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

Results

The outbreak of pneumonia was detected on the evening of July 7, when the Regional Department was notified that an increase in pneumonia cases had occurred in three hospitals in Murcia. Legionella antigen was detected in some patients' urine. Approximately 800 suspected cases were reported July 8-22; confirmed cases numbered 449. We estimate that 636-696 persons were affected. These estimates took into account the sensitivity of the antigen test in urine of 70%[4] and the background number of pneumonia cases estimated from the median of patients admitted with pneumonia in the region's hospitals during the summers from 1996 through 2000.

Onset of illness of the first confirmed case-patient was June 26. Until July 1, only a small number of cases occurred. After this date, the outbreak became explosive, with most cases occurring in <10 days (83% of confirmed cases had onset from July 2 through July 9). The last case-patient became ill on July 19 (Figure 1). The outbreak was considered compatible with massive exposure to a common source of contamination. When LD's incubation period was taken into account, the maximum emission was estimated to have occurred June 29-July 1 and to have ended completely July 9-17.

Confirmed cases of Legionnaires' disease by date of onset of illness, Murcia, Spain, June 26-July 19, 2001.

Hospital admission was necessary for 64% of all reported case-patients and 74% of confirmed case-patients. Six deaths from LD were confirmed to be directly related to this outbreak, five confirmed cases and one suspected. Therefore, the case-fatality rate was 1.1% for confirmed cases only and 0.9% for total estimated cases. For all confirmed case-patients, 74% were men and 26% women. The age range was from 19 to 91 years; 70% were ≥50, and 29% were ≥70. The incidence rate increased with age in both sexes and was higher in men in all age groups (Figure 2).

Confirmed cases of Legionnaires' disease within the city of Murcia, Spain. Specific incidence rates by sex and age (per 100,000).

Of the confirmed case-patients, 68% lived in Murcia city proper, 16% in the satellite districts within the municipality of Murcia, and 16% in other municipalities of the region. To evaluate the risk by quarters within the city of Murcia, the Standardized Incidence Ratio (SIR) was used. Three neighborhoods located in the northern part of the city had the highest incidence rate (4.9-6.7 per 1,000 population), significantly higher than the average for the city of Murcia ( Table 1 , Figure 3). According to epidemiologic interviews, 95% of the confirmed case-patients lived, worked, or visited in the northern districts in the 10 days before the outbreak began.

Confirmed cases of Legionnaires' disease within Murcia city, Spain. Standardized Incidence Ratio (SIR) by neighborhood. Circle represents hospital H.

L. pneumophila serogroup 1 was recovered from clinical samples of 19 patients; 18 samples were characterized. All were Pontiac (MAb 2+) Philadelphia MAb type and shared an identical molecular pattern by AFLP, PFGE-SfiI, and AP-PCR.

The descriptive study showed no common indoor source of exposure and determined that the outbreak was provoked by a common source located in the northern part of the city. The study hypothesis was that the outbreak had its origin in environmental contamination from cooling towers or other installations capable of producing and dispersing large quantities of aerosols potentially contaminated by legionellae.

A total of 85 cases and 170 controls were included in a case-control study. Participation in the case-control study was classified as recently proposed by Olsen et al.[21] The response rate among eligible cases was 89% (85/96) and 96% (85/89) among those eligible who were contacted. The response rate among persons selected as eligible controls was 51% (170/334) and 61% (170/279) among those eligible who were contacted. The distribution of cases and controls according to sex, age, and residence was identical. No significant differences were found between cases and controls in any of the variables considered as a risk for or predisposing factor to the disease, as shown in Table 2 . No differences were found with respect to education level or employment situation ( Table 3 ). A strong association between visiting the city of Murcia and being ill with LD was found (OR 14.1, 95% CI 4.2 to 45.9).

The zone of exposure, defined either by the block surrounding hospital H or by a circle of 200 m in radius around hospital H, was significantly associated with illness in all eight models of multivariate analysis ( Table 4 ). This zone of exposure also showed a much higher OR in every model. Thus, LD was 4.8-11.4 times more likely to develop in persons who passed through the zone around hospital H during the risk period than in persons who did not travel through this zone, independent of their having passed through the other zones. These results were also independent of the number of times the patient had visited the city.

An association between the illness and walking through the zone was observed in the multivariate analysis for another three zones of exposure. However, none of these zones appeared in more than two of the eight models, and each had an OR that was lower or much lower than that for the zone around hospital H in the corresponding model.

During the epidemiologic study of this community outbreak of LD, a nosocomial outbreak of LD at hospital H was discovered. In all 11 definite or probable nosocomial cases, some portion of the previous 10-day period of hospitalization coincided with the period when the cooling towers could have been active.

L. pneumophila was not recovered from water samples from the drinking water supply network in the city of Murcia. L. pneumophila serogroup 1 Pontiac (MAb 2+) was recovered from 22 installations (cooling towers of 11 buildings in the city and water storage tanks from 3 buildings). Ten of 11 cooling towers contained a Philadelphia MAb-type strain, but only two colonies, recovered in October from a cooling tower of hospital H, were indistinguishable from the patient strains by AFLP (Figure 4). Identical results were obtained when PFGE-SfiI and APPCR were applied.

Amplified fragment length polymorphism (AFLP) gel Regional ing outbreak human and environmental Legionella pneumophila serogroup 1 isolates. M, molecular weight marker (Ladder Mix, MBI Fermentas, UK). Lanes 1 and 2, two colonies from a cooling tower of the hospital H. Lines 3 and 4, human isolates. Lanes 5 and 6, human isolates. Lanes 7-11, different environmental isolates from several Murcia installations.

Data provided by the Weather Centre of Murcia showed that during the last days of June and early July some atmospheric thermal inversion occurred every day, except one. Winds were predominantly from the northeast quadrant with a very low average speed (9 kph) and very high temperatures (33.5°C-35°C).

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