Review
Environmental sources of community-acquired legionnaires’ disease: A review

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Abstract

Background

Most Legionnaires’ disease in the US and abroad is community-acquired and believed to be sporadic, or non-outbreak associated. Most patients are exposed to numerous water sources, thus making it difficult to focus environmental investigations. Identifying known sources of sporadic community-acquired Legionnaires’ disease will inform future sporadic Legionnaires’ disease investigations as well as highlight directions for research. The objective is to summarize and rank sporadic Legionnaires’ disease sources based on the level of linkage between the environmental source and cases.

Methods

A PubMed search was conducted using the search terms legion* and (origins or source or transmission) and (sporadic or community-acquired). Studies of nosocomial and/or outbreak-associated disease were excluded from this review. Definite, probable, possible and suspect ranks were assigned to sources based on evidence of linkage to sporadic Legionnaires’ disease.

Results

The search yielded 196 articles and 47 articles were included in the final review after application of exclusion criteria. A total of 28 sources were identified. Of these, eight were assigned definite rank including residential potable water and car air-conditioner water leakage. Probable rank was assigned to five sources including solar-heated potable water and soil. Possible rank was assigned to nine sources including residential potable water and cooling towers. Suspect rank was assigned to 20 sources including large building water systems and cooling towers.

Conclusion

Residential potable water, large building water systems and car travel appear to contribute to a substantial proportion of sporadic Legionnaires’ disease. Cooling towers are also a potentially significant source; however, definitive linkage to sporadic cases proves difficult. The sources of sporadic Legionnaires’ disease cannot be definitively identified for most cases.

Introduction

Legionnaires’ disease (LD) is a common form of community-acquired bacterial pneumonia (Cunha et al., 2016). This waterborne disease is caused by Legionella species, which naturally exist in lakes, and rivers, and amplifies in building water systems, and other man-made structures. Persons develop LD by inhaling aerosolized Legionella-contaminated water or aspirating potable contaminated water. LD disproportionately affects elderly and immunocompromised individuals. Chronic medical conditions such as diabetes mellitus and COPD are also associated with increased risk, as is current, and former smoking (Cunha et al., 2016). Almost all reported LD patients require hospitalization, and case fatality rates range from 15 to 20% (World Health Organization, 2007).

From 2000 to 2014, the reported annual rate of legionellosis in the US, which includes both LD and the milder Pontiac Fever, increased almost 300% from 0.42 to 1.62 per 100,000 (Garrison et al., 2016). US surveillance from 2005 to 2009 revealed that only 4% of cases during that time period were associated with known outbreaks. The same trend was observed in other parts of the world (Che et al., 2008).

The source of LD outbreaks is commonly pursued in order to stop the outbreak; however, the source of sporadic or isolated cases is rarely pursued (Garrison et al., 2016). Sporadic LD is defined as an isolated, single case with no known associations with other cases, and thus no known associations with outbreaks. The source of LD, as a waterborne disease, can be very difficult to determine given the plethora of water sources to which a person may be exposed during the 10-day incubation period.

LD is a reportable condition in the US that healthcare organizations and providers are required to report to health departments. The intensity of sporadic LD investigations varies by jurisdiction. The Centers for Disease Control and Prevention (CDC) recommends investigation of sporadic community-acquired LD if two or more cases share a common exposure (Adams et al., 2015). Jurisdictions may elect to investigate sporadic community-acquired cases; however, limited public health resources often limit these investigations. CDC also requests that all state, and local health departments complete a case report form which includes information on outcome, occupation, travel, healthcare, whirlpool spa exposures, and use of respiratory equipment (Garrison et al., 2016). CDC uses this information to determine travel, occupational, and healthcare-associated risk. In-depth investigation and analysis of sporadic case relatedness is performed by state and local health departments.

Knowledge about the sources of sporadic LD assists public health practitioners to efficiently conduct Legionella source investigations. The most recent review of sporadic LD sources was published in 1995 (Bhopal, 1995). This review summarized known sources of sporadic disease including residential potable water sources, and cooling towers; however, the author emphasized that additional large-scale research needed to be conducted before drawing conclusions about the major sources of sporadic LD (Bhopal, 1995). The purpose of the current review is to update current knowledge about known sources of sporadic LD and to identify those sources that pose the greatest risk.

Section snippets

Methods

This narrative review was initiated through a literature search of MEDLINE conducted on August 9, 2017. PubMed was used to identify potential articles for inclusion using the following search criteria:

legion* and (origin or (source or transmission)) and (sporadic or (community and acquired))

All 196 articles generated through this search were considered for inclusion through title and abstract review (Fig. 1). Exclusion criteria in the final review included description of sources of waterborne

Results

Of the 196 articles identified, 138 were excluded based on title and abstract screening (Fig. 1). The full text of 58 articles was reviewed, which led to the identification of 16 additional articles through reference review, for a total of 74 articles Of these, 27 were excluded because confirmed or probable linkage between the source and sporadic disease was not specifically studied and/or the source of disease was not explicitly discussed. Thus, we included 47 articles in this review (Fig. 1).

Discussion

The source of most sporadic LD cases is difficult to confirm. Nevertheless, as evidenced by this review, many potential sources have been linked to cases. The studies of possible and suspect rank included many more cases, but the types of analyses are less resource-intensive compared to the methods required to molecularly confirm sources. Reasons for the difficulty in confirming sources of sporadic disease include limited public health resources to conduct environmental investigations for

Conclusion

A variety of sporadic LD sources have been identified that have the potential to cause isolated incidents and more wide spread clusters. The literature reviewed indicated that important sources of sporadic LD are residential potable water, large building water systems, vehicle travel, and cooling towers. Nevertheless, sporadic LD sources remain to be identified as even intense environmental investigations sometimes still do not identify a source. Additional research is required to further

Conflict of interest

none

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    Present address: Department of Civil, and Environmental Engineering, Department of Engineering, and Earth Sciences, College of Engineering, University of Notre Dame, 156 Fitzpatrick Hall, Notre Dame, IN, 46556, USA.

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