Our study shows that routine introduction of monochloramine into a municipal water system can reduce Legionella colonization of buildings served by the water system by 69% within a 1-month period. This finding represents the first demonstration of a community-wide intervention with the potential to reduce the incidence of Legionnaires' disease associated with potable water.
Colonization by Legionella
spp. was significantly associated with the presence of amoeba in the chlorine phase of the study. This is not surprising, since Legionella
infection of and multiplication within these protozoa are the primary means by which Legionella
amplifies in the environment. Ours is one of few studies to document the Legionella
-amoeba association in building environments, especially on such a large scale. The strong association between the presence of amoebae and Legionella
during the CL phase supports the theory that protozoa represent the bacterium's primary means of multiplication and suggests that amoeba shelter the bacteria from disinfection by chlorine (6
). On the other hand, monochloramine significantly decreased the presence of Legionella
, suggesting that monochloramine was able to penetrate amoeba-laden biofilms while having no direct effect on the amoebae themselves.
Several lines of evidence support the conclusion that the observed reductions in Legionella
colonization were actually caused by the conversion to MC. MC was detectable in 88% of water heaters and 93% of distal sites after the change, while free CL was undetectable in water heaters and at the point of use. Other factors known to impact the growth of Legionella
, such as temperature and pH, did not change substantially between the two phases of the study. Random fluctuation in colonization status is an unlikely explanation for our observations; such changes should lead to previously culture-negative sites becoming culture positive with roughly the same frequency as culture-positive sites becoming culture negative. Instead, we observed a dramatic and statistically significant reduction in the proportion of sites colonized. Finally, these findings are supported by previous work documenting that MC reduces the risk of Legionella
colonization in hospitals (11
) and the risk of outbreaks of nosocomial Legionnnaires' disease (13
We observed differences in the effectiveness of MC in different types of buildings. Legionella colonization was more likely to be eliminated from hotels and single-family homes than from county government buildings, independent of other factors. One possible explanation for these differences is that hotels and single-family homes might have more consistent water usage, which could facilitate the delivery of residual monochloramine to all parts of the building water system. These differences could also be explained by unmeasured factors, such as the complexity of the building water systems, the pervasiveness of biofilm, or differences in maintenance. These factors could be important in predicting the impact of community-wide introduction of monochloramine.
We also observed an important association between water source and building colonization. Buildings served by water source B were almost seven times more likely, after adjusting for other factors, to be colonized with Legionella during the chlorine phase than buildings served by other water sources. To our knowledge, an association between a particular water source and colonization with Legionella has not been previously described. Furthermore, during the MC phase, none of the colonized buildings received their water from source B. One possible explanation for this observation is that the main difference between source B and the other water sources is that source B water passes through a hydrogen sulfide removal plant and is treated with a corrosion inhibitor. Higher turbidities observed in the plant effluent suggest that biofilm grown in the hydrogen sulfide removal towers sloughs off and enters the distribution system, thereby releasing larger quantities of Legionella downstream. Regardless of the precise nature of this association, it implies that there might be source-specific factors that predict colonization of buildings by Legionella and that these factors can be overcome by MC.
Despite the positive findings of our study, Pinellas County Utilities, in cooperation with other investigators, observed several negative and potentially detrimental outcomes following the conversion to MC (16
). The proportion of buildings that were colonized by mycobacteria increased from 19.1% during the CL phase to 42.2% during the MC phase. In the PCU distribution system, the number of samples that contained detectable levels of coliforms also increased from two samples during the CL phase to 20 samples during the MC phase. The long-term health effects of these changes are unknown. However, in light of these findings, PCU now recognizes that implementation of chloramination may have negative as well as positive impacts.
In 2002, nine cases of legionellosis among persons residing in Pinellas County, Florida, were reported to the CDC (CDC, unpublished data), a number too small to determine whether the change to MC had an impact on human disease. There are several explanations for the low incidence of disease. First, Legionnaires' disease, like other causes of community-acquired pneumonia, is underdiagnosed, largely because diagnostic testing is done infrequently in favor of empirical therapy (3
). Also, in recent years, cultures for Legionella
have become less common as Legionella
urinary antigen testing (4
), which detects only Legionella pneumophila
serogroup 1 (Lp1), has become more common. Of note, 28 (78%) of 36 Legionella
strains isolated from buildings in our study were species and serogroups other than Lp1 (Table ). Third, some cases of legionellosis could have occurred among travelers who left Pinellas County to return home before they were diagnosed.
Legionella species isolated from hot water heaters and distal sites
We sampled buildings served by a water utility in one county in southwestern Florida, which might differ from other counties in terms of its water distribution system, baseline water quality, patterns of water use, or other factors. Therefore, our results might not be generalizable to utilities in other counties in Florida or in other areas of the country. In addition, due to a planned change in PCU's source water 4 months after the conversion to MC, we conducted our survey over a short period of time, so long-term consequences on water quality, Legionella colonization, or colonization by other pathogens are unknown. We sampled a relatively small number of buildings, and therefore, we were unable to detect statistically significant changes in particular species of Legionella, including Legionella pneumophila serogroup 1, the most common species and serogroup that causes Legionnaires' disease.
Our study is the first to document a positive impact of conversion to MC on Legionella colonization in potable water systems of public and private buildings. As other municipalities consider which residual disinfectant might bring their trihalomethane levels in line with the Stage 1 Disinfectants and Disinfection Byproducts Rule, these data offer some insights into the potential effect of MC on colonization of buildings with Legionella. While these findings imply that MC might hold promise as a tool for the community-wide prevention of legionellosis, they must be interpreted in light of the overall impact on water quality. Additional larger-scale studies in other parts of the country are needed to document positive and negative aspects of routine, community-wide implementation of MC disinfection.