The structure and infectivity of the oocysts of a new species of Cryptosporidium from the feces of humans are described. Oocysts are structurally indistinguishable from those of Cryptosporidium parvum. Oocysts of the new species are passed fully sporulated, lack sporocysts. and measure 4.4-5.4 microm (mean = 4.86) x 4.4-5.9 microm (mean = 5.2 microm) with a length to width ratio 1.0-1.09 (mean 1.07) (n = 100). Oocysts were not infectious for ARC Swiss mice, nude mice. Wistar rat pups, puppies, kittens or calves, but were infectious to neonatal gnotobiotic pigs. Pathogenicity studies in the gnotobiotic pig model revealed significant differences in parasite-associated lesion distribution (P = 0.005 to P = 0.02) and intensity of infection (P = 0.04) between C. parvum and this newly described species from humans. In vitro cultivation studies have also revealed growth differences between the two species. Multi-locus analysis of numerous unlinked loci, including a preliminary sequence scan of the entire genome demonstrated this species to be distinct from C. parvum and also demonstrated a lack of recombination, providing further support for its species status. Based on biological and molecular data, this Cryptosporidium infecting the intestine of humans is proposed to be a new species Cryptosporidium hominis n. sp.

. Midpoint-rooted neighbor-joining tree reconstructed from variation in the rpoB B gene. Kimura 2-parameter distances were calculated for each pair of sequences. The percentage of bootstrap replicates (n = 1,000) in which a given node was recovered is indicated. Five hundred base pairs of the rpoB gene were sequenced from Sarcocystis sp. and from representatives of S. neurona, S. falcatula, and S. lindsayi in the sea otter (see Table I for details on isolates). The Neospora caninum and Toxoplasma gondii homologs were obtained from GenBank (accession nos. AF095904 and AF138960, respectively). a polymerase chain reaction (PCR) by using degenerate primers designed to amplify the rpoB gene, encoded by the plastid genome of apicomplexans-primers Fl (5'-gcg gtc cca aaa ggg tca gtg gat atg atw twt gaa gat gc) and R3 (5'-gcg gtc cca aaa ggg tca gtc ctt tat ktc cat rtc t). The resulting 504-bp PCR products were directly sequenced using BigDye chemistries and an ABI 3100 automated fluorescent sequencer. Homologous sequences were characterized from isolates of S. neurona, S. falcatula, and S. lindsayi, the origins of which are summarized in Table I. These were aligned to each other and to homologs from Neospora caninum and Toxoplasma gondii by using CLUSTAL W 1.8 (Thompson et al., 1994), available on the bioinformatics server of the Baylor College of Medicine. Relationships of these sequences were investigated by constructing a gene genealogy by calculating Kimura 2-parameter distances from 1,000 bootstrap replicates of the alignment and using the Neighbor-Joining algorithm using MEGA 2.1 (Kumar et al., 2001).The rpoB sequence obtained from the otter isolate was placed as a basal member of a clade that also contained the other examined isolates belonging to Sarcocystis but that included neither N. caninum nor T. gondii (Fig. 3). Concordant topologies were obtained when the minimum evolution and maximum parsimony criteria were used (data not shown). Several nucleotide substitutions distinguish this otter specimen from the isolates representing other species of Sarcocystis. In contrast, the rpoB of isolates representing S. falcatula are comparatively homogeneous. Thus, morphological and genetic evidence indicates that sea otters, in addition to being at the risk of exposure to S. neurona parasites, serve as host to at least 1 other species of parasites belonging to the genus Sarcocystis. LITERATURE CITEDDUBEY

Genetic and phylogenetic characterization of Cryptosporidium isolates at two loci (18S rRNA gene and heat shock gene) from both Australian and United States dogs demonstrated that dog-derived Cryptosporidium isolates had a distinct genotype which is conserved across geographic areas. Phylogenetic analysis provided support for the idea that the "dog" genotype is, in fact, a valid species.