Legionella bacterium attaching to an amoeba; the amoeba provides protection from harmful environmental conditions
Photo courtesy of Dr. Barry Fields, CDC
Two large Quebec City buildings and a Chicago hotel are reported sites of exposure to Legionella bacteria that resulted in over 100 recent cases of Legionnaires’ disease and 13 deaths.
In Quebec City, officials suspect building cooling systems became colonized with high numbers of the bacteria (CNN Health report), causing ten deaths and 165 cases of illness. The J.W. Marriot Chicago experienced an outbreak of 10 cases of Legionnaires’ disease between July 16 and August 15; three of these cases were fatal. The Chicago Tribune reported the primary source of the outbreak was a decorative fountain in the hotel’s main lobby (see Water Wall in Hospital Dispenses Legionella for a report on a 2010 Legionnaires’ disease outbreak). A September 1 Chicago Tribune report states that the fountain has been removed.
Legionella was first detected as the pathogen responsible for an outbreak of pneumonia among attendees to a 1976 Philadelphia American Legion convention; 29 attendees died after being infected by exposure to bacteria through the hotel air-conditioning system. Legionella is transmitted by inhaling the bacteria in the mist of the water, produced typically in the indoor water of hot tubs, decorative water walls or fountains, swimming pools and cooling towers of large buildings, in addition to air-conditioning systems. According to the Mayo Clinic website, Legionella can travel through the air for as much as four miles. Most people exposed to the bacteria do not become ill, according to CDC, but of those who do, the majority can be treated successfully with antibiotics. Whereas healthy people usually recover from Legionnaires’ disease (see CDC Patient Fact Sheet), it causes pneumonia-like symptoms and can lead to death in the elderly, smokers and people with weakened immune systems.
Legionella: Modus Operandi
The key to controlling Legionnaires’ disease is understanding Legionella’s watery ecological niche. Legionella thrives in recirculating water systems maintained between 35 and 55 degrees Celsius (95 to 131 degrees Fahrenheit). It dwells in biofilms, intricately guarded microbial communities that form on wet surfaces. Biofilms provide excellent “cover” for Legionella, supplying both nutrients and protection from harm. According to Kwaik et al., Legionella’s ability to infect and even multiply within certain single-celled organisms such as some amoeba (see photo above) contributes to its hardiness and provides a “Trojan Horse” mode of transportation.
Two of the most important factors in controlling Legionella in water circulating systems are minimizing scale buildup in the system and eliminating zones of temperature that support proliferation of the bacteria. Scale provides the type of uneven surface that supports biofilm development, a safe haven for bacteria. Temperature zones in the 35 to 55 degree Celsius range creates Legionella comfort zones, enhancing its growth potential.
The Environmental Protection Agency (EPA) lists the following methods for disinfecting water circulating systems to control Legionella: thermal (super heat and flush); hyperchlorination; copper-silver ionization; ultraviolet light sterilization; ozonation and instantaneous steam heating systems. In Legionella: Drinking Water Health Advisory, EPA recommends a disinfection strategy of combining two or more of these methods because “some methods have not always proven completely successful or have not provided permanent protection from recolonization.”
In their recent review of Legionella disinfection methods in hospital drinking water, Lin et al. (2011) recommend individual methods be validated in a stepwise fashion. These steps could be applied to any institution for the purpose of minimizing Legionella exposure:
- Laboratory evidence demonstrates Legionella disinfection
- Anecdotal reports of success in controlling Legionella in individual hospitals
- Peer-reviewed and published reports of success in controlling Legionella growth and preventing cases of Legionnaires’ disease in individual hospitals on a prolonged basis
- Reports confirming success from multiple hospitals over a prolonged period of time
Given what we know about Legionella and its complex survival mechanisms, a thoughtful approach is warranted to stay one step ahead of Legionnaires’ disease.
Joan B. Rose, PhD, is the Homer Nowlin Chair in Water Research at Michigan State University and a member of the Water Quality and Health Council.