Overview of microsporidia and microsporidiosis

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 pathogens associated with a wide range of clinical syndromes in persons with HIV/AIDS, travelers, children, organ transplant recipients, and the elderly. ThemostcommonmicrosporidiainfectinghumansareEnterocytozoon bieneusi and members of the Encephalitozoonidae. Theseinfectionsareoftenoverlookedduetothesmallsizeoftheinfectious agents. Albendazole is effective for treating infections by Encephalitozoon spp., but no effectivedrughasbeenidentifiedfortreatingE. bieneusi infections. Furthermore, it is dif-ficulttostudyE. bieneusi because tissue culture and small animal models that simulate human infections are lacking. There is still debate about whether microsporidian infections remainpersistent 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 spore shedding may be sporadic. The microsporidia havealso 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, opportunistic infection, emerg-ing infection, therapeutics, diagnostic testing, genomics, proteomics