Since summer 2006, the majority of outbreaks in Hungary have been caused by the new variant of the virus GGII.4 2006b, which was identified early this year in several European countries, including Hungary (in April). A rise in norovirus circulation had been expected in subsequent seasons following the appearance of this new variant.

In the second quarter of 2006, an especially high virus circulation was noted in north east Hungary. This was was thought to be due to the outbreak of drinking water-related diseases which occurred in June 2006 in Miskolc (the capital of Borsod-Abaúj-Zemplén county in the north east), affecting 3600 people. Although the outbreak had multiple causes, the predominant agent was found to be norovirus. Following this outbreak, the number of norovirus outbreaks in the area increased. In the third quarter of 2006, increased norovirus activity spread to the neighbouring county of Heves and to central part of Hungary, and in October and November 2000 it reached the northern lowlands and the northern and western Trans-danubian region.

So far, in 2006, norovirus outbreaks have been reported as occurring in similar settings to outbreaks in recent years: 42% (54 outbreaks) took place in hospitals, 31% (39 outbreaks) in residential homes for old people, 13% (16 outbreaks) in settings with children (such as nurseries and schools) and the remaining 15% (19 outbreaks) in other places. In most outbreaks, including the recent ones in November 2006, the transmission was person to person; other modes of transmission played a negligible role.

Compared with previous years, the overall number of norovirus outbreaks reported in Hungary between January and November 2006 is almost 2.5 times higher than during the same period in 2005, and exceeds the median value for the years 2002-2004.

In the first quarter of both 2005 and 2006, a similar number of outbreaks was reported (62 and 68 respectively). However, in the second quarter of 2006, the number of outbreaks almost doubled when compared with the same period of 2005, and in the third quarter and October and November of 2006, it was four times higher than in the same period of 2005. (Figure 1)

The total number of probable and laboratory confirmed norovirus outbreaks in Hungary in 2005 did not exceed the median value for 2000-2004 (consequently, 2005 was considered to be a year with low norovirus activity), whereas from January to November 2006 the number of outbreaks in all periods was above the median of these previous years, and in October and November 2006 it was three times higher than the median (Figure 1).

Figure 1. Total number of probable and confirmed norovirus outbreaks, Hungary, January 2005 - November 2006.

Norovirus surveillance in Hungary began in 1998, and since 2002, Hungarian national authorities have participated in the ‘Food-borne Viruses in Europe’ network (http://www.eufoodborneviruses.co.uk/). After the routine bacteriological investigation of an outbreak has been completed, representative samples undergo ELISA analysis and both positive and negative samples are sent to the national reference laboratory for gastroenteric viruses for further investigation, where a PCR test and comparative genetic analysis of the viruses identified are performed [1].

Thanks to this procedure, norovirus has been established as the cause of 569 gastroenteritis outbreaks in Hungary since 1998. In a further 583 outbreaks, the norovirus was thought to be the probable cause on the basis of clinical and epidemiological data alone, because laboratory investigation either was not performed or yielded negative results (Figure 2).

Figure 2. Gastroenteric outbreaks caused by norovirus or non-bacterial unknown agent 1998 - 2006

According to the surveillance data gathered so far, 2002/2003 has been the most active norovirus season since surveillance began. The sharp rise in number of norovirus outbreaks in that period was due to the increased circulation of the then newly-identified variant (G II.4 2002) which was detected in Europe in 2002, including in Hungary [2]. The increase in the number of outbreaks seen in 2006 seems to follow a similar pattern and be due to the newly identified genotype GII.4 2006 variants, but as the 2006/2007 season is not yet over, it will be possible to draw the final conclusions only after the surveillance data has been completed and put in a larger international context.