surveillance of viral gastroenteritis outbreaks was initiated just over a decade ago at the national institute for public health and the environment in the netherlands ( rivm ) .
outbreaks were typically reported by the municipal health service or the food inspection services to the rivm , and samples were collected in close collaboration with these agencies . to determine the role of nov and
possible differences between different nov strains in gastroenteritis outbreaks , a minimal set of epidemiologic data ( setting , date of onset , number of persons affected , most probable mode of transmission , and number of hospitalizations ) was collected for reported outbreaks .
these data were supplemented with results of molecular biologic detection and typing techniques to enable more in - depth analysis of surveillance data .
preliminary typing of strains was performed by sequencing region a ( 280 nt in the polymerase gene ) of the virus .
a systematic selection of strains was also typed by sequencing region c ( 277 nt in the capsid gene ) ( 1,2 ) . because nov activity is much higher in winter months
, seasons were analyzed from july through june , rather than per calendar year . from december 18 , 1993
to december 26 , 2005 , a total of 1,032 gastroenteritis outbreaks were reported to the rivm .
, 695 ( 74% ) outbreaks met our inclusion criteria for a nov outbreak ( 25% of samples positive by reverse transcription 
overall , we observed an increasing trend in the number of reported outbreaks per year ( figure , panel a ) . in the 199596 ,
200102 , 200203 , and 200405 seasons , more nov outbreaks were reported ( 66 , 90 , 154 , and 161 , respectively ) .
ggii virus strains were predominant in all years and caused 577 ( 91% ) of 631 outbreaks with known genotypes , compared with 36 ( 6% ) of 631 outbreaks caused by ggi or ggiv viruses and 18 ( 3% ) of 631 outbreaks caused by mixed infections with viruses of different genotypes ( table ) .
* mw , mainly waterborne ; f , foodborne ; ptp , person - to - person ; u / o , unknown or other ; t , travel associated ; s / d , school or daycare center ; r / c / c , restaurant , canteen , or caterer ; h , hospital ; ph , private house ; ri , residential institution .
ggii.4 strains have been detected since 1995 , with the highest proportions observed in years with high numbers of outbreaks . in the epidemic seasons of 199596 , 200203 , and 200405 the percentages of outbreaks caused by ggii.4 were 82% , 83% , and 89% , respectively , compared with an overall average of 68% ( figure ) . in seasons after these epidemics
, the percentage caused by ggii.4 decreased to 39% in 199697 , 55% in 200304 , and 32% in the first half of 200506 .
multiple nov genotypes co - circulated throughout the years of the study , but in postepidemic years , outbreaks caused by non - ggii.4 strains were more common ( figure , panel c ) .
the high number of outbreaks in 200102 may be partially explained by emergence of a new variant of ggii.4 in the spring of 2002 ( 4 ) , which caused uncharacteristically high numbers of outbreaks between april and june .
the epidemic increases in the number of outbreaks and seasonality of outbreaks were mainly attributable to ggii.4 .
strains with genotypes other than ggii.4 were found at similar levels throughout the year ( data not shown ) .
of ggii.4 outbreaks by the dotted line ( values on left y - axis ) , bars indicate percentage of ggii.4 outbreaks of the total no . ( values on right y - axis ) , and arrows indicate epidemic seasons .
shading of the bar indicates the percentage of ggii.4 , ranging from white ( 0%20% ) , in steps of 20% , to black ( 80%100% ) .
a total of 548 ( 79% ) outbreaks were reported in healthcare settings ( hospitals and residential institutions ) compared with 102 ( 15% ) in nonhealthcare settings ; for 45 ( 6% ) outbreaks no data were available ( table ) .
a total of 407 ( 81% ) of 502 outbreaks with genotyping information in healthcare settings were caused by ggii.4 nov strains ( table ) . in the nonhealthcare settings ,
ggii.4 was significantly less prevalent ( 39 [ 43% ] of 91 , p<0.0001 ) , resulting in a relative risk at least 2.17 higher for acquiring a ggii.4 infection in a healthcare setting than in other settings . a mode of transmission was reported for 272 ( 39% ) outbreaks .
data confirmed that the main transmission route was person to person ( 60% , 163/272 ) ( table ) ( 5,6 ) .
ggii.4 strains were found in 73% of person - to - person outbreaks compared with 44% of food - related outbreaks ( p<0.0001 ) . when outbreaks for which no genotype was known were counted as non - ggii.4 strains , the relative risk of finding ggii.4 in an outbreak caused by person - to - person transmission was 2.3 greater than finding it in a foodborne outbreak or relative to other genotypes .
multiple ggii nov strains were found in 18 outbreaks ( 3 foodborne , 1 waterborne , 4 person - to - person , and 10 with unknown modes of transmission ) .
detailed molecular epidemiologic data from long - term surveillance on nov outbreaks are rare because nov molecular detection techniques became available only in the mid-1990s . in our 12-year surveillance study , we observed large differences in the magnitude of the annual winter peak of nov infection .
although this finding has been observed in studies covering a shorter period ( 3,57 ) , our data suggest an increase in infections with ggii.4 in recent years , particularly in healthcare settings .
however , the number of outbreaks reported in healthcare settings is likely overrepresented in our study because of mandatory reporting of illness in such settings .
an actual increase in the number of outbreaks can not be proven based on passive surveillance data alone , but an increase is strongly suggested by increased prevalence of ggii.4 in recent years and supported by reported shifts in the predominant ggii.4 variant associated with large numbers of outbreaks ( 8) .
the overall dominance of ggii.4 suggests that this genotype is more transmissible than other genotypes in healthcare settings , where close contact of many persons favors person - to - person transmission .
transmission may also be affected by poorer hygiene or greater susceptibility to infection . increased transmissibility could result from increased levels of shedding of ggii.4 or altered stability of virus particles outside the host compared with other genotypes .
alternatively , changes in circulating viruses may lead to differences in host cell binding or immune recognition , thereby changing the dynamics of infection or size of the population at risk .
our group and other researchers have reported the emergence of distinct ggii.4 lineages in 199596 , 2002 , and 2004 ( 2,4,9,10 ) .
this suggests that the changing phenotype of ggii.4 strains results in increased numbers of outbreaks ( 10 ) . a detailed characterization of ggii.4 strains is ongoing to determine the molecular mechanisms involved in observed epidemiologic patterns .
the marked decrease in the percentage of ggii.4 strains during seasons after epidemic seasons caused by variant strains suggests that populations may acquire immunity against these predominant strains .
the value of this surveillance dataset will increase with its continuation , as well as with its expansion as part of a european surveillance network ( http://www.eufoodborneviruses.co.uk/ ) .
future research will be directed at understanding the molecular basis for observed changes in the epidemiology of nov and control of its spread .
 from 1994 through 2005 , gastroenteritis outbreaks caused by norovirus generally increased in the netherlands , with 3 epidemic seasons associated with new ggii.4 strains . increased percentages of ggii.4 strains during these epidemics , followed by a sharp decrease in their absolute and relative numbers , suggest development of immunity . 
The Study
Conclusions