Under-notification of giardiasis in Auckland, New Zealand: a capture–recapture estimation

Estimation of the degree of undercount is important for disease surveillance. Capture-recapture techniques are now being used to evaluate the completeness of disease ascertainment. This study estimated the level of under-notification of giardiasis in the Auckland adult population using a capture-recapture method. Two independent datasets of giardiasis cases > or = 15 years were generated from the 1998-1999 Auckland Giardiasis Study (AGS) case database and cases notified to Auckland Regional Public Health Services (ARPHS) for the same period of time. Cases were matched and under-notification was estimated using a two-sample capture-recapture method. During the 12-month period, 199 cases participated in the AGS and 413 cases were notified to ARPHS. The capture-recapture calculation indicated that only 49% of cases were notified. Under-notification by a factor of 2 obscures the true burden of giardiasis. Socio-economic conditions and water quality may influence disease notification inversely. Capture-recapture techniques are useful in evaluating the completeness of surveillance.