Department of Public Health, Malta
Acknowledgement to the contribution of the health inspectorate at the Department of Public Health, and the support of the Pathology Department (Bacteriology & Virology), St Luke’s Hospital, Malta.
Small round structured viruses (SRSV) are known to cause epidemic gastroenteritis in institutions and account for 54% of person to person outbreaks and 6% of foodborne outbreaks in England and Wales (1). Infection is commonly transmitted from person to person and also by food, water and inanimate objects. The incubation period is 10 to 50 hours, and the period of infectivity lasts from the acute illness until 48 hours after symptoms have ceased (2). This report describes a large outbreak of SRSV gastroenteritis that occurred in a non-boarding private school (school A) in Malta. The Department of Public Health in Malta was alerted on 23 November 1995 and conducted an investigation to identify the source of the outbreak and to control its spread.
The Maltese Islands cover 316 square kilometres, with a resident population of over 376 000 (3), making it one of the world’s most densely populated countries. Its climate is typical of the Mediterranean, with wet, windy winters and hot, dry summers. Prevalent disease and mortality patterns are similar to those in the developed world. The movement of people to and from the Island is intense. The school where the outbreak occurred had 1600 pupils from preparatory classes (age 5) to sixth form (age 18 years).
Epidemiological study: A case was defined as a pupil with vomiting or diarrhoea with or without other symptoms on 22 to 24 November 1995. To assess the size of the outbreak absenteeism in the school was analysed from 23 November until 4 December. Cases of gastroenteritis were also sought in the general community and in other schools. Three hundred and forty-three pupils were enrolled in a case control study: 126 cases (43%) among 296 absentees and 217 controls chosen by systematic sampling from healthy pupils. Cases were interviewed by telephone on 22 and 23 November and controls on 24 to 26 November using a questionnaire that included basic demographic data, symptoms, and potential risk factors (the consumption of food items and tap water).
Environmental investigation: The water distribution system, the supplier of food for school catering and the tuck shop and standards of hygiene were reviewed.
Laboratory investigation: Stool specimens were submitted for standard bacteriological testing for organisms including salmonella, shigella, campylobacter and Escherichia coli O157, and rotavirus testing using latex agglutination. Nine stools were sent to the Enteric and Respiratory Virus Laboratory, Central Public Health Laboratory (CPHL), London, UK, for viral investigations.
Epidemiology: On a typical winter day (6 November) 42 pupils were absent from 23 classes, but on the 23 and 24 November 296 pupils were absent or sent home from school with gastrointestinal symptoms. Absenteeism was widespread throughout school A extending to the sixth form. By 4 December (11 days after the day of peak incidence), attendance had returned to normal and absenteeism had fallen to 59 from 33 classes. No staff absences were noted and subsequent case finding showed that none had been affected. One hundred and sixty-seven of the 343 pupils who were interviewed (49%) had had symptoms and 104 (30%) fitted the case definition. Symptoms were mainly vomiting, abdominal pain, nausea, and to a lesser extent, diarrhoea and fever (table 1).
Cas selon les symptômes / Cases by symptoms (n=104)
|Symptôme / Symptom
Vomissements / Vomiting
Douleurs abdominales / Abdominal pain
Nausées / Nausea
Diarrhée / Diarrhoea
Fièvre-frissons / Fever-chills
|Proportion affectée / Proportion affected
A precise date of onset was obtained for 114 pupils who had suffered vomiting and diarrhoea. The outbreak peaked on 23 November and lasted three days (22 to 24 November) (figure 1). Cases tended to be younger than controls: <10 years 33.4%; 10-14 years 57.3%, >14 years 9.4%; compared with 29.2%, 48.8%, and 22.2%, respectively, for controls (p=0.03).
No statistically significant relationship was observed between ingestion of food or water at school A and subsequent development of diarrhoea or vomiting. Similar illness patterns were noted from 28 November among pupils at another school (school B) 21 kilometres away. A sample of absentees described brief self limiting gastrointestinal symptoms and some reported that other family members had been similarly affected. Flu-like symptoms were also reported later.
Laboratory: Standard bacteriological investigations (13 patients) and tests for rotavirus (9 patients) were all negative (C. Barbara, Virology Dept, Malta, personal communication). Six of the nine stools investigated for SRSV RNA by RT-PCR were positive. Testing with the Ni/E3 primer pair (which detects most prevalent strains in the UK) only gave weakly positive reactions; the GI/GII/E3 primer set however characterised the strain as genogroup 1 (D. Brown, CPHL, London, UK, personal communication).
Environmental investigation: Preliminary investigations showed that pupils tended to drink bottled water brought from home. Tap water was supplied to the school from roof tanks. No common public swimming or showering facilities were used by cases. No parties or other festivities were known to have taken place. No cooking was done on school premises itself but a small tuck shop obtained foodstuffs from outside caterers which were reheated and sold.
While initial investigations were underway, the school tuck shop suspended the sale of prepared and reheated food stuffs. The caterer was also temporarily suspended from further food preparation until supervised cleaning was completed. Symptomatic pupils were excluded until completely recovered. Preventive measures were also taken in school B. One class, from which half the pupils were absent, was closed on 1 December to avoid further spread. The school authorities were told to ventilate and clean the classroom, and disinfect toilet seats, door knobs, taps, and hand rails. Similar measures were taken in other classrooms over the weekend.
This is the first documented outbreak of virologically confirmed SRSV-1 infection in Malta. The epidemic curve suggested a point source outbreak but no common vehicle (water or food) was implicated. Epidemiological investigation of the two schools affected suggested that person to person spread was probably the main mode of transmission. Episodes were self limiting and harm reduction was undertaken by partial suspension of classes and environmental disinfection.
(1) Djuretic T, Wall PJ, Ryan MJ, Evans HS, Adak GK, Cowden, JM. General outbreaks of infectious intestinal disease in England and Wales 1992 to 1994.Commun Dis Rep CDR Rev 1996; 6: R57-63.
(2) Benenson AS. Control of communicable diseases in man. Fifteenth edition, Washington: American Public Health Association, 1990
(3) Census of Population and Housing – 1995. Preliminary report. p.10. Malta: Central Office of Statistics. March 1996