Intended for healthcare professionals


International investigation of outbreaks of foodborne disease

BMJ 1996; 313 doi: (Published 02 November 1996) Cite this as: BMJ 1996;313:1093
  1. Robert V Tauxe,
  2. James M Hughes
  1. Chief Foodborne and Diarrhoeal Diseases Branch, Division of Bacterial and Mycotic Diseases, National Center for Infectious Diseases
  2. Director National Center for Infectious Diseases, Centers for Disease Control and Prevention, Public Health Service US Department of Health and Human Services, Atlanta, Georgia 30333, USA

    Public health responds to the globalisation of food

    In 1992, the United States Institute of Medicine published a report entitled Emerging Infections: Microbial Threats to Health in the United States.1 The report, developed by an expert committee chaired by Drs Joshua Lederberg and Robert Shope, emphasised the fact that the world is truly a global village with respect to microbes. One factor is the change in the food industry. Foods, like many pathogens, are more likely than ever to cross national borders, and a single meal can combine products from many countries. In the United States and many other countries, importing of food from other parts of the world has increased greatly.2

    One unintended consequence is that when food becomes contaminated the resulting outbreak can span continents. Indeed, Salmonella agona first spread around the world as a consequence of the use of contaminated Peruvian fish meal in chicken feed.3 More recent foodborne outbreaks of international scope include a multistate outbreak of Cyclospora cayetanenis infection in the United States and Canada related to raspberries imported from Guatemala,4 salmonellosis in the United States and several Scandinavian countries traced to alfalfa sprouts grown from imported seeds,5 6 shigellosis in northern Europe traced to lettuce from southern Europe,7 shigellosis in Europe traced to prawns from Asia,8 and cholera in the United States traced to fresh coconut milk imported from Asia.9 In this week's BMJ (pp 1105, 1107) a two part report describes the recognition, investigation, and control of a large international outbreak of salmonellosis caused by a snack food produced in Israel that caused illness there and in at least two other countries (England and the United States).10 11

    In the past, foodborne outbreaks were often thought of as local events, affecting a group of people who all ate at one restaurant or attended one social event. These outbreaks are the easiest to detect because the victims themselves and the doctors they consult en masse can quickly perceive the clustered nature of the illnesses. Contamination of centrally produced foods that are widely distributed can lead to a diffuse outbreak, affecting people scattered over a wide area. More sophisticated public health surveillance is needed to detect this signal amid the background noise of many unrelated illnesses. For this reason, routine subtyping of some foodborne pathogens, coupled with analysis of surveillance data at a regional and national level, has become a powerful tool for detecting such outbreaks. For salmonella infections in England and Wales, routine serotyping and subtyping by phage susceptibility at the Public Health Laboratory Service, which celebrates its golden jubilee this year, has identified many outbreaks. When combined with epidemiological investigations, which have also become routine, this powerful public health approach can protect a nation's inhabitants and have an effect far beyond national borders.

    Detection of a surge in one serotype is only the beginning. Systematic epidemiological interviews and comparison of reported exposures among affected people and a comparable group of healthy people can then define the critical differences in exposure. Once a sound epidemiological approach implicates a contaminated food, steps can be taken to remove it from the market to prevent additional cases. Detailed investigation of how the contamination could have occurred can lead to measures that will prevent similar episodes in the future. This productive collaboration between microbiologists, epidemiologists, and food scientists is the hallmark of successful investigations of foodborne outbreaks of disease.

    Special challenges arise when the people affected, the investigators, and the food manufacturers are in different countries. For effective collaboration there needs to be a common goal and common methods. Salmonella serotyping and phage typing provides one common language for comparing strains from around the world. Comparable methods of national surveillance, such as reporting of the age and sex of individual patients, and similar methods of investigating outbreaks, are also critical to assembling the whole picture. International collaboration also provides new opportunities to increase the effectiveness of investigating outbreaks. In the outbreak of S agona infections reported here, there were too few cases in the United States to detect the outbreak, though investigation of those few cases that did occur provided powerful confirmatory evidence that food was contaminated before it reached England. In Israel, ironically enough, there were almost too many cases. The exposure to the implicated snack food was so common among both cases and controls that the difference in exposure did not reach significance in the first Israeli case-control study, except that there was a clear increase in risk of illness associated with eating more of the snack food. Epidemiological investigations among groups that are universally exposed to the source are difficult at best, and sometimes establishing a dose-response relation is the best we can do.12 The combination of the epidemiological investigation in England, the helpful “outlier” cases in the United States, and the detailed epidemiological and plant investigations in Israel made the food supply of all three countries safer.

    The S agona outbreak provides several critically important lessons including the need for ongoing analyses of surveillance data, prompt epidemiological investigation, timely communication, close collaboration between epidemiologists and public health microbiologists, and prompt notification of colleagues in other countries potentially affected by distribution of a contaminated product. Clinicians also have an important part to play by alerting public health officials if they suspect a common source outbreak and ensuring that faecal specimens are submitted to a microbiology laboratory. Finally, the outbreak highlights the critical role of public health laboratories in disease control. These lessons are applicable to global strategies for addressing emerging infections.13 14 The challenges presented by emerging infectious diseases demand that national and international organisations and agencies work closely together to strengthen global surveillance and capacity for response and to ensure timely communication when outbreaks with international implications are detected.


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