Molecular epidemiology of human enteric caliciviruses in The Netherlands

In The Netherlands, illness due to Norwalk-like viruses (NLVs) of the famil y Caliciviridae is quite common. NLVs cause >80% of the outbreaks of gastro enteritis reported to municipal public health centers and at least 5% of th e cases of gastroenteritis for which a general practitioner is consulted. I n addition, up to 18% of community cases of gastroenteritis in the 1998/ 19 99 winter season have been attributed to NLVs. Patterns of disease activity differ remarkably with "normal" years, when outbreaks occur that are cause d by different types of NLV, and "epidemic" years, when outbreaks appear to be caused by a single strain. This observation suggests selection of antig enic variants with increased virulence or altered modes of transmission. In addition, since caliciviruses related to the NLVs from humans have been de tected in stool specimens from calves at 45% of the dairy farms in The Neth erlands, the possibility of spillover of epidemic strains from an animal re servoir to humans should be considered.