Inability of chlorine dioxide to reduce Legionella counts to zero?

I have a few questions regarding a facility using chlorine dioxide (ClO2). Our infection control group has changed the rooms that are cultured for Legionella to reflect more of the high risk areas (ICU, heart transplant wards, HIV, etc.). The first round of testing revealed a few positives in these areas. All the counts were very low, only 0.2 to 1.3 cfu/mL. Swabs taken from these locations were negative. We claimed that these low numbers certainly reduced risk; however, the physicians wanted to reach zero colonization all the time. We believe that it is impossible to reach zero. Are we correct in assuming this?

You are correct. Unfortunately, this is a major flaw for the CDC guidelines for transplant units, which does specify a zero cfu limit. It is nearly impossible to achieve zero colonization all the time. Quantitation of cfu/mL is not an accurate indicator for disease, but proportion of sites yielding Legionella is.


Risk of nosocomial Legionnaires’ disease is better predicted by the proportion of water system sites testing positive for Legionella than by the concentration of Legionella bacteria. (Kool JL, et al. Infect Control Hosp Epid 1999; 20:797-05.)

No correlation between quantitative counts and Legionella cases. There was a correlation between site positivity (>30%) and cases (Best et al. Lancet 1983;307-310.)

No correlation between quantitative counts and Legionella cases (Kohler JR et al. J Hosp Infect 1999; 41:301-311).

In our recent experience, no hospital-acquired cases of Legionnaires’ disease were diagnosed at two hospitals during our evaluation of Cl02 despite the persistence of Legionella pneumophila in the water system. However, it took many months to significantly reduce the level of Legionella in the system; the percent positivity rate was <30%. Moreover, it took many months of chlorine dioxide disinfection to significantly reduce the level of Legionella in the system of both hospitals (Sidari JAWWA 2004).