ABSTRACT
The term microsporidia encompasses a wide variety of pathogens that are characterized by their intracellular location and their ability to form spores that can infect eukaryotic cells across the entire animal phyla. 1,2 The most important agent causing human disease belongs to the genus Enterocytozoon, with E. bieneusi being the only representative. 3 Unlike other members of microsporidia, for example, those belonging to genus Encephalitozoon, that are known to disseminate to extraintestinal organs like the central nervous system, ocular structures, urinary tract system, etc., the infection by E. bieneusi is limited to the intestinal tract in most of the cases, 2 and only a few reports of pulmonary involvement are documented. 4 Since the target site of infection is the enterocytes, the most commonly encountered symptoms are chronic diarrhea and malabsorption. While the disease may be mild and self-limiting in immunocompetent individuals, immunocompromised patients bear the maximum brunt of the disease, which may cause significant morbidity and mortality. Interestingly, the very first case of E. bieneusi was reported from an AIDS patient from Haiti in 1985. The patient presented with nonresolving diarrhea and wasting, and a new microsporidian agent, E. bieneusi, was described on the basis of morphological characteristics. 5 Over the following 30 years, this organism has been recognized as a ubiquitous enteric pathogen with extensive genetic diversity. It can infect a wide range of hosts and has been isolated from fecal samples of humans, domestic and wild mammals, as well as birds. Molecular studies over the last decade have revealed that the genetic structure of E. bieneusi, although small and compact, is actually quite diverse. 6–9 There exist several genotypes that are not only responsible for distinctive biological attributes but also impart a wide host range to the organism, with some genotypes restricted to humans, and others commonly occurring between humans and animals and between humans and the environment. 10,11 While the former are important in anthroponotic transmission, the latter have significant health concerns regarding the zoonotic and environmental spread of E. bieneusi.
