Risk for Travel-associated Legionnaires’ Disease, Europe, 2009

Julien Beauté

, Phillip Zucs, Birgitta de Jong, and for the European Legionnaires’ Disease Surveillance Network

Author affiliations: European Centre for Disease Prevention and Control, Stockholm, Sweden; European Legionnaires’ Disease Surveillance Network, Stockholm

Abstract

Legionnaires’ disease is underreported in Europe; notification rates differ substantially among countries. Approximately 20% of reported cases are travel-associated. To assess the risk for travel-associated Legionnaires’ disease (TALD) associated with travel patterns in European countries, we retrieved TALD surveillance data for 2009 from the European Surveillance System, and tourism denominator data from the Statistical Office of the European Union. Risk (number cases reported/number nights spent) was calculated by travel country. In 2009, the network reported 607 cases among European travelers, possibly associated with 825 accommodation sites in European Union countries. The overall risk associated with travel abroad was 0.3 cases/million nights. We observed an increasing trend in risk from northwestern to southeastern Europe; Greece had the highest risk (1.7). Our findings underscore the need for countries with high TALD risks to improve prevention and control of legionellosis; and for countries with high TALD risks, but low notification rates of Legionnaires’ disease to improve diagnostics and reporting.

Legionnaires’ disease (LD) is a severe multisystem disease, typically manifesting as pneumonia; it is caused by gram-negative bacteria, Legionella spp., which are found in freshwater environments worldwide, and can be can be transmitted by aspirating contaminated drinking water or by inhaling airborne droplets from contaminated sources such as building ventilation units and spas (1). A travel-associated Legionnaires’ disease (TALD) surveillance system at the European Union (EU) level has been in place since 1987 (2); EU surveillance of Legionnaires’ disease, regardless of patient travel history, began in 1996 (3). The rationale for the surveillance of travel-associated cases of Legionnaires’ disease (TALD) is that it enables the timely detection of clusters and the identification of the source of infection, which would otherwise probably remain undiscovered (4). Since April 2010, both surveillance systems have been operated by the European Legionnaires’ Disease Surveillance Network (ELDSNet) under the coordination of the European Centre for Disease Prevention and Control (ECDC).
Since 2005, 5,000–6,000 cases of Legionnaires’ disease have been reported each year by the 27 EU Member States, Iceland, and Norway (5,6). Annual case-fatality rates of notified Legionnaires’ disease have been ≈10%, and ≈20% of reported cases are travel-associated (5–7), which is similar to the situation described in the United States (8). There is evidence that the risk for Legionnaires’ disease might be higher under certain environmental conditions; warm and wet weather has been associated with higher incidence rates in the Netherlands and the United Kingdom (9,10). Using that evidence, Hicks et al. reported that an increased case count would be expected after heavy rains during the warm season (11). However, in Europe, an increasing trend from northern to southern countries is not clearly evident in surveillance data.
Because many cases of Legionnaires’ disease might go undiagnosed by clinicians, and because some clinicians do not report confirmed cases to national authorities, surveillance systems are likely to miss a sizeable proportion of Legionnaires’ disease deaths (1). The ECDC estimated that as few as 10% of Legionnaires’ disease cases are reported in Europe and reported that notification rates differ substantially across countries and that causes of undernotification are likely to vary (6). Because notification rates tend to reflect the quality of national surveillance rather than the local risk for Legionnaires’ disease, it is difficult to estimate and compare risk for Legionnaires’ disease across countries.
The surveillance of Legionnaires’ disease associated with international travel might, at least partly, overcome this limitation. A similar approach has been used in previous studies to estimate and compare the risks for foodborne and waterborne diseases in Europe (12,13). Tourism is a massive industry in Europe, and its summer peak coincides with the peak of Legionnaires’ disease. Cases of TALD in travelers who contract the disease in the country they visit are mostly reported by the patient’s country of residence independent of the quality of surveillance in the travel destination country. These cases may therefore enable not only a better estimation of local disease risk but also an assessment of the local quality of reporting. Finally, Legionnaires’ disease surveillance in Europe has traditionally focused more on TALD, because of the added value of improved prevention and control of international clusters. TALD surveillance in Europe can therefore be assumed to be less prone to underascertainment than the surveillance of Legionnaires’ disease not associated with travel.
This analysis is intended to assessing the risks for TALD in European countries on the basis of travel patterns. A secondary objective is to provide an estimate of the extent of underascertainment by country of destination.