Estimating the real incidence of invasive listeriosis through an integrated surveillance model in use in Lombardy (Italy, 2006-2014)

The annual incidence of listeriosis in Italy is lower (0·19-0·27 per 100 000 inhabitants per year) than in Europe (0·34-0·52 per 100 000 inhabitants per year). Since the observed incidence of listeriosis may be biased downward for underdiagnosis or under-reporting, this work aims to estimate the real incidence of listeriosis during a 9-year period in the Lombardy region, Italy. Data on listeriosis cases were collected from national mandatory notification system (MAINF) and Laboratory-based Surveillance System (LabSS). The two sources were cross-matched and capture-recapture method was applied to estimate the number of undetected cases and the real incidence of invasive listeriosis. Five hundred and eighty invasive listeriosis cases were detected by the two sources between 2006 and 2014: 50·2% were identified only via MAINF, 16·7% were recorded only via LabSS, overlaps occurred in 192 cases (33·1%). The mean annual incidence detected only by MAINF was 0·56 per 100 000 inhabitants, which rose to 0·67 per 100 000 considering also the cases detected by LabSS. The capture-recapture method allowed to estimate an incidence of 0·84 per 100 000. The high incidence of listeriosis may be due to improved sensitivity of the surveillance system, but also reflect a real increase, associated with an increased population at risk.