Severely impaired health status of non-notified Q fever patients leads to an underestimation of the true burden of disease.

Q fever patients are often reported to experience a long-term impaired health status, including fatigue, which can persist for many years. During the large Q fever epidemic in The Netherlands, many patients with a laboratory-confirmed Coxiella burnetii infection were not notified as acute Q fever because they did not fulfil the clinical criteria of the acute Q fever case definition (fever, pneumonia and/or hepatitis). Our study assessed and compared the long-term health status of notified and non-notified Q fever patients at 4 years after onset of illness, using the Nijmegen Clinical Screening Instrument (NCSI). The study included 448 notified and 193 non-notified Q fever patients. The most severely affected subdomain in both patient groups was 'Fatigue' (50·5% of the notified and 54·6% of the non-notified patients had severe fatigue). Long-term health status did not differ significantly between the notified and non-notified patient groups, and patients scored worse on all subdomains compared to a healthy reference group. Our findings suggest that the magnitude of the 2007-2009 Q fever outbreak in The Netherlands was underestimated when only notified patients according to the European Union case definition are considered.