Genetic diversity and molecular epidemiology of Norwalk-like viruses

Specimens (n = 287) from 59 gastroenteritis outbreaks collected from Februa ry 1997 to March 1999 were analyzed by reverse transcriptase-polymerase cha in reaction. The majority of outbreaks (88%) were associated with Norwalk-l ike viruses. Molecular analyses of strains from 46 outbreaks showed the coc irculation during the 1998-1999 winter of 2 genogroup II clusters, accounti ng for 57% and 28% of outbreaks, respectively. An important genetic diversi ty was observed during this 2-year period. Thirteen different genogroup II strains and 3 different genogroup I strains were found. Genogroup I strains , although from the same cluster, were highly divergent (9%-16%). Epidemiol ogic and molecular data indicate that several introductions did not result in any major shift of prominent strains, whereas 1 apparently established i tself. Some point mutations allowed corroboration of epidemiologic links an d strongly suggest that, in several instances, sharing staff and/or transfe r of patients between health care institutions can create a significant ris k for Norwalk-like virus dissemination.