V. Vaillant 1, S. Haeghebaert 1, J.C. Desenclos 1, P. Bouvet 2, F. Grimont 3, P.A. Grimont 2 3, A.P. Burnens 4
1 National Network of Public Health, Saint-Maurice, France
2 National Reference Centre of Salmonella and Shigella, Institut Pasteur, France
3 National Reference Centre of Enteric Molecular Typing, Institut Pasteur, Paris, France
4 National Reference Laboratory for Foodborne Diseases, Berne, Switzerland
On 20 December 1995, the National Network of Public Health (Réseau National de Santé Publique – RNSP) was notified by the Salmonella and Shigella National Reference Centre (Centre National de Référence – CNR) that a greater than expected number of human isolates of Salmonella dublin had been made in November and December 1995. The RNSP had already learnt through the European surveillance network, Salm-Net, of an outbreak of S. dublin infection that occurred in Switzerland in November. This epidemic had been attributed to the consumption of cheese made from raw cow’s milk in the Doubs region of France. The correlation of these two pieces of information suggested that this cheese might be associated with the French outbreak.
On 21 December 1995, the RNSP began an investigation to confirm the extent and impact of the outbreak in France, identify the source and vehicle of transmission, and propose appropriate control measures.
A case was defined as a person living in France from whom a strain of S. dublin was isolated between 1 November and 31 December 1995 during an acute illness (gastroenteritis or septicaemia). Cases were identified by the CNR.
In a preliminary survey on 21 and 22 December, several cases were found to have eaten cheese of the same brand as that implicated in the Swiss outbreak. The null hypothesis that cases and controls did not differ in exposure to the cheese was tested in a case control study carried out on 28 and 29 December. Two controls matched to each case by age (± 5 years) and place of residence were sought by random digit dialling. Cases and controls were interviewed on the telephone about their consumption of beef, dairy products, and cheese in the three days before the onset of symptoms.
The lysotypes of French human strains and strains isolated from the cheese at the origin of the Swiss outbreak were determined and compared by the CNR for molecular enteric typing.
Twenty-five symptomatic cases were detected by the CNR in November and December 1995. Twelve patients were admitted to hospital, five (20%) who had coexisting chronic illness died. The epidemic curve suggested a point source of contamination (figure 1). The highest incidence was observed in the Franche-Comté region, which borders with Switzerland, and where the cheese implicated in the Swiss outbreak was produced (figure 2).
Analysis of the case control study of 11 cases and 22 controls showed that the risk of S. dublin infection was significantly greater in people who had eaten the cheese than in those who had not (odds ratio infini, 95% confidence interval 1.5 – infini, p = 0.009). No other risk factor was implicated.
S. dublin lysotype D43 was isolated from the cheese responsible for the Swiss outbreak and from the 21 French patients (figure 1).
Control measures had been set up in the cheese producer’s plant by the veterinary services of the Doubs region at the end of November 1995, following a positive test for S. dublin carried out by the producer. The number of S. dublin strains received at the CNR quickly fell to their previous level after this date (figure 1).
The results of the epidemiological and microbiological investigations and the decline in the number of cases after control measures were taken in the cheese producing plant suggested that the French and Swiss epidemics were attributable to the consumption of the same cheese from the same producer.
This outbreak investigation clearly illustrates the impact both of contamination and of the measures taken. It also showed the value of international surveillance networks in outbreak investigations (1).
The outbreak raises the question of the safety of eating cheese made from raw milk (2,3). It is not planned to propose pasteurisation of raw milk cheese in France for cultural, social, and economic reasons (2). It is therefore important to reiterate the measures to prevent infections associated with the consumption of these products: to strive collectively for healthy, uncontaminated products through cattle breeding, production and distribution, and, at an individual level, to discourage the people most vulnerable to infection from consuming such products.
1. Hastings L, Burnens A, de Jong B, Ward L, Fisher I, Stuart J, et al. Salm-Net facilitates collaborative investigation of an outbreak of Salmonella tosamanga infection in Europe. Commun Dis Rep CDR Rev 1996; 6: R100-2
2. Desenclos JC, Bouvet P, Benz-Lemoine E, Grimont F, Desqueyroux H, Rebière I et al. Large outbreak of Salmonella enterica serotype paratyphi B infection caused by goat’s milk cheese, France, 1993: a case finding and epidemiological study. BMJ 1995 ; 312: 91-4
3. Maguire H, Cowden J, Jacob M, Rowe B, Roberts D, Bruce J, et al. An outbreak of Salmonella dublin infection in England and Wales associated with a soft unpasteurised cow’s milk cheese.Epidemiol Infect 1992 ; 109: 389-16