Incidence of viral and protozoal infections reported to HPS in 2006
Norovirus
Norovirus (NV) is the current name for the virus first recognised in 1968 in an outbreak of winter vomiting in a school in the town of Norwalk, Ohio, USA. The name 'Norwalk' virus was coined at that time. Since then, other names for NV infection have included: Small Round Structured Virus (SRSV) and Norwalk-like virus (NLV). Infection can be acquired through consuming contaminated food or water, or more commonly from an infected person via the faeco-oral route, through fomites, or by aerosol spread, particularly when a patient vomits.
In 2006 there were a 2324 laboratory reports of NV, an increase of 772 (50%) on the 1552 reports in 2005. It is the highest level reported to HPS and continues the general upward trend observed since 2002 (Figure 1).
The overall rate of NV in 2006 was 45.6 per 100,000 compared to 30.6 per 100,000 the previous year. There was a wide variation in rates across the NHS boards, with rates ranging from 0 in both Shetland and the Western Isles to 135.7 per 100,000 in Forth Valley. Despite the overall increase in the rate of NV infection, the rate actually declined in four boards: Borders, Grampian, Orkney and Western Isles. The rates for the island NHS boards should be viewed with caution due to the effect of their small population size. Some of the difference between the NHS boards may be due to differences in clinical or reporting practices.
Provisional information from ObSurv (the surveillance system for all general outbreaks of infectious intestinal disease in Scotland) is that there were 254 outbreaks of NV (confirmed or suspected) in 2006, this compares to 192 such outbreaks in 2005, and a previous high of 206 outbreaks in 2003.
Rotavirus
In 2006, 1700 reports of rotavirus were received at HPS, an increase of 98 (6%) on the 1602 reports in 2005. Despite this increase the level is still less than the 1775 reported in 2004. Since 2000 reports have remained fairly stable, varying between 1564 and 1775 each year (Figure 2).
The overall rate of rotavirus in 2006 was 33.4 per 100,000 compared to 31.5 per 100,000 the previous year. Rates varied across the NHS boards from 0 in Western Isles to 87.6 per 100,000 in Grampian. Despite the overall increase in the rate of rotavirus infection, the rate actually declined in six boards and remained the same in two.
During 2006 there were two general outbreaks of rotavirus reported to HPS, both associated with residential institutions.
Other viruses
Only serotypes 40 and 41 subgenus F of adenovirus are recognised gastro-intestinal pathogens, but data collected by HPS includes all reports of adenovirus identification and is therefore likely to be an overestimate of the burden of adenoviral gastroenteritis. During 2006 there were 1053 reports of adenovirus, a difference of only 11 reports (1%) on the 1064 reports the previous year.
In 2006, there were 29 reports of astrovirus - a difference of only one report on the 28 in 2005.
During 2006 reports of hepatitis A declined by eight (30%) from 26 to 18.
Cryptosporidium
A total of 610 isolates of Cryptosporidium sp. have been reported to HPS in 2006. This compares with 709 in 2005, a decrease of 14%. This decrease was not statistically significant HPS receives on average of between 600-800 cases per year. There was one small outbreak, associated with a swimming pool. This compares with three outbreaks notified to HPS in 2005.
Since April 2005, Cryptosporidium isolates have been sent to the Cryptosporidium Reference Laboratory Service in Swansea for speciation. For 2006, 254 (41%) isolates could not be speciated. However, 177 (29%) of these isolates were identified as C. parvum, with 174 isolates (28%) being reported as C. hominis and the remaining isolates speciated as C. meleagridis (4 cases) and C. felis (1 case). The seasonality of infection was markedly different in that the majority of reports to week 28 were C. parvum, whilst the majority of reports in the subsequent weeks to week 52 were C. hominis. This probably reflects the former species being associated with zoonotic infection in the April/May period during calving/lambing and the latter species being associated with person-person spread and returning travellers from abroad.
Giardia
A total of 200 isolates of Giardia sp. have been reported to HPS in 2006. This compares with 197 in 2005, an increase of 1.5%. This increase was not statistically significant although it is the second consecutive year that the numbers of reports of Giardia have increased. We do not know the reasons for this small increase but it may in part be due to better detection of the organism via ELISA, rather than via traditional microscopy techniques.
There were no outbreaks of Giardia reported in 2006 and there have not been for several years. However, since most cases of Giardia appear to be sporadic (although some could also be mini-outbreaks) and laboratory identification procedures and stool selecting criteria differ between Scottish NHS diagnostic microbiology laboratories, it is highly likely that Giardia is under-reported compared with Cryptosporidium. Standardisation of laboratory testing procedures for Giardia in Scotland would enable more meaningful surveillance to be performed at a national level, as would enhancing the quality of reported exposure history data.