Two forms of linear DNAs have been found in simian (SFV1) and human (HSRV) spumaviruses: a linear duplex unsensitive to nuclease S1 and a sensitive structure with a single-stranded gap. Two nuclease S1 sensitive sites, mapping at the same position for both viruses, have been identified in the gapped structure. Using different molecular subgenomic clones of HSRV as probes in Southern blot analysis, one S1 site was localized in the 3'LTR and the other near the middle of the molecule at about 6.5 kbp from the 5' end of the viral genome. The latter site was shown to correspond to a single stranded region within the linear duplex DNA. Nucleotide sequence analysis revealed that the polypurine tract (PPT) usually found at the 5' boundary of the 3'LTR of retroviruses, is duplicated in HSRV at the 3' end of the pol gene, near the gap. This suggests that the synthesis of plus strand DNA is discontinuous, generating the gap.

Accidental ingestion of natural waters while bathing carries a risk of infection by waterborne protozoa such as Cryptosporidium, Giardia and, possibly, microsporidia. In order to evaluate this risk, we conducted a one-year prospective study of two recreational lakes and three river sites located near Paris, where bathing and boating are frequent. Twenty-litre water samples were collected monthly from each site. Concentrated samples were submitted to immunomagnetic separation followed by immunofluorescence (IMS-IF) for Cryptosporidium and Giardia detection. PCR and PCR restriction fragment length polymorphism (PCR-RFLP) were used for the genetic characterization of Cryptosporidium species on IMS-IF-positive samples. PCR were systematically performed to detect Enterocytozoon bieneusi. Bacteria counts were also determined. IMS-IF revealed low counts of Giardia cysts and Cryptosporidium oocysts in the recreational lakes, with occasional peaks (max. 165 cysts/10 L and 9 oocysts/10 L). By contrast, the river sites were consistently and sometimes heavily contaminated throughout the year. Enterocytozoon bieneusi was found in only two river samples. PCR-RFLP genotyping showed the presence of C. hominis and C. parvum. No correlation was found between the presence or counts of parasites and bacteria, except between the presence of Giardia and high counts of Escherichia coli and enterococci. Based on a previously developed model for quantitative risk assessment of waterborne parasitic infections, we estimated that the mean risk of infection by Cryptosporidium and Giardia associated with swimming was <10(-4) in the recreational lakes, and frequently higher at the river sites.