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. 1998 Jul 11;317(7151):145.

Risk factors for winter outbreak of acute diarrhoea in France

Winter outbreaks of diarrhoea occur in United Kingdom too

Tamara Djuretic 1,2,3, Mary E Ramsey 1,2,3, Paddy C Farrington 1,2,3, Douglas M Fleming 1,2,3, David Brown 1,2,3
PMCID: PMC1113503  PMID: 9657801

Editor—Letrilliart et al conclude that the winter epidemic of diarrhoea in France in 1995-6 was not associated with consumption of tap water or shellfish, that person to person spread was implicated, and that the epidemic was of viral aetiology.1 Despite considerable reservations about the validity of their evidence we agree that the epidemic may have been largely attributable to viral diarrhoea.

Figure.

Figure

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Mean weekly incidence of infectious intestinal disease seen in general practice, by four week period and age group (from Royal College of General Practitioners’ weekly return service, 1992-6)

References

  • 1.Letrilliart L, Desenclos JC, Flahault A. Risk factors for winter outbreak of acute diarrhoea in France: case-control study. BMJ. 1997;315:1645–1649. doi: 10.1136/bmj.315.7123.1645a. . (20-27 December.) [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 2.Communicable Disease Surveillance Centre. Shellfish and outbreaks of gastroenteritis in England and Wales: 1996 and 1997. Commun Dis Rep CDR Wkly. 1998;8:21. [PubMed] [Google Scholar]
  • 3.Parrino TA, Schreiber DS, Trier JS, Kapikian AZ, Blacklow NR. Clinical immunity in acute gastroenteritis caused by the Norwalk agent. N Engl J Med. 1977;297:86. doi: 10.1056/NEJM197707142970204. [DOI] [PubMed] [Google Scholar]
  • 4.Fleming DM, Crombie DL, Ross AM. Annual report of the weekly return service for 1996. Birmingham: Royal College of General Practitioners; 1997. [Google Scholar]
  • 5.Djuretic T, Ramsay M, Gay N, Wall PG, Ryan M, Fleming D. An estimate of the proportion of diarrhoeal disease episodes seen by general practitioners attributable to rotavirus in children under five years of age, England and Wales. Acta Paediatr (in press). [DOI] [PubMed]
BMJ. 1998 Jul 11;317(7151):145.

Authors did not rule out shellfish as a factor

J S Osika 1,2, M M Muganwa-Kamya 1,2

Editor—Letrilliart et al claim to have excluded consumption of shellfish from the factors accounting for the winter outbreaks of diarrhoea in France.1-1 We did not find evidence in their paper to support this claim.

Firstly, the selection of cases was biased. The authors should have attempted to include all cases who consulted the doctors throughout the one month study rather than selecting only the first three cases who consulted each doctor.1-2 The first three patients with diarrhoea seen by a doctor during an epidemic of diarrhoea are unlikely to be representative of all the patients with diarrhoea seen by the same doctor during one month of the epidemic. Such a sample of cases is even less likely to be representative of cases in the community.

Secondly, the interpretation of the results given in the paper does not support the claim that consumption of shellfish was excluded as the source of the winter outbreaks. The finding that the cases with diarrhoea were more likely to have had contacts with other people with diarrhoea suggests that these cases were mostly secondary cases. Nothing was mentioned about the index cases, who are the key to the “cause” of the outbreak.

Index cases may have consumed raw shellfish and got infected with, for example, a Norwalk-like virus (small round structured virus).1-3 Transmission from person to person may have led to the secondary cases. Norwalk-like viruses can be transmitted effectively from person to person as well as through a contaminated food source (such as raw oysters) or water.1-3 The authors did not report other clinical symptoms that accompanied diarrhoea in their patients. Vomiting is one of the common symptoms of gastroenteritis due to Norwalk-like virus and the source of the name “winter vomiting disease.”

Finally, seasonality of gastroenteritis in the community is not limited to France. In England and Wales, general outbreaks of infectious intestinal diseases (community and institutional outbreaks combined) during 1992-4 peaked during summer.1-4 Salmonella infections, the commonest cause of these outbreaks, had peaks in July, while infections with Norwalk-like virus, the second commonest cause, had peaks in October.1-4

References

  • 1-1.Letrilliart L, Desenclos J, Flahault A. Risk factors for winter outbreak of acute diarrhoea in France: case-control study. BMJ. 1997;315:1645–1649. doi: 10.1136/bmj.315.7123.1645a. . (20-27 December.) [DOI] [PMC free article] [PubMed] [Google Scholar]
  • 1-2.Kelsey JL, Thompson WD, Evans AS. Methods in observational epidemiology. New York: Oxford University Press; 1986. [Google Scholar]
  • 1-3.Viral Gastroenteritis Subcommittee of the PHLS Virology Committee. Outbreaks of gastroenteritis associated with SRSVs. PHLS Microbiol Digest. 1993;10:2–8. [Google Scholar]
  • 1-4.Djuretic T, Wall PG, Ryan MJ, Evans HS, Adak GK, Cowden JM. General outbreaks of infectious intestinal disease in England and Wales 1992 to 1994. Commun Dis Rep. 1996;6:R57–R63. [PubMed] [Google Scholar]
BMJ. 1998 Jul 11;317(7151):145.

Authors’ reply

Laurent Letrilliart 1,2, Antoine Flahault 1,2, Jean-Claude Desenclos 1,2

Editor—We we do not understand what Fleming et al mean by the statement that “the apparent epidemic peaks in the incidence of infectious intestinal disease in January of most years are not clear”; these peaks are clearly shown in figure 1 of our paper, in which the visual impression is supported by state of the art time-series analysis.2-1 We agree with Fleming et al that breaking down this figure by age group is common in epidemiology, as the Royal College of General Practitioners does routinely. We do not, however, believe that it was necessary in this paper, which was based on a case-control design; those interested can find this additional approach illustrated in Valleron’s paper.2-2 That shows that, contrary to Fleming et al’s assertion, diarrhoea in each age group, and not only young children, has a seasonal pattern, despite the differences in incidence. Osika and Muganwa-Kamya comment on the representativeness of our patients. Our efforts focused on the appropriate selection of controls so that biases were avoided and the relative risks were estimated as accurately as possible. Nevertheless, the cases that we included in the study were comparable for age, fever, history of contact, and geographical distribution with the cases reported in the Sentinelle system during the same period. The predictive value of vomiting in diarrhoea due to Norwalk-like viruses seems to us debatable.2-3 The hypothesis that some cases may be attributable to person to person transmission by index cases contaminated by infected shellfish does not contradict our results. We stated only that most cases of acute diarrhoea are not due to consumption of shellfish. From our results we calculated the maximum proportion of cases of acute diarrhoea attributable to consumption of raw shellfish to be 3.4% (95% confidence interval 0% to 10.8%), which is not significant.

References

  • 2-1.Flahault A, Garnerin P, Chauvin P, Farran N, Saidi Y, Diaz C, et al. Sentinelle traces of an epidemic of acute gastroenteritis in France. Lancet. 1995;346:162–163. doi: 10.1016/s0140-6736(95)91214-2. [DOI] [PubMed] [Google Scholar]
  • 2-2.Valleron AJ. Six ans de surveillance épidémiologique des diarrhées aiguës en médecine générale francaise. MTP. 1998;1:55–63. [Google Scholar]
  • 2-3.Blacklow NR, Greenberg HB. Viral gastroenteritis. N Engl J Med. 1991;325:252–264. doi: 10.1056/NEJM199107253250406. [DOI] [PubMed] [Google Scholar]

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