A 298 bp region of the Cryptosporidium parvum 18S rDNA and a 390 bp region of the acetyl-CoA synthetase gene were sequenced for a range of human and animal isolates of Cryptosporidium from different geographical areas. A distinct genotype is common to isolates from cattle, sheep and goats and also an alpaca from Peru and is referred to here as the ' calf '-derived Cryptosporidium genotype. Another genotype of ' human '-derived isolates also appears to be conserved amongst human isolates although humans are also susceptible to infection with the ' calf ' Cryptosporidium genotype. Mice and pigs carry genetically distinct genotypes of Cryptosporidium. Three snake isolates were also analysed, 2 of which exhibited C. muris genotypes and the third snake isolate carried a distinct ' mouse ' genotype.

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The Cryptosporidium ITS1, 5n8S and ITS2 rDNA regions from a number of Cryptosporidium isolates from different hosts and geographical areas were cloned and sequenced in order to investigate the extent of sequence heterogeneity between human and cattle-derived isolates from different geographical locations and also between isolates of Cryptosporidium from different hosts such as cats, pigs, mice and a koala. Calf-derived isolates from different continents were virtually identical as were human-derived isolates from the UK and Australia. Genetic differences between Cryptosporidium isolates were extensive and were in fact greater than the level of nucleotide divergence between Toxoplasma gondii and Neospora caninum rDNA sequences. Based on the sequence information derived from this study, PCR-RFLP of the ITS1 region was undertaken in order to directly amplify and genotype Cryptosporidium isolates from different hosts. This PCR-RFLP approach can now be used for molecular epidemiology studies, circumventing the need for costly sequencing and allowing a wider range of genetically different isolates to be examined.

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