Diagnosis and molecular typing of Enterocytozoon bieneusi: the significant role of domestic animals in transmission of human microsporidiosis
Introduction
Nearly 1,500 microsporidian species in over 200 genera have thus far been reported in a broad range of eukaryotes; most of those species appear well adapted to specific hosts but some have wide host ranges and present serious public health problems (Stentiford et al., 2016; Vavra and Lukes, 2013; Weiss and Becnel, 2014). Microsporidia form an ancestral branch within fungi (Heitman, 2010; James et al., 2013), recent genomic evidences support the organisms might be true fungi (Quandt et al., 2017). Of the 17 microsporidian species known as human pathogens, Enterocytozoon bieneusi is the most prevalent one that infects the gastrointestinal tract and causes parasitic diarrhea mainly in immune-deficient individuals (Li et al., 2019c; Matos et al., 2012; Weiss and Becnel, 2014). E. bieneusi has strong host dependence probably due to its reduced genome (~6 megabases in length estimated) lacking key genes necessary for self-energy production (Akiyoshi et al., 2009; Keeling and Corradi, 2011; Keeling et al., 2010). E. bieneusi is typically transmitted through the fecal-oral route; the lifecycle of this organism begins with the ingestion of environmentally resistant spores by susceptible human and animal subjects, followed by host cell invasion via the cross-pierced polar tube, intracellular replication, mature spore release, and a new circulation in uninfected host (Li et al., 2019c). The repeated detection of E. bieneusi in humans, nonhuman mammals, birds, food, and water implies the likelihood of occurrence of zoonotic, foodborne, and waterborne transmission (Decraene et al., 2012; Galvan et al., 2013; Guo et al., 2014; Hu et al., 2014; Huang et al., 2017; Jedrzejewski et al., 2007; Lee, 2008; Li et al., 2012; Li et al., 2019c; Santin and Fayer, 2011).
Basic researches on biology and pathogenicity of E. bieneusi have been greatly hampered due to its quite small-sized structure and the lack of sustainable in vitro culture method (Visvesvara, 2002), while there have been important global advances in molecular epidemiology in recent years. Those concerned with genomic DNA isolation, PCR amplification, polymorphic analysis of the amplicons, genotype determination, and phylogenetic analysis of E. bieneusi strains from various sources are particularly notable. This review briefly introduces the achievements on diagnosis and molecular typing of E. bieneusi, summarizes genotypes identified thus far in major domestic animal groups around the world, examines phylogenetic patterns and phenotypic profiles of those genotypes, and evaluates the role of those usual animals in zoonotic transmission of this ubiquitous parasite.
Section snippets
Diagnosis
The most recognizable life stage of E. bieneusi is the spore, yet diagnosis by routine microscopy is often missed as the spore is very small in size, around 1μm. In addition, the identification of E. bieneusi spores in stools is often affected by interfering substances such as debris, bacteria, fungi, and some other common mammalian microsporidia, notably Encephalitozoon spp. (Garcia, 2002; Schwartz et al., 1994; Weber et al., 1994). Several staining techniques have been developed to diagnose
Genotyping and phylogeny
In contrast to traditional techniques, molecular typing of microorganisms generally allows a more detailed and comprehensive analysis of the structure of microbe populations. A more precise identification of microbe strains and a more reliable assessment of their prevalence and geographical distribution in various hosts became possible. Comparison of DNA sequences through phylogenetic analysis makes possible to assess the genetic relationships between isolates. As mentioned above, early
Potential zoonotic origins
Both domestic and wild mammals can host human-pathogenic E. bieneusi genotypes (Li et al., 2019c). Although the focus of this review is on the former, the role of wildlife in the epidemiology of E. bieneusi should not be neglected. In many cases, domestic and wild animals share the same outdoor areas in both urban and rural environments, so that interspecies transmission of zoonotic genotypes may occur during their interactions with each other (Dashti et al., 2020). Livestock animals including
Potential transmission routes
A foodborne outbreak of E. bieneusi infection in persons visiting a hotel in Sweden was inferred to be associated with consumption of cheese sandwiches and salad; the isolates were later typed as genotype C (Decraene et al., 2012). Genotype C may originate from fecal pollution by humans as its host range has so far been exclusively confined to humans as evaluated before (Li et al., 2019c). The occurrence of E. bieneusi spores in retail fresh food produce like berries, sprouts, and green-leafed
Conclusions and perspectives
In conclusion, zoonotic transmissibility of E. bieneusi has been explained in detail here, and the possible presence of foodborne and waterborne spore dispersal has been discussed as well. The distribution preference, phylogenetic pattern, and host specificity of E. bieneusi genotypes and the associated zoonotic implications varied a lot among different species of domestic animals (Fig. 1). Almost all the domestic animals investigated here can be the hosts of three of the most important
Acknowledgements
This work is funded by the Natural Science Fund of Heilongjiang Province for Excellent Young Scholars, grant number YQ2020C010.
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