ASCARIS INFECTIONS IN HUMANS FROM NORTH-AMERICA - MOLECULAR EVIDENCE FOR CROSS-INFECTION

Cases of human Ascaris infection occur sporadically in areas such as N . America and Western Europe, where this parasite is thought to be non -endemic. Clinical case histories suggest that many of these cases may be cross-infections from pigs. I describe patterns of variation in th e ribosomal DNA. of Ascaris from 9 such cases. For comparative purpose s, I also describe patterns of variation in parasites obtained from pi gs and humans from worldwide locations. A Hae III restriction site dis tinguishes two classes of rDNA repeats; repeats bearing this restricti on site were found in > 96% of parasites from pig populations worldwid e and in all 9 worms from humans in N. America. In contrast, repeats b earing this restriction site were detected in < 2% of parasites from h umans in endemic areas. The molecular data clearly incriminate pigs as the source of infection in the N. American cases. I discuss evolution ary and public health implications of incomplete host fidelity in Asca ris.