A new weapon found against Legionella?

Water treatment method may prevent outbreaks

Making a striking connection that was previously unrecognized — and still is not completely understood — researchers have found that a longstanding water treatment method using monochloramine, a combination of ammonia and chlorine, may herald a major breakthrough in the fight against Legionnaires’ disease.

Each year, 8,000 to 18,000 cases of Legionnaires’ disease occur in the United States, with some 10% to 20% of cases as part of outbreaks, the researchers report.1 Case-fatality rates in outbreaks typically are 20% to 40%. The disease is caused by Legionella bacteria, which can proliferate in pipes and tanks of water systems. Transmission occurs through inhalation of an aerosol containing the bacteria or by aspiration of contaminated water. Most outbreaks have been traced to either drinking water or cooling towers, and ecological and laboratory studies show that sporadic cases are caused by the same sources. Most nosocomial outbreaks have been linked to hospital drinking water systems that have been colonized by Legionella species, the authors report.

While underscoring the need for additional research to verify the findings, the authors draw the compelling conclusion that if all municipalities in the nation used monochloramine for water treatment, some 3,600 to 8,100 cases of Legionnaire’s disease would be prevented annually and 900 to 2,025 lives saved. Those projections are based on a conservative estimate that roughly half the number of annual cases of Legionnaire’s disease can be traced to drinking water, though some experts say the actual percentage is much higher and other suspected sources like cooling towers are not significant sources of transmission. Regardless, the researchers estimate that 90% of the outbreaks caused by drinking water would be eliminated by using the treatment method.

"It seems the majority of outbreaks are traced to water and not cooling towers," says lead author Jacob Kool, MD, assistant professor of clinical medicine at the University of Southern California in Los Angeles and a former epidemic intelligence service (EIS) officer with the Centers for Disease Control and Prevention. "[But even if] only half of them are caused by drinking water, then 90% of that half can be eliminated if this monochloramine really works that well."

Monochloramine — formed when ammonia and free chlorine are mixed in water — has been used for drinking water disinfection since 1916. About one-quarter of U.S. cities currently use the method for "chloramination" (as opposed to chlorination) of water systems. Municipal drinking water treatment typically includes initial disinfection to kill organisms in the water and residual disinfection to maintain biocidal activity throughout the water distribution system, Kool and his co-authors explain.

Monochloramine’s disinfecting action is slower than that of free chlorine, so it is less useful for initial disinfection. On the other hand, it is more stable than free chlorine, so a disinfecting residual can be maintained over long distances in a distribution system. In addition, if its efficacy is borne out, the researchers suspect it will be traced to an ability to penetrate biofilm and eradicate Legionella.

"Chlorine remains on the outside of biofilm and cells, but monochloramine penetrates into cells and biofilm," Kool adds. "In that way, it seems clear that it could reach Legionella much easier because Legionella hides’ in biofilm. That is something that is well-known and recognized as a problem; it impedes disinfection with chlorine. There is also the hypothetical idea that maybe monochloramine penetrates amoebae. It is well known that Legionella hides in amoebae as well, to escape superheating and hyperchlorination."

31 of 32 outbreak hospitals used chlorine

The authors conducted a case-control study to compare disinfection methods for drinking water supplied to 32 hospitals that had outbreaks of Legionnaires’ disease with disinfection methods for water supplied to 48 control hospitals. They found that hospitals supplied with drinking water containing free chlorine as a residual disinfectant were 10.2 times more likely to have a reported outbreak of Legionnaires’ disease than were those that used water with monochloramine. A review of 32 nosocomial outbreaks reported from 1979 to 1997 found that 31 of the hospitals used chlorine for disinfection rather than monochloramine.

The findings were supported by laboratory tests finding that Legionella were frequently recovered from chlorinated municipal water but not from chloraminated water in recent investigations by the CDC. Indeed, Kool first noticed the connection with monochloramine while investigating outbreaks of Legionnaire’s disease in Texas as a CDC EIS officer.

"In one county where they used chlorine in the water, we found that all hospitals actually had some Legionella in their water," he tells HIC. "But two other counties had no Legionella in their water, so we tried to get more information about that. We heard from the water treatment plant that they were using monochloramine. I had never heard of it before, and I asked my colleagues at CDC if they had. No one had. That actually piqued my curiosity, because if we had had outbreaks in monochloramine-using cities, we would have heard about it. None of my colleagues had ever investigated an outbreak of Legionnaires’ disease in a city that uses monochloramine."

The paper was reviewed and co-written by officials with the CDC respiratory diseases branch and hospital infections program, but Kool says the findings will have to be thoroughly verified before any new recommendations are made. The CDC currently recommends that hospitals with Legionella problems decontaminate the water and then implement control measures aimed at preventing regrowth. Long-term control measures include increasing the temperature of the hospital’s hot water and continuously injecting additional chlorine. Although the methods are effective, a high water temperature increases the risk of scalding, and continuous high concentrations of chlorine can cause corrosion of plumbing systems.

"Everyone is waiting for confirmatory studies — you don’t want to base policy decisions on just one study," Kool says. "There is still the hypothetical possibility that monochloramine has some effect on lab work, making it harder to isolate Legionella than in water that contains chlorine. The CDC and I are working on follow-up studies."

Hospitals could use booster’ stations

If the findings are verified, it may be possible for individual hospitals with longstanding problems with nosocomial Legionella infections to convert their water systems even if their municipality does not use monochloramine, Kool notes. A chloramination "booster" station could be installed in a chlorine-using water system to chloraminate only a small part of that system, such as a hospital campus.

"It would definitely be possible," he says. "The question is, is it feasible? It would involve more monitoring because you’re not just adding chlorine, but also ammonia. The ratio of ammonia to chlorine has to be exactly right. So it would involve some more training of hospital staff and some more monitoring. But we are actually looking into that right now. We are already working with one hospital that is considering trying this to see if that helps them control their Legionnaires’ disease problem."


1. Kool JL, Carpenter J, Fields BS. Effect of monochloramine disinfection of municipal drinking water on risk of nosocomial Legionnaires’ disease. Lancet 1999; 353:272-277.