Research
Burden of shigellosis in Thailand Pornthip Chompook et al.
of bacillary dysentery among children less than 5 years old
was 2.7/10 000 children per year. But in contrast to government
data, the incidence of shigellosis detected by active
surveillance among children in this age group in the 1980s in
urban Bangkok was more than 100-fold higher at 640/10 000
children per year (3).
The observed discrepancy between government statistics
obtained by passive surveillance and shigellosis rates detected
by active surveillance highlights a fundamental problem in the
surveillance methods used. In studies using passive surveillance,
cases are detected when patients present to a health-care provider
participating in surveillance. In active surveillance studies,
health-care providers or other study staff visit each member of
the study population at regular intervals and enquire about
disease episodes occurring since the last visit. Passive surveillance
should ideally detect all treated episodes. But there is
always a risk that patients may seek treatment from health-care
providers who are not participating in surveillance activities.
Thus, rates estimated by passive surveillance may underestimate
true incidence rates.