Legionellosis Outbreak: A Moving Target

Determining the source of an infectious outbreak can be difficult in and of itself, but a moving reservoir adds to the challenges. That's what researchers encountered when investigating cases of Legionnaires' disease that kept presenting in one city in Spain. Legionella pneumophila is a Gram-negative bacterium identified as the causative agent of an outbreak of pneumonia that occurred in a Philadelphia hotel during a Legionnaires convention in 1977; the outbreak affected 221 persons, of whom 34 died. Although other Legionella spp. can cause the disease, L. pneumophila is responsible for 90 percent of the cases of legionellosis globally. This species is one of the most common causes of community-acquired bacterial pneumonia and the second most common cause of severe pneumonia. L. pneumophila is a waterborne bacterium that can cause respiratory illness when a susceptible person inhales contaminated, aerosolized water. Infection sources are usually human-made aquatic habitats, such as potable water supplies, whirlpool spas, cooling towers, showers, decorative fountains, and hoses.

In a study in the September issue of Emerging Infectious Diseases, the CDC's monthly journal, researchers Mireia Coscollá, José Fenollar, Isabel Escribano, and Fernando González-Candelas explain that from 1999 through 2005 in Alcoi, Spain, incidence of legionellosis was continually high. They explain further, "Over the next four years, incidence was lower, but an increase in July 2009 led health authorities to declare an epidemic outbreak. A molecular epidemiology investigation showed that the allelic profiles for all Legionella pneumophila samples from the 2009 outbreak patients were the same, thus pointing to a common genetic origin for their infections, and that they were identical to that of the organism that had caused the previous outbreaks."

The researchers reported that their analyses indicated a milling machine used in street asphalt repaving and its water tank as the most likely sources. As opposed to other machines used for street cleaning, the responsible milling machine used water from a natural spring. When the operation of this machine was prohibited and cleaning measures were adopted, infections ceased.

Coscollá, et al. (2010) note, "Since 2000, a specific epidemiologic surveillance system for legionellosis has been in place in the Hospital Virgen de los Llirios in Alcoi, Spain. Every patient with signs of pneumonia is scanned by chest radiography and urine analysis for L. pneumophila serogroup 1 antigen. This surveillance system enables early detection and better prognosis for L. pneumophilainfected patients. It also helps distinguish between sporadic cases and outbreaks, thus enabling early start of epidemiologic investigations. At the end of July 2009, the epidemiology surveillance system detected two cases of legionellosis in persons who had stayed in Alcoi during their incubation periods. New cases appeared during the first week of August, at which time an epidemic outbreak was declared and an epidemiologic investigation was started. Patients in the outbreak were questioned about clinical and personal aspects. Spatialtemporal analysis was used to identify the most likely areas of exposure for infection. L. pneumophila was isolated from environmental samples obtained in those areas. Because the usual facilities and municipal water systems associated with risk were not contaminated, other facilities not previously linked to legionellosis outbreaks were considered. To verify the common genetic origin of the outbreak and its environmental source, we performed an epidemiologic molecular analysis using sequence-based typing for clinical and environmental samples."

The researchers say they performed a rigorous environmental-related investigation: "Systematic environmental investigations are regularly performed in the municipal water distribution system, but when this outbreak was detected, an active search for L. pneumophila was made in patients homes (bulk water and biofilms from showerheads and taps) and the water distribution system (bulk water); results were negative. None of the other usual sources (e.g., public fountains, cooling towers, humidifiers) were found to pose a risk. The absence of usual risk sources led us to consider other possible sources of aerosols, including moving devices used in street cleaning and asphalt repaving; the latter had been observed in the risk area during the epidemiologic inspection."

Among patients with positive urine antigen test results, 11 cases of legionellosis were confirmed and L. pneumophila was isolated from four, according to the researchers. They add, " All patients required hospitalization, and all except one recovered. (The patient who did not recover had severe signs and symptoms and subsequently died.) The main signs and symptoms were fever (100 percent incidence), pneumonia (100 percent ), headache (27.3 percent ), myalgia (27.3 percent ), diarrhea and/or vomiting (18.2 percent ), and confusion (45.5 percent ). The average age was 70 years, range 4988 years. More men than women were affected (male:female ratio = 4.5). Confirmed cases occurred from July 21 through September 17. According to the date of disease onset, the outbreak showed three epidemic waves: two cases in the second half of July, eight cases in the first half of August, and one case in the second half of August. No common indoor source of exposure was found, and the initial hypothesis was that the outbreak originated from environmental contamination of an unknown source capable of producing and dispersing large quantities of aerosols contaminated with L. pneumophila. The first two patients lived in the northern part of the city, which suggested that the source could be located in that area. The spatial distribution of patients buffers changed in August, thus indicating that the likely source of the outbreak had moved to the Santa Rosa quarter. The area of epidemic risk was modified accordingly, and the search for putative environmental sources focused on that neighborhood. L. pneumophila was not isolated from samples derived from the municipal water supply, traditional risk facilities, and patients houses. The environmental investigation was extended to other potential sources, especially portable cleaning devices such as sweepers, hydrocleaners, and water tanks used to clean the streets, all of which used water from the municipal water supply. At that time, the Santa Rosa quarter was being repaved, and one of the machines used in the repaving process was a tank truck that carried water used by a large milling machine. The water in the tank was obtained from a natural spring untreated with chlorine or anything else. Because the average daytime temperature in Alcoi during July and August is 27°C, the water in the machine might have been warm enough for L. pneumophila growth. The milling machine had been working north of Alcoi around July 15 and in the Santa Rosa neighborhood from July 31 through Aug. 20. This activity fits spatially and temporally with the incubation period of confirmed cases. This machine was identified and removed from service on Aug. 21, thus was able to cause the last infection detected on Aug. 23."

Reference: Coscollá M, Fenollar J, Escribano I and González-Candelas F. Legionellosis Outbreak Associated with Asphalt Paving Machine, Spain, 2009. Emerg Infect Dis. Vol. 16, No. 9, September 2010.