Molecular characterization of Cryptosporidium from selected hosts

Keith Sargent

In this study, domestic dogs and cats in the metropolitan area of the city of Perth, Western Australia, and wild mice caught at various agricultural sites in rural Victoria, were screened for the presence of the coccidian protozoan parasite Cryptosporidium. This study was undertaken to determine the prevalence of Cryptosporidium in these three groups of animals, and to genetically characterize isolates from positive hosts in order to examine the extent of genetic variation between isolates of Cryptosporidium, and to asses their potential for zoonotic transmission. It was hypothesized that Cryptosporidium is common in domestic and wild animals and differs genetically to isolates of Cryptosporidium from humans. Of 116 faecal samples collected from domestic dogs, none were found to be positive for Cryptosporidium. Of 162 faecal samples collected from domestic cats, two were positive for Cryptosporidium (a prevalence of 1.2%). Morphological characterization of oocysts of these isolates showed them to have an average size of 4.6 x 4.0 µm, smaller in size than C. parvum isolates typically seen in humans (5.0 x 4.5 µm). Sequence analysis of 18S rDNA sequences showed cat isolates of Cryptosporidium to have a distinct genotype from those previously reported to occur in humans and cattle. In addition, phylogenetic analysis with other C. parvum isolates and Cryptosporidium species, grouped the cat isolates as a distinctly separate group. These results suggest that cats may harbour a distinct "cat adapted" strain or species of Cryptosporidium. Of 182 mouse faecal samples screened, 11 were found to be positive for Cryptosporidium (a prevalence of 6.0%). Analysis of 18S rDNA sequences showed these isolates to have a distinct genotype from those reported in humans and cattle. The finding that mouse isolates of Cryptosporidium harbour a unique genotype to that seen in isolates recovered from humans, suggests that mice are unlikely to be an important zoonotic reservoir of infection for humans. In addition, a rDNA PCR-RFLP approach, designed for the purpose of characterizing C. parvum isolates, differentiated human and calf genotypes previously identified by rDNA sequencing. Furthermore this technique detected intergroup genetic variation within isolates recovered from humans and animals. The results of this study supported the general hypothesis under investigation. Results suggest that C. parvum is not a uniform species, that there are distinct genotypes of C. parvum present in isolates from animal and human hosts, and as such it would appear that there is a need for a reappraisal of the current taxonomy of the genus Cryptosporidium

Molecular characterization of Cryptosporidium from selected hosts

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