Yesterday’s issue of Eurosurveillance contained an excellent piece of epidemiological field work concerning an outbreak of listeria in Belgium in 2006 and 2007. Edited excerpts below:

A total of 11 cases appear to have been involved in this episode (six in 2006 and five in 2007). Of the positive cultures of these patients, nine were from blood and two from cerebrospinal fluid; four cases were pregnant women or newborns. One pregnant woman had a twin stillbirth. The episode was not geographically clustered, as the isolates were received from three different regions in Belgium: Flanders, Wallonia and Brussels.

The episode was first recognised by the BLRC in November 2006. Only on four occasions were patients asked about their food habits. No standardised questionnaire was used. Suspected food samples were taken from the patients’ refrigerators or from the same batch of the suspected food at the retail level. Smoked salmon was sampled because in a case of preterm birth, the mother remembered having eaten smoked salmon. Raw beef brains were the only suspected food item in a case of septicaemia. The woman with the twin stillbirth reported having eaten pre-packed lasagne; this food item was suspected after some of her housemates presented with gastroenteritis. However, L. monocytogenes could not be detected in any of these samples.

The combination of serotyping, metal resistance typing and PFGE led to the identification of 11 identical isolates. The episodic strain was of serovar 4b, sensitive to arsenic and cadmium and belonged to pulsovar A. Six of them were isolated within a period of a few weeks which is exceptional for a small country like Belgium. Besides the cluster isolations in 2006, the episodic strain was isolated from a further five patients in 2007, indicating a long extension of the episode which went on until July 2007.

The source of contamination was not detected. Two factors may have contributed to this failure: no systematic interviewing of the patients and unsuccessful food sampling. During this episode only four patients were contacted by community health inspectors and only three different food samples were taken which proved to be negative for L. monocytogenes in 25 g.

Serovar 4b is not unusual. In Europe and North America, most published outbreaks of listeriosis in the past 20 years have involved 4b. In addition, strains of serovar 4b tend to be overrepresented in perinatal listeriosis, suggesting that they may have special virulence attributes for pregnancy and breach of the blood-placenta barrier. In the cluster described here, four of the 11 cases were pregnancy-related.

It is presumed that the episodic strain was particularly virulent because it involved a relatively high number of pregnancy-related cases and meningitis cases, four of 11 and two of 11 respectively. According to annual data from the BLRC, strains from cases with maternal-neonatal listeriosis represent 10% of the total number of clinical strains; a similar proportion is observed for cases with meningitis.

This episode would have passed unnoticed had not the BLRC performed strain typing. Efficient monitoring of listeriosis requires systematic interviewing of the patients using a standardised questionnaire. Close cooperation between community health inspectors, the Belgian Federal Agency for the Safety of the Food Chain (FASFC) and the BLRC would result in a rapid linking of sporadic cases and enhance the chance of finding the infection source in outbreaks.