Outbreak of shigellosis in a secondary school in Kaunas, Lithuania

Forty nine students and a teacher fell ill with shigellosis in a Kaunas secondary school. Clinically they had headache, nausea, vomiting, stomacache, high temperature and diarrhoea. Cabbage salad was the transmission factor of the infection and the source of the infection - two foodhandlers in a school kitchen from whom was isolated Shigella sonnei.

Fifty-three children and three adults (one teacher and two workers at the school canteen) at the Kaunas Dainava secondary school sought medical advice between October 28th and November 10th, 1999. Initially, the two canteen workers denied clinical symptoms, and were thus excluded from the active epidemiological investigation. Forty-nine of the children (92.4%) fell ill during the course of two days (October 28th and 29th). Thirty-three of the children and the teacher were admitted to the Hospital of Infectious Diseases, Kaunas District. The other patients were treated at home.

All patients had symptoms of headache, nausea, vomiting, stomachache, high temperature (39-40oC), and voluminous and frequent diarrhoea.

After interviewing the patients, it was established that all of the children with the exception of two had eaten lunch at the school canteen. The first patients to fall ill, four children from classes I-II and one child from class VII, had their initial symptoms on the evening of October 27th, 7-8 hours after the school lunch.

The epidemiological curve showed a great increase in the number of infectious diseases on October 28th. From the data given by the school administration, more than 50% of all children having the school lunch were students from the primary classes (classes I-IV). Most often they received free lunch with coupons. Of the patients who were children, 53% were from the senior classes and 47% from the primary classes. Thirty-three of the sick children were boys (62%) and 20 were girls (38%).

Environmental investigation

On site inspection was conducted and revealed severe violations of the hygienic rules at the school canteen including insufficient volumes of hot water and lack of freezing facilities. Two sick kitchen workers (chief of production and scullery maid) were still working, disregarding hygienic requirements and technical regulations for preparing food.

The assortment of dishes prepared daily at the school canteen was adequate; a few different salads, 3-4 main courses with sauce or garnish, and various drinks (juice concentrates diluted with boiled water). Dishes were being prepared throughout the working day. The temperature inside the dishes was not measured. Various salads and garnishes to the main course were kept at warm temperatures creating favourable conditions for the propagation of microorganisms. There were no means of hand desinfection and drying in the toilet used by the canteen personnel.

Samples of water and food, swabs from surrounding things were taken for microbiological investigation, but the causal agent of the enteric infection was not detected.

Case control study

A case control study was set up to identify the source of the outbreak. In the case control study the 56 cases were compared with 1386 healthy individuals at the same school. The odds ratio (OR) for falling ill after having the school lunch increased from 2,0 on October 26th to 20 on October 27th. The lunch menu on October 27th included milk, soup, wieners with mashed potatoes, carrot salad with garlic, red beets salad with beans, fried fish, tea, orange juice drink, roll "Aguonele", fresh cabbage salad. Those foodstuffs most often mentioned by the patients were included in the analysis (table 1). Salad served separately or together with mashed potatoes as a garnish to wieners was prepared on the morning of October 27th and kept at room temperature. Children having wieners with cabbage salad or just cabbage salad were at increased risk of falling ill (OR 19.7 and 6.0, respectively). Moreover, 49/56 patients (87%) had wieners garnished with fresh cabbage salad and 32/56 patients (57%) had cabbage salad separately.

Table 1. Risk factors for shigellosis in 56 patients and 1386 healthy controls from the same school.  

We also interviewed children from three classes that were not served wieners with cabbage salad for lunch on October 27th; class 11a had no ill student, and classes 11b and 12b, had one ill student each (table 2). Only two students from these classes (11b and 12b) had this dish and both fell ill.

Table 2. Students in three classes who were not served wieners garnished with cabbage salad on the October 27th.

Microbiological investigation

Shigella sonnei 12IIg was identified in samples from the two kitchen workers with symptoms, from the teacher, and from 49 of the children. In four of the symptomatic children no specific microbiological cause could be identified.

Measures taken

A number of measures were taken to stop the outbreak: 1) The canteen was closed for cleaning and desinfection from 2 November to 5 November; 2) Specialists from Kaunas Public Health Centre (PHC) carried out the final desinfection at the school canteen, the quality being tested by microbiological and chemical investigation; 3) the chief of production and the assistant headmaster responsible for feeding the children were fined; 4) the kitchen workers were repeatedly sent to complete the hygienic teaching courses; 5) it was recommended that the head of the canteen should reduce the assortment of dishes; 6) the Kaunas PHC requested that Department of Education connect hot water, and hot flowing water was connected to the school from the city net on November 10th; 7) out-patient cards of non-hospitalised students were checked at the Dainava Children's Polyclinic, and all homes of children that had been in contact with patients were visited and information on prevention (oral and booklets) were provided; 8) the shigellosis outbreak and the preventive measures were discussed at a faculty meeting at the Dainava School; and 9) the Kaunas Health Department, television and other media were informed.

Conclusions

We have described an outbreak of shigellosis. Although Shigella sonnei was not isolated from any foodstuff the results of the epidemiological investigation indicate that cabbage salad was probably the source of this outbreak.