Laboratory Diagnosis of Microsporidia

Microsporidiosis is an emerging and opportunistic infection associated with a wide range of clinical syndromes in humans. This review highlights the research on microsporidiosis in humans during the previous 2 years. The reduced and compact microsporidian genome has generated much interest for better understanding the evolution of these parasites, and comparative molecular phylogenetic studies continue to support a relationship between the microsporidia and fungi. Through increased awareness and improved diagnostics, microsporidiosis has been identified in a broader range of human populations that, in addition to persons with HIV infection, includes travelers, children, organ transplant recipients, and the elderly. Effective commercial therapies for Enterocytozoon bieneusi, the most common microsporidian species identified in humans, are still lacking, making the need to develop tissue culture and small animal models increasingly urgent. Environmental transport modeling and disinfection strategies are being addressed for improving water safety. Questions still exist about whether microsporidia infections remain persistent in asymptomatic immune-competent individuals, reactivate during conditions of immune compromise, or may be transmitted to others at risk, such as during pregnancy or through organ donation. Reliable serological diagnostic methods are needed to supplement polymerase chain reaction or histochemistry when spore shedding may be sporadic.

Microsporidia have emerged as causes of infectious diseases in AIDS patients, organ transplant recipients, children, travelers, contact lens wearers, and the elderly. These organisms are small single-celled, obligate intracellular parasites that were considered to be early eukaryotic protozoa but were recently reclassified with the fungi. Of the 14 species of microsporidia currently known to infect humans, Enterocytozoon bieneusi and Encephalitozoon intestinalis are the most common causes of human infections and are associated with diarrhea and systemic disease. Species of microsporidia infecting humans have been identified in water sources as well as in wild, domestic, and food-producing farm animals, raising concerns for waterborne, foodborne, and zoonotic transmission. Current therapies for microsporidiosis include albendazole which is a benzimidazole that inhibits microtubule assembly and is effective against several microsporidia, including the Encephalitozoon species, but is less effective against E. bieneusi. Fumagillin, an antibiotic and anti-angiogenic compound produced by Aspergillus fumigatus, is more broadly effective against Encephalitozoon spp. and Enterocytozoon bieneusi but is toxic when administered systemically to mammals. Gene target studies have focused on methionine aminopeptidase 2 (MetAP2) for characterizing the mechanism of action and for identifying more effective, less toxic fumagillin-related drugs. Polyamine analogues have shown promise in demonstrating anti-microsporidial activity in culture and in animal models, and a gene encoding topoisomerase IV was identified in Vittaforma corneae, raising prospects for studies on fluoroquinolone efficacy against microsporidia.

The genome of the microsporidia Encephalitozoon cuniculi is widely recognized as a model for extreme reduction and compaction. At only 2.9 Mbp, the genome encodes approximately 2,000 densely packed genes and little else. However, the nuclear genome of its sister, Encephalitozoon intestinalis, is even more reduced; at 2.3 Mbp, it represents a 20% reduction from an already severely compacted genome, raising the question, what else can be lost? In this paper, we describe the complete sequence of the E. intestinalis genome and its comparison with that of E. cuniculi. The two species share a conserved gene content, order and density over most of their genomes. The exceptions are the subtelomeric regions, where E. intestinalis chromosomes are missing large gene blocks of sequence found in E. cuniculi. In the remaining gene-dense chromosome 'cores', the diminutive intergenic sequences and introns are actually more highly conserved than the genes themselves, suggesting that they have reached the limits of reduction for a fully functional genome.