No 45 - 2012

VTEC O157 outbreak September-October 2012

Escherichia coli (E. coli) forms part of the normal intestinal flora. However, specific types of E. coli are entero-pathogenic and frequently cause gastroenteritis. Verocytotoxin-producing E. coli (VTEC) is one of the entero-pathogenic VTEC is defined by the capacity to produce specific toxins, Vtx1 or Vtx2, that are central in the pathogenic process of the bacteria. Another term for VTEC is STEC (Shiga-toxin producing E. coli). EHEC (enterohaemorrhagic E. coli) is also frequently used synonymously with VTEC.

Typical symptoms of VTEC infection are bloody diarrhoea and severe abdominal pain. Some infected person’s progress to haemolytic-uraemic syndrome (HUS) characterised by acute renal failure, microangiopathic haemolytic anaemia and thrombocytopaenia. The risk of progression depends on specific VTEC virulence factors (e.g. the toxins). By typing the genes for the virulence factors, VTEC can be divided into HUS-associated VTEC and low-risk VTEC. Thus, the VTEC prototype, E. coli O157 (the burger bacterium), includes both HUS-associated and low-risk types. This EPI-NEWS describes the first foodborne outbreak caused by a strain of HUS-associated VTEC in Denmark.

The outbreak

In late September, Statens Serum Institut was approached by the Paediatric Department at Copenhagen University Hospital, Rigshospitalet; the Department had treated three children for HUS within a few weeks. On 5 October another case was notified. Four HUS cases within a short period of time represented an unusual increase as the annual number of HUS cases in Denmark typically ranges from two to six.

A subsequent epidemiological investigation examining potential common exposures for the four HUS cases did not establish any such links as neither the patients nor their families had participated in common gatherings, etc. In week 42, the SSI became aware of another HUS case and of three cases of VTEC O157 of a HUS-associated type. Based on these findings, an epidemiological investigation was initiated in order to determine if the cases represented an outbreak – and, if so, to establish the source of infection. The working hypothesis was that the observed cases were part of an E. coli O157 outbreak. As a consequence the first HUS case, which was of the O145 type, was not considered part of the possible outbreak.

Epidemiological and microbiological tests

As from mid-September, a total of nine VTEC O157 infections had been diagnosed. Tests performed on the available bacterial strains confirmed that it was of the same type and with an unusual toxin profile including the genes eae, vtx1a and vtx2a. The microbiological evidence indicated that the cases should be considered part of an outbreak. Due to their geographic distribution, it was likely that the outbreak vehicle would be a common food item.

In all, four of the nine cases developed HUS. Furthermore, three HUS cases were observed in patients without bacteriological confirmation. Subsequent serological results for two of the three cases later indicated O157 infections. Including these cases, the outbreak comprised a total of 12 cases. Patient age ranged between three and 35 years of age (the age of HUS patients ranged from three to 20 years), and seven were female. Patients belonged to a total of eight families and HUS was observed in seven out of eight families.

The date of disease onset ranged from 18 September to 12 October 2012. However, the latest occurring case was, in all probability, a secondary case (person-to-person infection), and the disease onset date for the latest primary case therefore 4 October.

Characteristically, the affected families had young children, enjoyed traditional Danish foods and lived in provincial towns. There were no vegetarians among the patients and patients’ households had done their shopping in a variety of different supermarket chains.

Common to all cases were the fact that they had ingested ground beef during the incubation period, six in the form of minced meat patty. In two cases, there were reports on the ground beef not being cooked thoroughly. On this basis, the outbreak team found it probable that the source of the infection could be ground beef prepared at home, and the Danish Veterinary and Food Administration is currently working to shed further light on this hypothesis. It seems probable that the outbreak has now ended.

VTEC: Diagnosis and treatment

The Danish Society for Clinical Microbiology and Statens Serum Institut recommend testing for VTEC of all patients below the age of seven years with diarrhoea and all patients with bloody diarrhoea regardless of their age. However, not all microbiology laboratories routinely offer VTEC diagnostics, which contribute to making VTEC an under-diagnosed infection. It is therefore essential that the physician submitting the sample specifically request it be tested for VTEC/diarrhoea-producing E. coli. VTEC as well as HUS are individually notifiable diseases. These notifications are an important means to ensure early tracing of infections and prevention of secondary infection. Guidance on the notification of disease caused by haemolytic uraemic syndrome (HUS) and verotoxin producing bacteria (VTEC) - In danish.

Preventing and treating dehydration may maintain the kidney perfusion, which is decisive to reduce the risk of progression into HUS. Conversely, treatment with antibiotics and anti-diarrhoeals should be avoided. Studies indicate that antibiotic treatment may increase the risk of HUS in children with VTEC infection as the bacterium may increase toxin production when treatment is initiated.

Commentary

VTEC, including E. coli O157, is primarily a foodborne infection. VTEC is found naturally in cattle and other ruminant animals. The source of infection is frequently beef - including ground beef - which has not been cooked thoroughly, or products made from beef such as beef-salami. Other foods to suspect include non-heat treated dairy products or other foods that may have been contaminated by cow manure. These include, among others, ready-to-eat vegetables, salad greens, vegetable sprouts and fruit. Contaminated drinking or bathing water or direct contacts to animals (e.g. at farms open to visits from the general public) are also possible sources of infection. Finally, the bacterium may transfer through person-to-person contact. The risk of person-to-person transfer is greater among children in day-care institutions than among other children, but transfer may also take place within families.

The first Danish foodborne VTEC outbreak was described in 2003-4, EPI-NEWS 23/04. It was an E. coli 0157 outbreak with a low-risk type. Epidemiological investigations established that infection had, in all probability, spread from one specific dairy. 2007 saw an outbreak with a different low-risk type, E. coli O26. The source of infection in this outbreak was a specific type of beef salami, EPI-NEWS 16/07. In 2011, Germany observed a very large outbreak of the HUS-associated E. coli type O104. The source of infection was fenugreek sprouts, EPI-NEWS 27-33/11. A total of 26 Danes were recorded as being symptomatic. The Danish cases were primarily infected during travels to Germany.

The current outbreak is the first in Denmark with a foodborne HUS-associated type of E. coli. A remarkable aspect of this outbreak is the high number of HUS cases in relation to the number of patients with VTEC diarrhoea. This indicates that the outbreak strain may be a particularly virulent variant of the bacterium.

The outbreak shows that rapid and timely VTEC diagnostics are essential, and that all HUS cases should be notified without undue delay. The outbreak was relatively limited in scope as the source of infection was presumably ground beef, which has a limited shelf life and is primarily used in dishes that require thorough cooking. If the source of infection had been a food article with a longer shelf life, or had been continual in nature, the scope and consequences of the outbreak would have been more severe.

(B. Søborg, L. Müller, S.G. Lassen, S. Ethelberg, K. Mølbak, Department of Infectious Disease Epidemiology, F. Scheutz, Microbiology and Infection Control).

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7 November 2012