T. Ziese, MD, Y. Anderson, B. de Jong, S. Löfdahl, PhD, M. Ramberg
Swedish Institute for Infectious Disease Control, Department of Epidemiology, Stockholm, Sweden.
Escherichia coli O157 infections present in different ways, including mild diarrhoea, severe haemorrhagic colitis, and the haemolytic uraemic syndrome (HUS). Between 2% and 21% of cases have been observed to develop HUS, whose case fatality rate is 3% to 7% (1). E. coli may be isolated from faeces. The diagnosis of HUS is clinical, and may be corroborated by isolation of the organism or the identification of antibodies to E. coli O157 lipopolysaccharide in the blood. Several outbreaks of E. coli O157 infection have occurred in Great Britain, Canada, and the United States in recent years.
The Swedish Institute for Infectious Disease Control (Smittskyddsinstitutet – SMI) received no more than three reports of enterohaemorrhagic E. coli O157 infections (EHEC) each year from 1988 to 1994. About half of these were due to E. coli O157. In July 1995, two cases (two siblings) with diarrhoea caused by E. coli O157 were observed and reported in the Journal of the Swedish Medical Association. By late October, the SMI was receiving reports of up to 10 cases of EHEC infection each week. Most of the EHEC cases were confirmed by a polymerase chain reaction (PCR) test at the SMI. They belonged to the serotype O157 and produced Vero cytotoxin 2.
By the end of February 1996, a total of 110 cases had been reported (figure), 29 (26%) of whom developed HUS. No deaths occurred. Half of the cases were under 5 years of age. Equal numbers of both sexes in cases were under 20 years; two thirds of the cases aged 20 or over were women.
Reports were received from most counties and it was assumed that the outbreak was caused by a common source with a nationwide distribution. The SMI began a case control study. The case definition included clinical features (diarrhoea or HUS) and laboratory confirmed infection with E. coli O157 in stool specimens. If several members of a household were affected, only the first case (index case) was included in the study. Two controls were matched to each case by sex, age, and region from the Swedish central population register. A detailed questionnaire was administered over the telephone. It included demographic information and over 150 questions about the consumption of food items. By 1 December, the case control study had not implicated any single food and a new questionnaire, including questions about food handling, was therefore introduced.
Over 200 food samples from refrigerators and freezers in patients’ homes and from stores where food had been bought were investigated for the presence of E. coli O157, with negative results. In parallel, the SMI developed and established a PCR for detailed subtyping of E. coli O157. At the beginning of the outbreak, the subtyping was restricted to E. coli O157 and showed that different subtypes of E. coli O157 were involved. To clarify the distinction between strains, a new classification was introduced and is currently being evaluated.
The SMI, the National Food Administration, and the consultants in communicable disease control (CCDCs) took the opportunity of the great public interest during this first EHEC outbreak in Sweden to stress the importance of good hygienic standards in the kitchen to prevent infection with EHEC. In February 1996 the SMI, the National Board of Health, and the National Food Administration began a surveillance study of the presence of E. coli O157 in imported and Swedish meat in February. Some 1600 samples of beef will be investigated to estimate the frequency of E. coli O157.
The outbreak is now considered to have ended, as the EHEC cases reported in April are believed to have acquired the infection abroad. The results of the case control study are not yet complete, as not all the laboratory analyses are available. As a result of this outbreak, most of the microbiological laboratories now look routinely for EHEC in faecal specimens from patients with bloody diarrhoea. Because of the possibility of further outbreaks, infections with EHEC became notifiable in Sweden in January 1996.
Cases of reported EHEC by week of onset – Sweden, July 1995 to February 1996
1. Wall PG, McDonnell RJ, Adak GK, Cheasty T, Smith HR, Rowe B. General outbreaks of Vero cytotoxin producing Escherichia coli O157 in England and Wales from 1992 to 1994. Commun Dis Rep1996; 6: R26-33