No 3 - 2012

Shigellosis 2008-2010

Shigellosis is caused by bacteria from the genus Shigella. Shigella infections only occur in humans and spread by person-to-person transmission or via contaminated food or water. Four different Shigella species are distinguished: S. dysenteriae, S. boydii, S. flexneri and S. sonnei. The latter is endemic in Denmark.

Laboratory findings of Shigella are notifiable to the Department for Microbiological Monitoring and Research, SSI, and also clinically notifiable to the Department of Epidemiology. Data from both systems are now publicly available at ssi.dk/data.

Shigellosis was most recently reported in EPI-NEWS 06/08.

Notified cases

In the 2008-2010 period, a total of 494 cases of shigellosis were notified to Statens Serum Institut either via the laboratory notification system, the clinical monitoring system or both systems, Table 1.
 
A total of 478 cases (97%) were reported via the laboratory notification system, while only 285 (58%) were reported via clinical monitoring. Among the 478 cases reported by the laboratory notification system, 291 (61%) infections were S. sonnei, 134 (28%) S. flexneri, 26 (5%) S. boydii, 16 (3%) S. dysenteriae, while the species of another 11 (2%) had not been determined. The number of S. flexneri, S. boydii and S. dysenteriae cases have remained relatively stable over the past ten years, while the number of S. sonnei cases has fluctuated considerably, Figure 1.

The 2007 increase was caused by a major S. sonnei outbreak from imported baby corn, EPI-NEWS 35/07.

Description of shigellosis cases

Among the 494 notified cases of shigellosis, 284 (57%) were females and the median age was 32 years (range 0-85 years). Among the 285 clinically notified cases, 55 (19%) were admitted to hospital. This group primarily comprised patients from the youngest age group, 0-10 years.
Only 50 patients (18%) were infected in Denmark, and among these the youngest age group was also predominant. The majority of cases acquired in Denmark were S. sonnei (66%), followed by S. flexneri (10%), Figure 2.

Travel-associated cases

A total of 225 cases (79%) were infected outside of Denmark. A fourth of these were infected in Egypt (56) after which the most frequently reported countries were: India (21%), Syria (4%) and Pakistan (4%). The remaining 102 cases were distributed on 54 countries and five continents.

Occupational infection

A total of ten cases of presumed occupational infection were notified; four in 2008, three in 2009 and three in 2010. Four were infected while stationed abroad, one was a sewage/sanitation worker, two were foodindustry workers, one worked at an asylum centre, one at a department of clinical microbiology, and one was infected during a business travel.

Outbreaks

In 2008-2010,  three shigellosis outbreaks were reported in the database of foodborne outbreaks (FUD). In May 2008-2010, 17 of 61 guests at a confirmation held at a local hall in North Jutland were infected with S. flexneri. It was not possible to determine if the source was a food ingredient or the outbreak was caused by person-to-person transmission.

The second outbreak was caused by S. sonnei in April-May 2009 when a total of ten patients from across the country were notified to SSI. The source was identified as sugar peas from Kenya, EPI-NEWS 36/09.

The third outbreak also occurred in April of 2009 when four of six guests at a private get-together were infected with S. sonnei. The source was presumably raw tiger shrimps. In addition to the outbreaks reported in the FUD, 13 clusters were identified via clinical notifications, primarily consisting of family members who fell ill after travelling together abroad.

Commentary

In the 2008-2010 period, the number of shigellosis cases remained relatively stable as only few and minor outbreaks were observed. 

Also, as previously seen, the majority of the notified patients were infected abroad. Monitoring of shigellosis in Denmark is important as it contributes to the investigation of outbreaks and initiation of preventive measures. This report shows that the laboratory notification system is nearly complete, but that only slightly more than half of the laboratory-diagnosed cases were notified by the treating physicians. Notification is mandatory on paper Form 1515 to the Department of Epidemiology and to the Medical Office of Health.

(L. Müller, C. Kjelsø, S. Ethelberg, Department of Epidemiology)

18 January 2012