Cryptosporidiosis has recently attracted attention as an
emerging waterborne and foodborne disease as well as an
opportunistic infection in HIV infected individuals. The lack of
genetic information, however, has resulted in confusion in the
taxonomy of Cryptosporidium parasites and in the
development of molecular tools for the identification and typing
of oocysts in environmental samples. Phylogenetic analysis of the
small subunit ribosomal RNA (SSU rRNA) gene has shown that the
genus Cryptosporidium is comprised of several distinct
species. Our data show the presence of at least four species:
C. parvum, C. muris, C. baileyi and C. serpentis (C.
meleagridis, C. nasorum and C. felis were not studied).
Within each species, there is some sequence variation. Thus,
various genotypes (genotype 1, genotype 2, guinea pig genotype,
monkey genotype and koala genotype, etc.) of C. parvum
differ from each other in six regions of the SSU rRNA gene.
Information on polymorphism in Cryptosporidium parasites
has been used in the development of species and strain-specific
diagnostic tools. Use of these tools in the characterization of
oocysts various samples indicates that C. parvum genotype
1 is the strain responsible for most human Cryptosporidium
infections. In contrast, genotype 2 is probably the major
source for environmental contamination of environment, and has
been found in most oysters examined from Chesapeake Bay that
serve as biologic monitors of surface water. Parasites of
Cryptosporidium species other than C. parvum have
not been detected in HIV+ individuals, indicating that the
disease in humans is caused only by C. parvum.
