Lessons From an Outbreak of Legionnaires' Disease on a Hematology-Oncology Unit

Abstract

OBJECTIVES To define the scope of an outbreak of Legionnaires' disease (LD), to identify the source, and to stop transmission. DESIGN AND SETTING Epidemiologic investigation of an LD outbreak among patients and a visitor exposed to a newly constructed hematology-oncology unit. METHODS An LD case was defined as radiographically confirmed pneumonia in a person with positive urinary antigen testing and/or respiratory culture for Legionella and exposure to the hematology-oncology unit after February 20, 2014. Cases were classified as definitely or probably healthcare-associated based on whether they were exposed to the unit for all or part of the incubation period (2-10 days). We conducted an environmental assessment and collected water samples for culture. Clinical and environmental isolates were compared by monoclonal antibody (MAb) and sequence-based typing. RESULTS Over a 12-week period, 10 cases were identified, including 6 definite and 4 probable cases. Environmental sampling revealed Legionella pneumophila serogroup 1 (Lp1) in the potable water at 9 of 10 unit sites (90%), including all patient rooms tested. The 3 clinical isolates were identical to environmental isolates from the unit (MAb2-positive, sequence type ST36). No cases occurred with exposure after the implementation of water restrictions followed by point-of-use filters. CONCLUSIONS Contamination of the unit's potable water system with Lp1 strain ST36 was the likely source of this outbreak. Healthcare providers should routinely test patients who develop pneumonia at least 2 days after hospital admission for LD. A single case of LD that is definitely healthcare associated should prompt a full investigation. Infect Control Hosp Epidemiol 2017;38:306-313.

FIGURE 1

Epidemic curve of healthcare-associated cases of Legionnaires’ disease (n =10) by week of symptom onset. ADPH, Alabama Department of Public Health. All cases were reported to the respective local health departments within 1 week of positive test results.

FIGURE 2

Probable incubation period (2–10 days prior to symptom onset), exposure to hematology-oncology unit, and positive Legionella test collection date by case patient (n = 10).

FIGURE 3

Schematic of the hematology-oncology unit showing the sites of positive environmental sampling results (figure not to scale). Of 27 rooms, 9 were occupied by case patients for at least 1 night; Legionella species were isolated from points-of-use in each of the 4 case patient rooms from which samples were obtained.