The Microsporidia and Microsporidiosis

Classification of microsporidia is based on life cycle and structural characters. This chapter describes the structure and application of specific techniques, which helps to determine an organism is a microsporidian. The cell was interpreted to be a uninucleate gamete destined for fusion with another such cell to restore by plasmogamy the diplokaryotic state of a meront. When the cells are covered by the electron-dense material, they are classified as sporonts, the initial stage of sporogony. Whether Encephalitozoon intestinalis rightfully belongs to the genus Encephalitozoon or should be retained in its original genus, Septata. The genus Encephalitozoon is monomorphic, and single nuclei occur in all life cycle stages. Especially in the case of microsporidia from water-dwelling invertebrate hosts, the spores bear ornamentations or mucous layers on their surfaces that can be used as taxonomic characters. The size of spores is an important structural character, but its exact recording is difficult, especially in microsporidia with very small spores. Measuring spores on photographs, made with a carefully calibrated microscope and photographic enlarger, is a suitable alternative if a special eyepiece or image analyzer is not available.

The recognition of microsporidia as opportunistic pathogens in humans has led to increased interest in their molecular biology. Much of the recent work has focused on determination of the nucleotide sequences for rRNA genes. These sequences have been used in the development of diagnostic tools for species identification. In addition, such rRNA gene sequences have facilitated the development of a molecular phylogeny of these organisms. Given the increasing evidence of the importance of the microsporidia as both human and agricultural pathogens, recent work has focused on the identification of microsporidial genes that could serve as potential therapeutic targets. Prior to the advent of comparative analysis of molecular data, the classification of eukaryotes was based on morphological, ecological, and physiological characteristics. Speculation based on this information is problematic because the homology among such characters is often not discernible and characters that could be considered were thought to be either shared or derived depending on the underlying hypothesis used in construction of the phylogeny. A much different view of the phylogenetic placement of the Microspora has been suggested based on analysis of β-tubulin genes. The molecular data present an excellent means of identifying a species and provide an excellent data set for proposing evolutionary relatedness through phylogenetic analysis. Molecular techniques for the diagnosis of microsporidiosis appear to have high sensitivity and specificity. These methods have proven extremely useful both in the identification of animal models as well as in investigations of the epidemiology of microsporidia pathogenic to humans.

The spores of microsporidia possess a unique, highly specialized structure, the polar tube, which is used to inject the parasite from the spore into a new host cell. Several theories have been proposed regarding the method by which the sporoplasm exits the spore and on the function of the polar filament or tube in this process. Electron-dense, particulate material fills the center of the filament. Weidner proposed that this material was unpolymerized polar tube protein (PTP). On the basis of ultrastructural observations, the eversion of the polar tube has been likened to a tube sliding within a tube. This chapter presents details on spore activation and discharge. When sporoblasts form, each one contains five to six coils of the preformed polar filament with the anchoring disk positioned at the anterior end. The major amino acids coded by the Encephalitozoon hellem and Encephalitozoon cuniculi PTP genes were proline and glycine. Application of the techniques of modern biology has resulted in the identification of several PTPs although the interactions and functional significance of these proteins remains to be determined.

This chapter describes the clinical and pathogenic features of microsporidiosis. It mainly focuses on intestinal disease. Most reports of intestinal microsporidiosis have involved human immunodeficiency virus (HIV)-infected individuals. Although the majority of cases have been diagnosed in homosexual males, diagnoses also have been made in heterosexual women and in children. Enterocytozoon bieneusi has been identified in patients with other immune deficiencies, as well as in immuno competent individuals who are asymptomatic or have a self-limited diarrheal illness. Microsporidia were originally believed to cause intestinal disease on the basis of their identification in abnormal small intestinal mucosa of severely immunosuppressed AIDS patients with chronic diarrhea and weight loss. The cardinal feature of intestinal microsporidiosis is injury to the small intestinal epithelium, leading to malabsorption. The various stages in the life cycle of E. bieneusi and Encephalitozoon intestinalis have been characterized by transmission electron microscopy (TEM) in several laboratories. Intestinal microsporidiosis affects the topography of the small intestinal surface. E. bieneusi is most often observed in the upper third of the villus and not in the crypt. There is no acute inflammation. Two agents, fumagillin and albendazole, have been shown to have activity against microsporidia both in vitro and in vivo. In contrast to E. bieneusi, E. intestinalis has a uniformly excellent response to albendazole therapy.

The most robust and widely practicable technique for the diagnosis of microsporidial infection is light microscopic morphological demonstration of the organisms themselves. Evaluation of patients with suspected microsporidiosis should begin with light microscopic examination of stool specimens and urine or cytological examination of other body fluids. The most common clinical finding of ocular microsporidiosis is keratoconjunctivitis. In patients with suspected ocular microsporidiosis, urine and respiratory secretions also should be examined for microsporidia. In patients with ocular microsporidiosis, urine and respiratory secretions should also be examined for the presence of microsporidial spores. Serologic assays (including carbon immunoassay [CIA], IFAT, enzyme-linked immunosorbent assay [ELISA], and Western blot [WB]) have been applied in detecting specific IgG antibodies directed to Encephalitozoon intestinalis spores in humans and to Encephalitozoon cuniculi spores in humans and several animal species. Aside from studies on ocular microsporidiosis in presumably otherwise healthy persons, detailed histopathologic investigation of human microsporidial infection has been performed only in immunodeficient individuals. One clue that microsporidiosis may be present is based on the observation that infected epithelial cells containing mature spores are often shed intact into the lumen from the underlying basement membrane. Upper and lower respiratory tract infections due to microsporidia are associated almost exclusively with disseminated disease produced by all three members of the genus Encephalitozoon. Microsporidia have been found in almost every organ system. Molecular techniques have been used to compare microsporidial isolates obtained from humans and animals in order to study possible sources of infection and reservoir hosts.

Encephalitozoon cuniculi was first reported in rabbits and laboratory rodents in the early 1920s, and this species remains the most frequently observed organism in nonhuman vertebrates. Serological surveys reported prevalence rates as high as 80% in hamsters, 30% in rats, and 85% in guinea pigs. This chapter describes microsporidial infections in a variety of domestic mammalian hosts such as mice, rats, guinea pigs and hamsters, and wild mammalian hosts such as rodents and rabbits. It discusses a few cases of naturally occurring microsporidiosis that have been reported in nonhuman primates. The chapter also talks about microsporidial infections in dometic animal hosts and wild carnivores. Until recently, the species of microsporidia in avian tissues had not been determined, and no parasite species had been categorized as an avian pathogen. Only a few cases of naturally occurring microsporidiosis, including several different parasite genera, are reported in amphibian or reptile hosts.

This chapter highlights the biological and life cycle features of entomogenous microsporidia and provides some basic information on their taxonomic distribution. The development of all insect-parasitic microsporidia is restricted to the cytoplasm of the host cell. The development of all insect-parasitic microsporidia is restricted to the cytoplasm of the host cell. Specialized relationships between microsporidia and the host at the cellular level have been termed xenomas. Weiser distinguished two main types of xenomas in insects, syncytial and neoplastic. The chapter presents the major and minor pathways of transmission for insect microsporidia. Alternatively, the criteria used to establish the genera of microsporidia from aquatic insects may not truly reflect phylogenetic diversity but adaptations to specific habitats and host systems. A section lists a few genera for which insects are not the type hosts. This is because many species have been reported to occur in insects (such as Thelohania, Pleistophora) or because there are possible links to insect microsporidia (such as Trichotuzetia). Finally, the chapter provides diagnostic information which is primarily restricted to the features of sporulation and the spore, with the addition of distinguishing life cycle characteristics when available. The type host and species are given followed by comments on distribution and other matters deemed of importance.