Cryptosporidium taxonomy: recent advances and implications for public health

Abstract

There has been an explosion of descriptions of new species of Cryptosporidium during the last two decades. This has been accompanied by confusion regarding the criteria for species designation, largely because of the lack of distinct morphologic differences and strict host specificity among Cryptosporidium spp. A review of the biologic species concept, the International Code of Zoological Nomenclature (ICZN), and current practices for Cryptosporidium species designation calls for the establishment of guidelines for naming Cryptosporidium species. All reports of new Cryptosporidium species should include at least four basic components: oocyst morphology, natural host specificity, genetic characterizations, and compliance with the ICZN. Altogether, 13 Cryptosporidium spp. are currently recognized: C. muris, C. andersoni, C. parvum, C. hominis, C. wrairi, C. felis, and C. cannis in mammals; C. baïleyi, C. meleagridis, and C. galli in birds; C. serpentis and C. saurophilum in reptiles; and C. molnari in fish. With the establishment of a framework for naming Cryptosporidium species and the availability of new taxonomic tools, there should be less confusion associated with the taxonomy of the genus Cryptosporidium. The clarification of Cryptosporidium taxonomy is also useful for understanding the biology of Cryptosporidium spp., assessing the public health significance of Cryptosporidium spp. in animals and the environment, characterizing transmission dynamics, and tracking infection and contamination sources.

FIG. 1.

Oocysts of C. parvum and some C. parvum-related species. Modified from reference 247.

FIG. 2.

Genetic relationship among named Cryptosporidium species and unnamed genotypes inferred by a neighbor-joining analysis of the partial SSU rRNA gene. Values on branches are percent bootstrapping using 1,000 replicates. Numbers following species or genotypes are isolate identifications used in the construction of the phylogenetic tree, whereas numbers in parentheses are the number of isolates sequenced. Modified from reference 255.

FIG. 3.

Genetic diversity in C. parvum and C. hominis from AIDS patients in New Orleans, based on neighbor-joining analysis of the partial GP60 gene. Values on branches are percent bootstrapping using 1,000 replicates. Five allele families of parasite are seen: Ia, Ib, and Ie are C. hominis allele families, and Ic and IIa are C. parvum allele families. Modified from reference 249.