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INTRODUCTION
Epidemiological evidence indicates that aerosols produced by cooling towers are one of the main causes of large community-wide outbreaks of Legionnaires' disease (LD) [1-13], however, prevention of these kinds of outbreaks continues to pose considerable difficulties. The concentration of the population in the cooling tower area of influence and the opportune implementation of control measures are some of the factors that determine the variability and duration of such outbreaks and the number of affected people. The case-fatality rate sometimes exceeds 10% [8, 12], and only rarely is <1% [4].
An outbreak of LD occurred in June 2006 in Pamplona (195 983 inhabitants), capital city of the region of Navarre, Spain. The outbreak was first detected on 1 June, when an increase in the number of cases of community-acquired pneumonia was reported in relation to district 2 of Pamplona (22 012 inhabitants), which is a commercial, financial and administrative area of the city and is visited daily by a large number of people who live in other neighbourhoods. Legionella pneumophila antigen in urine (Binax Now; Binax Inc., Scarborough, ME, USA) was detected in some of these cases [14]. Hence, the search for the source of infection was directed mainly to these and other nearby districts. The active search resulted in the detection of 146 cases of LD by 12 June. The epidemiological, microbiological and environmental investigations of this outbreak are described below.
METHODS
Case finding
From the time the outbreak was detected on 1 June 2006, an active prospective and retrospective search for cases clinically compatible with Legionella pneumonia in emergency services, hospitals and primary health-care centres was implemented, together with a protocol for radiological and microbiological confirmation and appropriate treatment of all cases. The alert was transmitted to the Spanish and European epidemiological surveillance networks [14], which resulted in two cases being detected associated with the outbreak diagnosed in other regions (Aragón and Asturias).
A case was defined as a patient with clinical symptoms of pneumonia, a compatible chest radiology study and microbiological confirmation of L. pneumophila antigen in urine (Binax Now). For several days before the outbreak, an elevated incidence of pneumonia in persons with a urine antigen test positive for Streptococcus pneumoniae and negative for Legionella had been observed. For this...