Epidemiology and Infection

A national outbreak of multi-resistant Salmonella enterica serovar Typhimurium definitive phage type (DT) 104 associated with consumption of lettuce

P. W. HORBY a1, S. J. O'BRIEN a1c1, G. K. ADAK a1, C. GRAHAM a2, J. I. HAWKER a3, P. HUNTER a4, C. LANE a5, A. J. LAWSON a6, R. T. MITCHELL a5, M. H. REACHER a1, E. J. THRELFALL a6, L. R. WARD a6 and On behalf of the PHLS Outbreak Investigation Team
a1 Gastrointestinal Diseases Division, PHLS Communicable Disease Surveillance Centre (CDSC), 61 Colindale Avenue, London, NW9 5EQ, UK
a2 PHLS Statistics Unit, PHLS Communicable Disease Surveillance Centre, 61 Colindale Avenue, London, NW9 5EQ, UK
a3 CDSC (West Midlands), University of Birmingham, Edgbaston, Birmingham B15 2TT, UK
a4 CDSC (North West), Public Health Laboratory, University Hospital Aintree, Longmoor Lane, Liverpool L9 7AL, UK
a5 Environmental Surveillance Unit, PHLS Communicable Disease Surveillance Centre, 61 Colindale Avenue, London, NW9 5EQ, UK
a6 Laboratory of Enteric Pathogens, PHLS Central Public Health Laboratory, 61 Colindale Avenue, London, NW9 5EQ, UK


Between 1 August and 15 September 2000, 361 cases of Salmonella enterica serotype Typhimurium definitive phage type (DT) 104, resistant to ampicillin, chloramphenicol, streptomycin, sulphonamides, spectinomycin and tetracycline (R-type ACSSuSpT), were identified in England and Wales residents. Molecular typing of 258 isolates of S. Typhimurium DT104 R-type ACSSuSpT showed that, although isolates were indistinguishable by pulsed-field gel electrophoresis, 67% (174/258) were characterized by a particular plasmid profile. A statistically significant association between illness and consumption of lettuce away from home was demonstrated (OR=7·28; 95% CI=2·25–23·57; P=0·0006) in an unmatched case–control study. Environmental investigations revealed that a number of food outlets implicated in the outbreak had common suppliers of salad vegetables. No implicated foods were available for microbiological testing. An environmental audit of three farms that might have supplied salad vegetables to the implicated outlets did not reveal any unsafe agricultural practices. The complexity of the food supply chain and the lack of identifying markers on salad stuffs made tracking salad vegetables back to their origin extremely difficult in most instances. This has implications for public health since food hazard warnings and product withdrawal are contingent on accurate identification of the suspect product.

(Accepted October 21 2002)

c1 Author for correspondence.