Q fever outbreaks: a syndromic approach for detection of hidden clusters.

OBJECTIVE: To explore - using syndromic data - whether there is evidence for local Q fever outbreaks before detection of the known 2007 outbreak in the Netherlands and whether syndromic surveillance could have accelerated their detection. BACKGROUND: For many years, Q-fever has been very rare in the Netherlands with around 15 notified cases per year, but since the end of May 2007, outbreaks of Q fever have been reported in rural areas in the southern part of the country, with 20-25% of the notified cases hospitalized. In 2007, 178 Q fever cases were notified, most of them occurring in May and June 2007. In 2008 and 2009 large outbreaks of Q fever recurred with increasing numbers of cases and in an expanding geographic area. Dairy goat farms are considered the most plausible source of infection for these outbreaks and Q fever has been first reported as cause of abortion waves on such farms in 2005. This triggered questions about possible undetected or misdiagnosed outbreaks preceding the 2007 outbreak, as most patients with severe acute Q fever present with pneumonia. In this study we explore retrospectively whether there is evidence for human Q-fever outbreaks in the Netherlands in 2005-2007, before detection of the 2007 outbreak. For this, we look for elevations in space and time of hospitalized patients with lower-respiratory infections (LRI) and other syndromes that can be associated with Q fever, and assess whether these local syndrome elevations could be due to Q-fever (or to other causes e.g. RSV/influenza). We then evaluate whether real-time syndromic surveillance on hospitalizations could have ac-celerated the detection of human Q-fever outbreaks. METHODS: Space-time scan statistics were used to look for clusters of lower-respiratory infections (LRI), hepati-tis and/or endocarditis in Dutch hospitalizations (2005-2007). We assessed whether these were plau-sibly due to Q fever, using criteria based upon age distribution and discharge diagnoses, overlap with notified Q fever cases, indications for other likely causes, and geographic overlap with contaminated farms. RESULTS: For seven out of twenty detected LRI clusters and one out of two hepatitis clusters, we considered Q fever a plausible cause. With real-time syndromic surveillance, two of these clusters could have been signaled in 2005, one in 2006, and four in 2007. CONCLUSIONS: We found substantial support for the occurrence of local Q fever outbreaks before detection of the 2007 outbreak and in a wider area. Real-time syndromic surveillance could have detected these clusters, prompting further investigations and additional labo-ratory tests. This might have resulted in up to two years earlier detection of Q fever outbreaks. Serology on historical blood samples from the detected cluster areas would be necessary to confirm that Q fever did cause these clusters. (aut. ref.