Winter outbreaks of diarrhoea occur in United Kingdom too

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.

Winter outbreaks of viral diarrhoea associated with shellfish have been described in the United Kingdom,2 but the French study's failure to show an association may have been because of the methods used. Infection with small round structured virus, the most frequently identified cause of foodborne viral gastroenteritis, induces short term immunity,3 which could produce perverse effects in a study of this design—for example, people who eat oysters frequently might be differentially immune owing to higher exposure to risk inducing an apparent protective effect for frequent consumption. Separate analysis of primary and secondary cases would be more appropriate to test the primary hypothesis that food and water consumed were responsible for the epidemic.

The descriptive epidemiology and the design of the study are inadequate to explain the hypotheses selected. …