Active surveillance may detect more disease-episodes.
However, these episodes will include mild episodes of enteric
infection that do not require treatment and may, from the
policy-maker’s perspective, be irrelevant. In addition, active surveillance
requires considerable logistical and financial resources.
On balance, experts frequently prefer passive surveillance to
estimate the relevant disease burden of diarrhoeal diseases, such
as shigellosis, as well as for trials of other vaccines to protect
against diarrhoeal diseases (4, 5). A second limitation that
may lead to an underestimate of true disease rates is the lack
of sensitivity of traditional microbiological culture methods.
Shigella is a sensitive organism that will perish in a less than
optimal environment. Delays in plating or being kept at an
unsuitable ambient temperature will result in a substantial
reduction in case-detection rates. Reliance on traditional culture
methods therefore leads to an underestimate of the actual
burden of shigellosis.
To estimate the burden of shigellosis in a semi-rural area
of Thailand more accurately, we conducted a comprehensive
passive surveillance study of treatment centres and corrected
our findings to account for missed cases and the use of traditional
microbiology methods that have limited sensitivity.