The cytoplasm of a microsporidian spore con- transmission (Didier and Weiss, 2006). Completion sists of a nucleus in a monokaryon or diplokaryon of the genome (Katinka arrangement, an anterior anchoring disk, a membra- et al., 2001) and the ongoing genome projects on nous lamellar polaroplast that appears to include an Anncaliia (syn. Brachiola, Nosema) algerae, Spraguea atypical Golgi apparatus, polar vesicles that are likely lophii, Antonospora (syn. Nosema) locustae, and E. to be reduced mitochondria called mitosomes, endo- bieneusi are offering new insights into the genom- plasmic reticulum, ribosomes, and a poster vacuole ics, proteomics, and basic biology of the microspo- (Vavra and Larsson, 1999; Vivares et al., 2002; Vavra, ridia (Keeling et al., 2005; Texier et al., 2005; Tzipori, 2005; Burri et al., 2006; Beznoussenko et al., 2007). 2007). This review summarizes recent research on Polar tube: Microsporidia possess a unique the microsporidia and microsporidiosis with an em- structure, the polar tube, which infects the host cell phasis on infections of humans. during germination (Keohane and Weiss, 1999). The coiled polar tube emanates from the anchoring disk The microsporidian spore and coils numerous times within the posterior region of the spore. A change in osmotic pressure results in General features. Unlike bacterial that are swelling of the posterior vacuole and causes the polar generated in response to environmentally stressful tube to evert, followed by transfer of the cytoplasmic conditions, microsporidia spores develop as the ma- contents through the 50–500 µM-long polar tube in- ture and infectious stage of the life cycle. Similarly to to the host cell (Weidner et al., 1994; Frixione et al., bacterial spores, microsporidian spores are resistant 1997) (Figure 1). The mechanism(s) of germination and survive for long periods of time in the environ- and polar tube formation remain to be determined. ment (Fayer, 2004). Spores of the microsporidian In addition, the mechanism by which the sporoplasm species that infect mammals are relatively small, penetrates its host cell has not been resolved. At measuring 1.0-3.0 µM by 1.5-4.0 µM. Spores are lease five polar tube proteins have been identified as surrounded by a glycoprotein outer layer and a chi- components of the microsporidian polar tube using tinous inner layer (Vavra et al., 1993; Metenier and proteomic approaches (Polonais et al., 2007, Weiss et Vivares, 2001; Southern et al., 2007). Several proteins al., unpublished data). These proteins appear to be have been identified in the spore wall and endospore partially conserved among the microsporidia species including SWP1, SWP2, SWP3 (EnP2) and EnP1 studied to date but share little or no homology with (Peuvel-Fanget et al., 2006, Xu et al., 2006). Some of other proteins researched through various databases, these proteins (i.e. EnP1) may be involved in spore suggesting that the polar tube proteins define a novel wall adhesion to host cells or mucin, thereby play- family of proteins (Polonais et al., 2005). Ptp1, which ing a role in the process of invasion (Southern et al., is the most abundant component of the polar tube, is Protistology • 245

Fig. 1. Life cycle of Encephalitozoon and Enterocytozoon species of microsporidia in humans. The majority of infections are believed to occur by ingestion or inhalation of infectious microsporidia spores which are the mature stages of these organisms. Infections are believed to occasionally occur by direct contact or trauma. Vertical transmission in humans has not been reported to occur. Spores are typically shed with the feces, urine, and possibly with respiratory secretions and mucus. This image is reprinted with permission from the DPDx website of the Centers for Disease Control and Prevention (http://www.dpd.cdc.gov/dpdx/). 246 • Elizabeth S. Didier and Louis M. Weiss modified by the addition of O-linked mannose resi- overlapping coding regions (Vivares et al., 2002; dues, which probably play a role in the adhesion of Keeling et al., 2005; Texier et al., 2005; Williams et the polar tube to host cells (Xu et al., 2003; Peek et al., al., 2005). The genome sizes of the Microsporidia are 2005). Ptp1 and ptp2 both contain cysteine residues currently estimated to range from 2.3 Mb on 11 chro- and appear to interact with each other. Ptp4 local- mosomes for E. intestinalis to 19.5 Mb on 16 chromo- izes to the end of the polar tube and may have a role somes for Glugea atherinae (Metenier and Vivares, in adherence to the host cell or in the final process of 2001). Microsporidia lack many of the genes encod- invasion (Polonais et al., 2005; Polonais et al., 2007). ing proteins in metabolic and regulatory pathways, Serum from patients known to have been infected and retain those related to transport of energy sourc- with microsporidia express antibodies which bind es and metabolites, presumably as a consequence of to the polar tube, suggesting that ptps may serve as host cell dependence (Vivares et al., 2002; Keeling serological diagnostic antigens (van Gool et al., 2004; and Slamovits, 2004, 2005; Texier et al., 2005). In Peek et al., 2005). addition, sporoblast and spore stages were found to express proteins that are protective from environ- Phylogeny and taxonomy considerations mental exposure (Brosson et al., 2006). More than a dozen genes encoding mitochondrion-derived pro- The taxonomic classification of Microsporidia teins have been found and mitochondrial HSP70 has and the species within this phylum were histori- been localized to the mitosome, thus supporting the cally based primarily on morphology, ultrastruc- likelihood that microsporidia evolved from ancestors ture, biology, and habitat features, but more re- which contained mitochondria (Vivares et al., 2002; cently molecular phylogenetics have been applied Williams and Keeling, 2003; Thomarat et al., 2004; for this classification (Larsson, 2005; Vossbrinck Gill and Fast, 2006). Genome sequence projects are and Debrunner-Vossbrinck, 2005). Early molecular underway for several microsporidians, including E. phylogeny studies comparing rDNA sequences sug- bieneusi, Antonospora (syn. Nosema) locustae, and gested that microsporidia were among the earliest Anncaliia (syn. Brachiola) algerae; continued com- or deep-branching because they lacked parative genomic and proteomic analyses are expect- typical mitochondria, Golgi, and peroxisomes, and ed to yield additional information about the phylog- they possessed small ribosomes like those of pro- eny and taxonomy of the microsporidia (Keeling et karyotes (Vossbrinck et al., 1987). However, mi- al., 2005; Texier et al., 2005; Tzipori, 2007). A web crosporidia were found to exhibit many fast-evolv- resource for microsporidia genomics has been devel- ing genes and a long-branch attraction artifact of oped (http://www.biohealthbase.org/) to assist the such faster-evolving genes brought into question research community in the analysis of data generated these early interpretations. The Microsporidia are on these pathogens. now considered to be highly-diverged, well-adapted, and specialized parasites that are related or belong Clinical characteristics to the Fungi or perhaps represent a sister group to the Fungi (Fedorov and Hartman, 2004; Keeling and The relevance of microsporidiosis in humans was Slamovits, 2004; Thomarat et al., 2004; Gill and Fast, recognized in the mid 1980's in association with op- 2006). In a recent report, eight theorized placements portunistic enteric infections and persistent diarrhea within the Fungi have been rejected based on broad in persons with HIV/AIDS (Orenstein et al., 1990). species sampling and comparisons with several Debate followed about whether microsporidia were genes, which has resulted in the hypothesis that the truly pathogenic, since they were also detected in Microsporidia should be placed either at the base of persons without signs of diarrhea (Rabeneck et al., the fungal tree within the Chytridiomycota or within 1995; Didier, 2000). This was most likely a reflec- the (James et al., 2006). tion that the immune status of the host plays a role Microsporidia are peculiar because they contain in the expression of clinical signs during infection some of the smallest genomes of eukaryotes due to (Didier, 2000; Khan and Didier, 2004; Didier and both gene reduction and compaction (Keeling and Weiss, 2006). Immune-competent laboratory ani- Slamovits, 2004; Keeling et al., 2005; Keeling and mals infected with E. cuniculi exhibited clinical signs Slamovits, 2005; Texier et al., 2005). The E. cuniculi during the early acute stage of infection that typi- genome, for example, consists of 2.9 Mb on 11 chro- cally resolved even though the infection persisted, mosomes with approximately 2000 tightly packed whereas immune-deficient athymic and SCID mice genes, which have few introns, are shorter than the infected with E. cuniculi succumbed (Khan and corresponding proteins in other eukaryotes and have Didier, 2004; Mathis et al., 2005). AIDS patients Protistology • 247 with < 100 CD4+ T cells per mm3 blood, were most There still exist significant clinical questions likely to experience persistent diarrhea, weight loss, about the persistence of microsporidian infections and abdominal pain associated with E. bieneusi or E. developing in humans. Microsporidia infections intestinalis infections (Kotler and Orenstein, 1998). persist in immune competent mammals such as HIV-infected individuals receiving antiretroviral mice, rabbits, dogs, and rats, but no formal studies therapies, or non-HIV-infected individuals who have been performed to document whether they also were immunologically naive to microsporidia (i.e. persist in immune competent humans or nonhuman children or travelers) initially developed diarrhea primates. If microsporidia infections do persist in that subsequently resolved (Tumwine et al., 2005; otherwise healthy individuals, it is reasonable to Wichro et al., 2005). In humans, replication of mi- expect that the infections may reactivate and cause crosporidia (e.g. E. intestinalis, E. bieneusi) occurs in clinical signs of disease during conditions of im- the villus epithelium of the small intestine, resulting mune-compromise (eg. aging, chemotherapy) and in reduced villus height and surface area that appear that persistently-infected individuals can transmit to contribute to malabsorption and diarrhea (Kotler infections to others at risk. A case report of micro- and Orenstein, 1998; Weber et al., 2000; Morpeth sporidial keratoconjunctivitis being transmitted by and Thielman, 2006; Wiwanitkit, 2006; Batman et the donor corneal graft supports the latter possibil- al., 2007). E. bieneusi infections occasionally spread ity (Kakrania et al., 2006). Microsporidiosis is also to the hepatobiliary system to cause cholangitis and a being reported more frequently in solid organ trans- few pulmonary infections have been reported (Weber plant recipients, but it is not clear if these infections et al., 2000; Sodqi et al., 2004). Encephalitozoon spe- are transferred by the donor organ, reactivate from cies typically disseminate and infections have been latent infection in the recipient as a consequence of identified in nearly every organ system including a immunosuppressive therapy or are acquired from recently described fatal pulmonary infection in a the environment by the immune compromised re- bone marrow transplant recipient (Orenstein, 2003; cipient (Barsoum, 2006). Orenstein et al., 2005). Interesting reports of human infection, although less common, include a case of Diagnosis Trachipleistophora anthropophthera cornea infection in an AIDS patient (Juarez et al., 2005) and a fatal Prior to the widespread application of molecular case of myositis in a woman with rheumatoid arthri- diagnostics methods, transmission electron micros- tis, caused by Anncaliia (syns. Nosema, Brachiola) copy (TEM) was used to definitively confirm a diag- algerae, a microsporidian that typically infects mos- nosis of microsporidiosis based on observing a polar quitoes (Coyle et al., 2004; Visvesvara et al., 2005; filament within spores (Cali et al., 1991; Orenstein, Franzen et al., 2006). This A. algerae case raises the 2003). TEM is still important for describing ultra- possibility that some microsporidia infections in hu- structural features that, along with newly-applied mans may occur through vector borne transmission molecular biology approaches, contribute to the in addition to the commonly accepted mechanism of taxonomic organization of the microsporidia, as evi- water and food borne transmission of these patho- denced by the recent reclassification of Brachiola spe- gens. Emerging cases of microsporidia infections are cies to Anncaliia (Cali et al., 1991; Cali et al., 1993; being reported among contact lens wearers and cor- Larsson, 2005; Franzen et al., 2006). Histochemical nea transplant recipients (Fogla et al., 2005; Joseph methods have also been applied for detecting micro- et al., 2005; Kodjikian et al., 2005; Vemuganti et al., sporidia more efficiently in fluids (e.g. feces, urine, 2005; Joseph et al., 2006; Kakrania et al., 2006). and mucus) and tissues. These methods included the Transplacental transmission of E. cuniculi has been application of fluorescent brighteners (e.g. Calcofluor reported in carnivores and laboratory rodents, and was White, Uvitex 2B, Fungifluor) that target the chitin- recently considered to be responsible for the deaths of ous spore wall, modified (concentrated) trichrome newborn emperor and cotton-top tamarins in Europe staining used alone or in combination with Gram and the Americas (Guscetti et al., 2003; Reetz et al., stain, and the Warthin-Starry silver stain (Weber et 2004; Juan-Salles et al., 2006). Although transplacen- al., 2000; Garcia, 2002). Immunofluorescent anti- tal transmission has not yet been reported in humans, body staining for species-specific identification has it seems plausible that it may occur in them, since been applied using monoclonal and species-species there are physiological similarities between humans absorbed polyclonal antibodies (Mo and Drancourt, and nonhuman primates, microsporidia species that 2004; Singh et al., 2005). Currently, PCR-based infect nonhuman primates also infect humans, and methods are commonly used in research laboratories microsporidia are ubiquitous in nature. for detecting microsporidia, but are less often used 248 • Elizabeth S. Didier and Louis M. Weiss in commercial diagnostics labs. PCR methods ap- and Thielman, 2006). In regions of South America, plied for diagnostics typically utilize primers that Africa, and Asia, where antiretroviral therapies are target microsporidian rDNA genes (Franzen and not readily accessible, microsporidiosis has been Muller, 1999; Weiss and Vossbrinck, 1999). Recently, consistently identified in HIV-infected patients with an oligonucleotide microarrary system was reported AIDS and other risk factors including poor sanitary for simultaneous detection of four species of human conditions and exposure to animals (Dascomb et al., pathogenic microsporidia species in clinical speci- 2000; Mak, 2004; Bern et al., 2005; Chacin-Bonilla mens, which should increase diagnostic throughput et al., 2006; Morpeth and Thielman, 2006; Sarfati et (Wang et al., 2005). al., 2006; Wiwanitkit, 2006). Since nowadays mi- Microsporidia infections are increasingly report- crosporidia are specifically being looked for more ed in relatively immune-competent individuals such often, they are increasingly recognized in travelers, as children, travelers, and the elderly, so serological children, the elderly, and organ transplant recipients tests are being developed to ascertain whether sub- (Abreu-Acosta et al., 2005; Leelayoova et al., 2005; clinical or asymptomatic infections can be detected. Mungthin et al., 2005; Tumwine et al., 2005; Wichro These approaches include using whole organisms or et al., 2005; Barsoum, 2006; Nkinin et al., 2007). recombinant polar tube or spore wall proteins as an- Microsporidiosis, however, is still probably over- tigens, especially in cases where the microsporidian looked since the causative organisms are quite small species cannot be grown in culture (van Gool et al., and their detection requires considerable expertise 2004; Peek et al., 2005; Polonais et al., 2005; Xu and by the microscopist using histochemical methods. Weiss, 2005; Taupin et al., 2006). Interestingly, sera In the case of PCR, inhibitors commonly confound from humans infected with microsporidia bound to interpretation of results. In addition, microsporidia the polar tube and specifically to glycoepitopes found are often not included in the routine differential di- on this structure (Peek et al., 2005; Xu and Weiss, agnoses for diarrhea, and urine specimens are typi- 2005). cally not evaluated for microsporidia as a potential Serology has not been used routinely for diagnos- cause of systemic infections. With increasing aware- ing microsporidiosis in humans because a correlation ness and sensitivity in the diagnostics methods, a between antibody expression and concurrent infec- rise in the reported prevalence rates of microsporidi- tion has not yet been proven and variable antibody osis may be anticipated. levels are observed in immune-deficient individuals. It is still unclear how most infections are transmit- Many of the species of microsporidia that infect hu- ted to humans, but the genotypes of microsporidia mans tend to disseminate and infect the kidneys, so that infect humans have been identified in domestic, examination of urine, in addition to feces, is likely farm, wild, and aquatic animals, supporting the like- to improve detection of these infections. Since the lihood that microsporidiosis is zoonotic (Deplazes et shedding of microsporidia spores in feces or urine al., 2000; Graczyk et al., 2004; Mathis et al., 2005; may be intermittent or at levels below detection by Graczyk et al., 2007). Species of microsporidia that histochemistry or PCR, serological approaches may infect humans have also been identified in water become feasible for diagnosing infections in im- sources, and risk associations for infection with mi- mune-competent individuals if it can be determined crosporidia included occupational and recreational whether seropositivity is indicative of persistent in- contact with water (Didier et al., 2004; Graczyk and fection. Lucy, 2007). These findings contributed to micro- sporidia being included in the NIH Category B bio- Epidemiology and sources of infection defense list of pathogens (http://www3.niaid.nih.gov/ biodefense/bandcpriority.htm) and the list of EPA Infections due to microsporidia in humans have microbial contaminant candidates (http://www.epa. been reported world-wide and prevalence rates have gov/safewater/ccl/ccl2_list.html) of concern for wa- ranged from 0 to 50% depending on the geograph- terborne transmission. There also appears to be an ic region, method of diagnosis, and demographic association between microsporidia and food-borne characteristics of the population studied (Didier transmission as a consequence of contaminated irri- et al., 2004). Prevalence rates for microsporidiosis gation water, and organisms have been identified on were highest among HIV-infected individuals with lettuce, parsley, cilantro, and strawberries in Costa diarrhea and less than 100 CD4+ T cells per mm3 Rica (Calvo et al., 2004). These observations provid- blood; the use of antiretroviral therapies has re- ed the rationale for studies of the transport of micro- duced the prevalence of microsporidiosis in persons sporidia through sandy porous media for developing with HIV/AIDS (Lewthwaite et al., 2005; Morpeth mathematical models to assess the potential of mi- Protistology • 249 crosporidia contamination of potable water supplies status have reduced the occurrence of opportunistic (Brusseau et al., 2005). infections including microsporidiosis (Didier et al., 2005b; Morpeth and Thielman, 2006; Wiwanitkit, Immunology 2006). Furthermore, a recent study indicated that the aspartyl protease inhibitors used in the highly- Resistance to microsporidiosis depends upon active antiretroviral therapy cocktail also inhib- functional T lymphocytes, a conclusion based on ited the growth of E. intestinalis in tissue culture the greater severity of disease in AIDS patients with (Menotti et al., 2005). Albendazole, a benzimid- declining CD4+ levels and the development of azole that inhibits microtubule assembly, is effec- lethal microsporidia infections in experiments on tive against Encephalitozoon, but not E. bieneusi mice depleted of CD4+ and CD8+ T cells (Hermanek infections (MacDonald et al., 2004; Tremoulet et al., et al., 1993; Didier, 2000; Khan and Didier, 2004; 2004). Fumagillin, an antibiotic and antiangiogenic Moretto et al., 2004). Proinflammatory responses compound produced by fumigatus, was via Th1 cytokines, such as IFN-γ, TNF-α, and IL-12, more broadly effective against Encephalitozoon spp. as well as reactive oxygen and nitrogen intermedi- and Enterocytozoon bieneusi, but was toxic when ates are important for early stages of resistance to administered systemically (Molina et al., 2002). Encephalitozoon infections, as shown in experiments Current studies are focusing on compounds that tar- using murine models and ex vivo human studies get microsporidian polyamines (e.g. polyamine ana- (Khan and Moretto, 1999; Khan and Didier, 2004; logues), methionine aminopeptidase type 2 (e.g. fu- Franzen et al., 2005; Moretto et al., 2007). CD8αα+ magillin-related compounds and analogues), chitin intraepithelial lymphocytes were observed to in- (e.g. nikkomycins), and topoisomerases (e.g. fluoro- crease rapidly after oral administration of E. cuniculi quinolones) (Bacchi et al., 2001; Didier et al., 2005a; to mice and intestinal dendritic cells were observed Didier et al., 2005b; Zhang et al., 2005; Didier et al., to produce IFNγ that subsequently led to cytotoxic 2006). However, since E. bieneusi cannot be grown lymphocyte activity and immune-regulation via in long-term tissue culture, these studies have been IL-10 secretion (Moretto et al., 2004; Moretto et al., based on cultivatable species of microsporidia (e.g. 2007). Antibody responses seem to contribute to Encephalitozoon spp.). prolonging survival in SCID mice given E. cuniculi Concerns exist about the potential of water-borne per os (Sak et al., 2006). Virtually nothing is known and food-borne transmission of microsporidia. about protective immune responses to E. bieneusi Recent studies demonstrated successful disinfec- infections, in part because a tissue culture system is tion of E. intestinalis in water by chlorine and ozone lacking. SCID mice treated with anti-IFN-gamma and successful disinfection of E. cuniculi in food by that were inoculated with E. bieneusi became tran- high pressure processing. It was also shown that the siently infected but these severely immune-deficient exposure of E. cuniculi to bleach, ethanol, HiTor, or animals eventually cleared the microsporidia infec- Roccal was effective in reducing infectivity of these tions and did not exhibit any signs of disease, unlike organisms in tissue culture model systems (Becnel et immune-deficient humans infected with the same al., 1995; John et al., 2005; Jordan et al., 2005; Jordan microsporidian (Feng et al., 2006). This would sug- et al., 2006). gest that mice are not natural hosts for E. bieneusi. Naturally occurring and experimental E. bieneusi Conclusion infections that have been reported in SIV-infected and non-SIV-infected rhesus and pigtail macaques The recognition of microsporidia as causes of op- currently represent the only animal models that portunistic infections in AIDS patients brought about clinically simulate infections observed in immune- a greater appreciation of these organisms and their competent and immune-deficient humans (Tzipori ability to adapt and infect a wide range of animals, et al., 1997; Mansfield et al., 1998; Sestak et al., 2003; including humans. Improved diagnostic methods Green et al., 2004; Drosten et al., 2005). Studies on are likely to reveal a broader range of infections and immune responses to E. bieneusi, however, have not to assist in establishing molecular epidemiological yet involved nonhuman primate models. profiles for defining sources and modes of human in- fections' transmission. Current studies on compara- Therapy and disinfection tive genomics and proteomics of the microsporidia are expected to provide new insights into microspo- Antiretroviral therapies that inhibit HIV-infec- ridia biology and to promote the development of ef- tion and thereby partially reconstitute the immune fective preventive and therapeutic strategies. 250 • Elizabeth S. Didier and Louis M. Weiss

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Address for correspondence. L.M. Weiss. Departments of Medicine and Pathology, Albert Einstein College of Medicine, 1300 Morris Park Avenue Room 504 Forchheimer, Bronx, NY 10461, U.S.A. E-mail: [email protected]