DIARRHEA - DEFINING THE EPISODE

Background. There is still no consensus on the appropriate definition of an 'episode' of diarrhoea, even though it has been shown that the c hoice of definition has a major impact on reported incidence rates. Pr evious work has focused on the observed distribution of illness episod es in time but has not attempted to determine whether the patterns obs erved depart from those expected by chance. Methods, A simple theoreti cal model of the distribution of illness episodes is developed, based on the concept of a 'trigger event'. The model incorporates elements r elating to the duration of symptoms, inter-individual variation in inc idence rates and seasonality. Appropriate parameters for the model are derived from two empirical datasets. Results. It is shown that short intervals between one aetiologically distinct period of diarrhoea and the next will frequently occur by chance, especially in circumstances where high incidence rates and within-child clustering of illness prev ail. The duration of symptoms will have no effect on the length of int ervals between periods of illness, and seasonality is unlikely to have a major impact. Over 10% of all non-initial trigger events might be e xpected to occur during the course of a pre-existing period of diarrho ea, and would not therefore be identified in a study based on reported symptoms. Conclusions. The findings of previous studies, suggesting t hat 2 or 3 days without symptoms will generally mark a new episode of diarrhoea, are endorsed. Modelling the expected distribution of illnes s in time may help to highlight structural or analytical problems with empirical datasets.