Is lettuce the culprit?

Scientists in the US have for the first time identified a fresh product as the source of an outbreak of human Yersinia pseudotuberculosis infections, a recent medical report says.

The article, which is due to be published in the March 1 issue of The Journal of Infectious Diseases says the outbreak was identified in Finland and traced epidemiologically to farms producing lettuce there.

Y. pseudotuberculosis, first identified in 1883, causes infections characterized by fever and abdominal pain that are often confused with acute appendicitis. The microbe is well known in veterinary medicine as the cause of illnesses in hares, deer, and sheep, among other animals. Y. pseudotuberculosis infections in humans are relatively rare, and while foodborne transmission has long been suspected, attempts to trace the pathogen to a concrete source of contamination in the past have been unsuccessful.

In October of 1998, two microbiology laboratories in southern Finland discovered an alarming increase in infections during routine surveillance of laboratory-diagnosed infections. J. Pekka Nuorti, of the National Public Health Institute of Finland, and colleagues from the University of Helsinki, the National Public Health Institute of Finland, and the National Food Agency of Finland initiated epidemiological and environmental investigations that would eventually reveal the source as contaminated iceberg lettuce.

In a case-control study, 38 patients with confirmed infections were questioned about what and where they ate in the two weeks before the onset of their symptoms. The investigation led to four lunch cafeterias where the patients reported eating iceberg lettuce. The lettuce served in those cafeterias was traced to four farms in the southwest archipelago region of Finland.

While no lettuce remained from the shipments identified from the cafeterias, Y. pseudotuberculosis was discovered in soil, irrigation water, and lettuce samples from one of those farms. The investigators suspect that the pathogen was spread by the feces of roe deer, which have been carriers of the pathogen in the past. Deer faeces were found in and around the open, unfenced fields where the lettuce was grown.

In an accompanying editorial, Robert V. Tauxe, of the Centres for Disease Control and Prevention, notes that the next step in preventing future outbreaks of this kind might begin with studying the behavior of Y. pseudotuberculosis in lettuce plants and attempting to define whether deer or other animals are the specific reservoir of the pathogen.

Such investigations may lead to better methods of prevention--from fenced-in fields to vaccinations of implicated animal populations or the use of disinfecting strategies such as irradiation--and give those who enjoy fresh fruits and vegetables more security about what they eat.

Either way it seems that producers of a number of fresh foods, produced in environments where it is difficult to fence out all wild animals, will have to give further thought to this consideration if they are to maintain required health and safety standards.