Legionnaires' Disease Outbreak in Murcia, Spain

Ana García-Fulgueiras, Carmen Navarro, Daniel Fenoll, José García, Paulino González-Diego, Teresa Jiménez-Buñuales, Miguel Rodriguez, Rosa Lopez, Francisco Pacheco, Joaquín Ruiz, Manuel Segovia, Beatriz Baladrón, Carmen Pelaz

Disclosures

Emerging Infectious Diseases. 2003;9(8) 

In This Article

Methods

An active surveillance system to detect patients with any form of pneumonia was established on July 8 at all hospitals in the region of Murcia. Any reported case of pneumonia was considered a suspected case of LD if this diagnosis could not be ruled out. A confirmed case of LD was defined as a case of pneumonia with laboratory evidence of acute infection with Legionella including a) isolation of any species or serogroup of Legionella from respiratory secretions, lung tissue, or blood, b) a fourfold or higher rise in antibody titers from 1:128 against L. pneumophila SG1 by immunofluorescence or microaglutination in paired acute- and convalescent-phase serum specimens, or c) detection of L. pneumophila antigen in urine.

An epidemiologic questionnaire to elicit information on clinical aspects, predisposing factors, risk factors, place of residence, and recent urban mobility within the city of Murcia was administered to 662 persons with suspected cases, most within 24 to 48 hours after the case was reported. A computerized database was set up as well as maps showing geographically referenced cases and a spatial analysis by census division that used a geographic information system.[14]

Inclusion in the study was restricted to patients who had confirmed LD, were residents outside the city of Murcia, and had been reported July 8-20 as case-patients. Each case-patient was matched to two controls according to place of residence, sex, and age. Controls were randomly selected from the population of the same area of residence and health district as the matched patient.

A standardized questionnaire to interview patients and controls was designed. It focused on urban mobility and exposure to outside air within the northern part of Murcia 2 weeks before the patient's onset of illness. Patients and controls were interviewed in person at home between July 25 and August 8. Itineraries of all participants, including information about means of transport and frequency of trips, were outlined on a map of Murcia. In addition, any travel into or visit to 30 specific zones of the city in which putative sources of contaminated aerosols were located was recorded. The questionnaire also requested information about place of residence and work, occupation, education level, employment status, smoking habit, alcohol intake, chronic lung disease, diabetes, renal or heart disease, malignancy, immunocompromising disease, organ transplant, therapy with corticosteroids, and other risk factors for LD within 2 weeks before illness.

A multivariate analysis that used conditional logistic regression was conducted to calculate odds ratios (OR) with 95% confidence intervals (CI) as estimates of the relative risk for LD associated with a person's travel through each zone; we controlled for the confounding effects of traveling through other zones. Any zone of exposure that was significant in univariate analysis or showed biologic plausibility as a source was entered into the multiple analysis. The frequency with which participants visited Murcia city was also introduced into the multivariate analysis. Statistical analysis was conducted with STATA software.[15]

Exposure zones were analyzed in two ways after codification of the information obtained from each patient or control as he or she traveled or did not travel through a) the area defined by the block around a building with a cooling tower or the block around an ornamental fountain (in this way, 30 zones of the northern part of the city were coded), and b) the area delineated by a circle of 200 m radius around a cooling tower or a large ornamental fountain. Therefore, eight high-risk zones were studied.

In all cases, for each area of exposure, how the patient or control traveled through the area (i.e., walking [a category that also included bicycling or motorbiking] or driving [a car, bus, or truck]) was specified. This information was analyzed for the following: a) walking versus not passing through a zone or b) walking versus not passing through an area or traveling through it by car. Finally, for all possibilities, data were analyzed in two further ways: a) complete, which took into account all persons in the study, or b) restricted, which took into account only the trios of case-patients and their two paired controls in which all three persons stated that they had visited Murcia in the study period.

Possible sources of aerosols were inspected, and water samples were collected from the water supply network and from 339 installations (e.g., cooling towers, storage tanks, and decorative fountains). Cooling towers were identified by aerial inspection because no census of these installations was available. Attack rates by residence were used to determine in which locations inspections and environmental samplings could be conducted.

Environmental samples were processed according to ISO 11731/1998. Environmental and clinical L. pneumophila serogroup 1 isolates were typed by monoclonal antibody (MAb) with International and Dresden MAb panels[16,17] and compared by three molecular methods, amplified fragment length polymorphism (AFLP), pulsed-field gel electrophoresis (PFGE)-SfiI, and arbitrarily primed-polymerase chain reaction (AP-PCR).[18,19,20]

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