Norovirus strains belonging to the GII.4 genotype dominate as a cause of nosocomial outbreaks of viral gastroenteritis in Sweden 1997--2005. Arrival of new variants is associated with large nation-wide epidemics.
BACKGROUND
In recent years an increase of the incidence of nosocomial outbreaks caused by noroviruses has been observed throughout Sweden, with high peaks noted in the winter seasons 2002/2003 and 2004/2005, respectively.


OBJECTIVE
To phylogenetically characterize norovirus strains causing nosocomial outbreaks from 1997 to 2005 and estimate the impact of norovirus-like disease on the Swedish health care system during the peak season 2002/2003 when a new variant of norovirus occurred.


METHODS
Stool samples from 115 randomly selected nosocomial outbreaks occurring during 1997--2005 throughout Sweden were studied by RT-PCR and sequencing. In addition, to investigate the impact on the health-care system, a questionnaire was distributed to infection control units (n=90) serving all Swedish hospitals, nursing homes and other health-care institutions during the largest epidemic of nosocomial outbreaks.


RESULTS
Sequencing of 279 nucleotides of the norovirus RNA polymerase gene in stools containing norovirus RNA showed that strains belonging to the GII.4 genotype dominated. Each of the two large epidemics was due to a new variant within this cluster. The questionnaire revealed that 30,000-35,000 episodes of nosocomial norovirus-like infections occurred in 80 of 82 major Swedish hospitals affected in 2002/2003.


CONCLUSIONS
New norovirus variants within the cluster GGII.4 may have a major impact on the health-care system.
Johansen
Kari
K
Department of Virology, Immunology and Vaccinology, Swedish Institute for Infectious Disease Control, 171 82 Solna, Sweden. kari.johansen@smi.ki.se
Mannerqvist
Kerstin
K
Allard
Annika
A
Andersson
Yvonne
Y
Burman
Lars G
LG
Dillner
Lena
L
Hedlund
Kjell-Olof
KO
Jönsson
Klas
K
Kumlin
Urban
U
Leitner
Thomas
T
Lysén
Maria
M
Thorhagen
Margareta
M
Tiveljung-Lindell
Annika
A
Wahlström
Cecilia
C
Zweygberg-Wirgart
Benita
B
Widell
Anders
A
D017250:Caliciviridae Infections / Q000453:epidemiology* / Q000821:virology; D003428:Cross Infection / Q000453:epidemiology* / Q000821:virology; D004196:Disease Outbreaks; D005243:Feces / Q000821:virology; D005759:Gastroenteritis / Q000453:epidemiology* / Q000821:virology; D014644:Genetic Variation; D005838:Genotype; D006801:Humans; D017720:Molecular Epidemiology; D029322:Norovirus / Q000145:classification* / Q000235:genetics; D010802:Phylogeny; D011159:Population Surveillance; D020133:Reverse Transcriptase Polymerase Chain Reaction; D012621:Seasons; D017422:Sequence Analysis, DNA; D011795:Surveys and Questionnaires; D013548:Sweden / Q000453:epidemiology