[Investigation of a listeriosis cluster: how complete are laboratory-based reports?].

BACKGROUND

In Germany, listeriosis surveillance is based on mandatory laboratory notification of Listeria monocytogenes. Local public health departments (LPHDs) forward the received reports anonymously to state public health departments (SPHDs) which send them to the national public health centre. For the 28th reporting week (RW) 2003, LPHDs reported four cases of listeriosis to the SPHD of Hesse, cumulating to 19 listeriosis cases in 2003 compared to four cases in RW 1 - 28, 2002.

METHODS

To investigate this cluster, we asked LPHDs in Hesse and Lower Saxony, where a comparable increase in cases was observed, to collect food histories for the four weeks preceding illness for cases reported from 25. - 29. RW 2003. We performed laboratory-based active case detection by contacting laboratories accepting clinical specimens from Hesse, and evaluated reporting completeness at the state level for the period 1.1.2002 - 31.8.2003.

RESULTS

Food histories of the eight cases interviewed did not permit to incriminate any particular food item. Of the 49 listeriosis cases identified through either LPHDs or active case detection, 29 (59 %) were reported, forwarded to the SPHD and identified through active case detection and four (8 %) were reported and forwarded, but not identified through active case detection. Four cases (8 %) were reported to three LPHDs, but not forwarded to the SPHD. These were identified through active case detection. Further twelve cases (24 %) were exclusively identified through active case detection.

CONCLUSIONS

This assessment of reporting completeness suggests that the number of laboratory confirmed listeriosis cases exceeds the number registered at state level by at least 48 %. Completeness might be improved by electronic, automated reporting.