Description Of The Organism Classification

Cryptosporidium spp. are protozoan parasites of the phylum Apicomplexa, class Sporozoasida, subclass Coccidia, order Eucoccidiorida, family Cryptosporidiidae (four sporozoites within oocyst) (Tzipori and Griffiths, 1998). Like other related coccidial parasites, such as Eimeria, Cyclospora and Isospora, Cryptosporidium develops in the gastrointestinal tract of vertebrates. Unlike Sarcocystis and Toxoplasma, Cryptosporidium does not require extraintestinal development and is not included in the classification of cyst-forming coccidia. Cryptosporidium spp. also differ from other coccidians by their predominant

Principles and Practice of Clinical Parasitology

Edited by Stephen Gillespie and Richard D. Pearson © 2001 John Wiley & Sons Ltd

Fig. 6.1 Proposed phylogeny of C. parvum species. Recently, neighbor-joining analysis of small subunit rRNA sequences of C. parvum and other proposed Cryptosporidium species suggests that closely related strains from humans, monkeys, calves, mice, guinea pigs (C. wrairi), geese (C. meleagridis) and cats (C.felis) be considered a C. parvum group. It should be noted that C.felis has recently been identified in AIDS patients. Adapted by permission from Xiao et al. (1999b); reproduced by permission from Sears (2000)

Fig. 6.1 Proposed phylogeny of C. parvum species. Recently, neighbor-joining analysis of small subunit rRNA sequences of C. parvum and other proposed Cryptosporidium species suggests that closely related strains from humans, monkeys, calves, mice, guinea pigs (C. wrairi), geese (C. meleagridis) and cats (C.felis) be considered a C. parvum group. It should be noted that C.felis has recently been identified in AIDS patients. Adapted by permission from Xiao et al. (1999b); reproduced by permission from Sears (2000)

lack of host specificity, antibiotic resistance, use of the parasitophorous vacuole and ability to auto-infect the host.

Cryptosporidium spp. were traditionally named after the host species in which they were originally found. After further study, however, most original species names have been discarded. Up to eight current species of Cryptosporidum are recognized, four mammalian (C. parvum, C. muris, C. felis and C. wrairi), two avian (C. baileyi and C. meleagridis), one reptilian (C. serpentis) and one fish (C. nasorum). Synonyms for the human pathogen C. parvum include C. agni, C. bovis, C. cuniculus, C. garnhami and C. rhesi (Tzipori and Griffiths, 1998; Xiao et al., 1999a).

Taxonomic classification of Cryptosporidium into species remains controversial, given conflicting morphologic, genotypic and cross-transmission data. Recent evaluations of the genetic variability of Cryptosporidium by sequence analysis of the parasite small subunit (SSU) rRNA suggest that isolates from humans, monkeys, calves and mice were highly related and somewhat more distantly related to C. felis, isolated from cats (Xiao et al., 1999b) (Figure 6.1). It is of interest that genetic studies of the SSU rRNA have identified infection with C. felis and with an isolate from a dog in AIDS patients, suggesting greater variability in the C. parvum strains infecting at least AIDS patients than previously appreciated (Pieniazek et al., 1999).

Numerous studies now indicate that C. parvum isolates exhibit considerable molecular heterogeneity. Molecular diversity of C. parvum has been demonstrated by identification of parasite antigens, isoenzyme-typing studies and by studies evaluating genetic polymorphisms of C. parvum genes or DNA fragments. An important advance in the understanding of C. parvum is the genetic distinction between 'human' (genotype 1) and 'bovine or animal' (genotype 2) isolates (Table 6.1). This distinction has been made by evaluations of polymorphisms in C. parvum DNA by numerous techniques, including random amplified polymorphic DNA analysis (RAPD) (Morgan et al., 1995), evaluation of restriction enzyme sites in repetitive DNA sequences (Bonnin et al., 1996; Carraway et al., 1997), by comparison of the nucleotide sequences of the 18S rRNA gene and the internal transcribed spacer 1 (ITS1) from the ribosomal repeat unit (Carraway et al., 1996; Morgan et al., 1997) and in mutations in the gene encoding

Table 6.1 C. parvum: two distinct genotypes

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