Legionella species colonization in cooling towers: Risk factors and assessment of control measures

Mouchtouri, Varvara A.; Goutziana, Georgia; Kremastinou, Jenny; Hadjichristodoulou, Christos

doi:10.1016/j.ajic.2009.04.285

« Previous Next »AJIC: American Journal of Infection Control
Volume 38, Issue 1 , Pages 50-55, February 2010

Legionella species colonization in cooling towers: Risk factors and assessment of control measures

Varvara A. Mouchtouri, BSc, PhD
- Department of Hygiene and Epidemiology, Medical School, University of Thessaly, Larissa, Greece
- Department of Public and Administrative Health, National School of Public Health, Athens, Greece
Georgia Goutziana, MD
- Department of Hygiene and Epidemiology, Medical School, University of Thessaly, Larissa, Greece
Jenny Kremastinou, MD, PhD
- Department of Public and Administrative Health, National School of Public Health, Athens, Greece
Christos Hadjichristodoulou, MD, PhD
- Department of Hygiene and Epidemiology, Medical School, University of Thessaly, Larissa, Greece
- Address correspondence to Christos Hadjichristodoulou, MD, PhD, Department of Hygiene and Epidemiology, Medical School, University of Thessaly, Papakiriazi Str, 22, 41222, Larissa, Greece.

published online 21 August 2009.

Background

Cooling towers can be colonized by Legionella spp, and inhalation of aerosols generated by their operation may cause Legionnaires' disease in susceptible hosts. Environmental investigations of Legionnaires' disease outbreaks linked with cooling towers have revealed poorly maintained systems, lack of control measures, and failure of system equipment. The purpose of this study was to identify Legionella-contaminated cooling towers, identify risk factors for contamination, and assess the effectiveness of control measures.

Methods

A total of 96 cooling towers of public buildings were registered and inspected, and 130 samples were collected and microbiologically tested. Microbiological test results were associated with characteristics of cooling towers, water samples, inspection results, and maintenance practices.

Results

Of the total 96 cooling towers examined, 47 (48.9%) were colonized by Legionella spp, and 22 (22.9%) required remedial action. A total of 65 samples (50.0%) were positive (≥ 500 cfu L⁻¹), and 30 (23%) were heavily contaminated (≥ 10⁴ cfu L⁻¹). Of the 69 isolates identified, 55 strains (79.7.%) were L pneumophila. Legionella colonization was positively associated with the absence of training on Legionella control (relative risk [RR]=1.66; P=.02), absence of regular Legionella testing (RR=2.07: P=.002), absence of sunlight protection (RR=1.63: P=.02), with samples in which the free residual chlorine level in the water sample was<0.5mg/L (RR=2.23; P=.01), and with total plate count (P=.001). Colonization was negatively associated with chemical disinfection (RR=0.2; P=.0003) and with the presence of a risk assessment and management plan (RR=0.12; P=.0005). A statistically significant higher age (P=.01) was found in legionellae-positive cooling towers (median, 17 years; interquartile range [IQR]=5.0 to 26.0 years) compared with noncolonized cooling towers (median age, 6 years; IQR=1.0 to 13.5 years). After the 22 legionellae-positive cooling towers were disinfected with chlorine, 2 (9%) of them remained positive for Legionella spp with a concentration≥1000 cfu L⁻¹.

Conclusions

Cooling towers can be heavily colonized by Legionella spp and thus present a potential risk for infection. This study demonstrates the importance of a risk assessment and management plan. Water chlorination effectively reduces legionellae contamination. Proper training of cooling tower operators is paramount.

Key Words: Legionella, cooling tower, factor, disinfection, risk assessment