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    • Current evidence indicates that the main

    2018-11-06

    Current evidence indicates that the main reservoirs of zoonotic Cryptosporidium remain livestock, with the potential transmission of C. parvum (= C. pestis (see Slapeta, 2011)), although other species, and genotypes, have been reported in humans but only occasionally (Slapeta, 2013). Susceptibility to infection with other host adapted species and genotypes is largely governed by the immune status of the host (Slapeta, 2013). Interestingly, although cattle have been repeatedly implicated as sources of water-borne outbreaks, the application of genotyping procedures to the contaminating isolate(s) has often incriminated human effluent as the source (Hunter and Thompson, 2005; Thompson, 2003). The risk of infection appears greater within rural environments than within urban areas; presumably because of the increased opportunity for both direct and indirect transmission to occur in areas with poor sanitation and higher contact rates with domestic animal reservoirs of infection (Thompson and Smith, 2011).
    Diversity — taxonomic issues impede progress? The taxonomy of the genus Cryptosporidium has been controversial for many years with a number of taxonomic revisions that have seen species invalidated because descriptions were deemed inadequate in terms of morphological distinctness and/or concern that host occurrence was not worthy of species recognition (O\'Donoghue, 1995; Slapeta, 2013). With the advent of molecular tools, the number of species has increased dramatically, the majority described on the basis of genetic distinctness and host occurrence. In most cases, there are minimal morphological characters to distinguish species of Cryptosporidium. Slapeta (2013) drew attention to the fact that in the last decade there was approximately one new species named each year, and 10 species proposed for 2004–2013. The most recent review listed 30 species but in addition to species that have been recognised as a result of surveys of humans and domestic animals there is growing evidence of numerous genotypes, identified in wildlife and in environmental samples (Appelbee et al., 2005; Oates et al., 2012; Slapeta, 2013). Given that Cryptosporidium is a gregarine we can expect the number of species and genotypes to grow considerably since gregarines are considered to be the most diverse group of GW5074 (see below).
    Coccidial relationship challenged Significant observations and research findings that have influenced opinion concerning Cryptosporidium being placed within the Gregarines are summarised in Table 1. Although believed for many years to be coccidia, species of Cryptosporidium were always considered to be atypical for a number of reasons (see Introduction). In addition to lacking key morphological structures such as sporocyst, micropyle, and polar granules, (See Table 2.) a critical observation, although largely overlooked at the time, was a report of serological cross-reactivity with Monocystis, a gregarine (Bull et al., 1998). This relationship was reinforced when SSU-rDNA sequencing demonstrated that Cryptosporidium is more closely related to gregarines (Carreno et al., 1999). Most recently, Cavalier-Smith (2014) undertook a revision of gregarine higher classification, and the evolutionary diversification of sporozoa on the basis of gregarine site-heterogeneous SSU-rDNA trees. This has firmly placed Cryptosporidium within the gregarines, demonstrating that some ‘eugregarines’ and all ‘neogregarines’ are closely related to Cryptosporidium. Cavalier-Smith (2014) established a new subclass, the Orthogregarinia for Cryptosporidium and other closely related gregarines, with Cryptosporidium in its own subclass, the Cryptogregaria; defined as comprising epicellular parasites of vertebrates possessing a gregarine-like feeder organelle but lacking an apicoplast. In addition to the ‘molecular’ evidence, Cryptosporidium shares many biological features with gregarines, including its epicellular location, connection to the host cell via a myzocytosis-like feeding mechanism, heterogeneity of trophozoite cell shape, and other structural similarities (Aldeyarbi and Karanis, 2016; Barta and Thompson, 2006; Borowski et al., 2008, 2010; Clode et al., 2015; Valigurová et al., 2007, 2008). The gliding movements seen in different stages of Cryptosporidium is a behavioural feature similar to the gliding motility exhibited by gregarines (Borowski et al., 2008, 2010; Sibley, 2004; Valigurová et al., 2013). The ability to observe the life cycle and development of Cryptosporidium in in vitro culture has made an important contribution to recognising Cryptosporidum\'s gregarine similarities, not only by demonstrating previously unrecognised stages in the life cycle, incredible developmental plasticity and the occurrence of syzygy, but also the fact that Cryptosporidium is not an obligate epicellular parasite. Cryptosporidium has been shown to have the capacity to multiply epicellularly and extracellularly, again reflecting the fact that Cryptosporidium is closely related to gregarine protozoa (Hijjawi et al., 2004; Karanis et al., 2008, Koh et al., 2013, 2014; Rosales et al., 2005), which can also multiply by either means (Leander and Ramey, 2006).