Incidence and Prognostic Factors for Legionnaires' Disease in Denmark 1993–2006

Objectives. To describe the incidence of Legionnaires’ disease and factors affecting survival in Denmark from 1993 to 2006. Methods. Retrospective register study using data on episodes of Legionnaires’ disease from notification forms and the National Reference Laboratory, and information on comorbidity from national registers on discharge diagnoses and mortality. Results. The average annual incidence of laboratory confirmed cases was 1.5？per 100000 people. The 30-day case fatality was 19%, ranging from 7% in travellers to 37% in hospital-acquired cases. Patients older than 60 years and patients with underlying disease were more likely-to die, as were patients infected with L. pneumophila serogroup 6. The lowest mortality was seen for L. pneumophila serogroup 1. Conclusion. Increased awareness of the disease, better diagnostics, and appropriate testing and treatment strategies are needed to improve survival. In particular, we focus on the relevance of diagnostic methods, such as polymerase chain reaction (PCR), that detect other serogroups than the urinary antigen tests used that will only reliably detect antigen from L. pneumophila serogroup 1. PCR is faster and more sensitive than culture. 1. Introduction The term Legionnaires’ disease refers to pneumonia caused by legionella bacteria. When the disease was first described about 30 years ago, it was perceived as a very severe but rare form of pneumonia with a high case fatality rate (CFR) of at least 15%. Since then, several studies have shown that Legionnaires’ disease has a much broader spectrum, ranging from mild pneumonia in out-patients to the originally described severe disease, and that legionella is one of the most common causes of community acquired pneumonia requiring hospitalization [1–4]. More than 50 legionella species have been described, of which approximately 20 are known to have caused human disease, as they have been isolated in samples from pneumonia patients. However, the far most common cause of Legionnaires’ disease is L. pneumophila and especially serogroup 1. This is also the one most easily diagnosed because of the existence of the urinary antigen test (UAT) for legionella, which has the highest sensitivity for L. pneumophila serogroup 1 [1, 2, 5]. In Denmark, Legionnaires’ disease has been notifiable by treating clinicians to the Department of Epidemiology at Statens Serum Institut since 1980. In parallel, the National Reference Laboratory at Statens Serum Institut performs diagnostics and collects isolates of legionellae from the local clinical microbiology laboratories for