The Chicken–Egg Dilemma: Legionnaires’ Disease and Retrograde Contamination of Dental Unit Waterlines

To the Editor—On February 9, 2011, an 82-year-old Italian woman died from Legionnaires’ disease (LD). Other than 2 appointments at a dental office, she had not been exposed to any obvious source of Legionella infection in the 2–10 days before symptom onset that occurred on February 7. On February 17, an epidemiologic field investigation in the dental office, performed by the regional healthcare agency, detected Legionella pneumophila serogroup 1 (sg1) in water samples from the cold-water tap (1500 CFU/L), the dental turbine (62000 CFU/L), and the cup filler (4000 CFU/L) of a dental unit, which had been routinely disinfected with H₂O₂. Strain typing revealed that the isolates from the environment and the patient’s bronchial aspirate matched, suggesting that the dental unit waterlines (DUW) were the likely source of LD infection. ¹

In line with the guidelines for epidemiologic field investigation, ² immediate control measures were taken. On March 1, the dental healthcare worker was warned to quit practicing until thorough DUW disinfection with chlorination was performed and to reculture the DUW for the presence of L. pneumophila after the disinfection process. ¹ On March 15, after the disinfection, a single water sample was collected by the agency from an indeterminant site, Legionella was not detected (<50 CFU/L) ¹ (Table 1) but Pseudomonas aeruginosa (400 CFU/L) was found. To date, this is considered the only confirmed LD case attributable to DUW.

TABLE 1 Results of the Epidemiologic Field Investigation in the Dental Office Attended by the Patient who died from Legionnaires’ Disease on February 9, 2011 ^a

NOTE: DUW, dental unit waterline.

^a Detection of Legionella pneumophila serogroup 1: counts in CFU/L; lowest limit of detection, 50 CFU/L.

^b Pseudomonas aeruginosa (400 CFU/L) was detected.

This story, however, has an epilogue that was not reported and may subvert the alleged causal association between L. pneumophila in DUW and consequent LD development. Indeed, the dentist declared that the 2 appointments occurred on December 9 and January 24; during the latter appointment, the patient showed breathing difficulty, fatigue, and relayed a premonition of her imminent death. In addition, on March 1, the agency collected 6 more water samples in 4 treatment rooms of the dental office, and L. pneumophila was likely not detected because the results of these samples were not reported (Table 1).

These results are very important, because they suggest that L. pneumophila sg1 transmission from DUW to the patient cannot be confirmed and that an alternative hypothesis cannot be ruled out. Namely, the patient may have been infected before the dental treatment, which may have led to the retrograde contamination of room-1 water systems, including the DUW, through the outlet points. This hypothesis is plausible because the patient harbored L. pneumophila in her respiratory tract, ¹ and during dental treatments biological fluids and microorganisms from the respiratory tracts of dental patients are aspirated into the DUW through the outlet points. ³

Such an alternative hypothesis is corroborated by the consideration that the first appointment occurred 62 days before patient’s death, an interval largely exceeding the LD incubation period, and the patient’s condition during the second appointment suggests that she was already affected with LD and, therefore, was already colonized with L. pneumophila. Secondarily, legionellae were detected at a high level in the room-1 turbine on February 17 and not reported in 4 treatment rooms on March 1. There could be two explanations for the negative findings: (1) water-system chlorination was performed by the dentist before receiving the official notification from the healthcare agency; (2) retrograde DUW contamination had occurred through the outlet points (ie, turbine, cup filler, and faucet) during treatment, which produced transient L. pneumophila colonization in room 1 and no colonization in rooms 2–4. The latter hypothesis was corroborated by the fact that, despite the high microbial load detected in the DUW during the first inspection, no other cases of LD or Pontiac fever among the dental staff and patients were reported. ¹

It is not possible to exclude the possibility that the patient was exposed to other sources of Legionella infection in the days preceding the dental appointments because the patient’s history was limited to 10 days before symptom onset and LD incubation is frequently longer. ⁴

The present probable retrograde water system contamination from an infected patient, along with other episodes, suggests that it is time to reconsider the routes of Legionella transmission to patients and spread in the environment. Indeed, person-to-person LD transmission was recently demonstrated for the first time in Portugal where a patient developed LD taking care of her son previously infected 300 km away from home with L. pneumophila ST1905. ⁵ In addition, the outlet points of 2 Italian hospitals were more frequently and heavily contaminated with L. pneumophila sg1 than the building water systems. This finding was explained by retrograde contamination. ⁶ Finally, a dentist who died from LD had positive results for Legionella dumoffii, L. pneumophila, and L. longbeachae, and these microorganisms were detected in the DUW and in the domestic water system, implying that the infected dentist was associated with at least 1 retrograde contamination. ⁷

Legionella pneumophila is detectable in the DUW, ⁸ which suggests that patients and staff could be at risk for LD. However, no LD clusters and outbreaks have been associated with dental treatments, and there are only 2 suspected sporadic cases including this one. ^{1 , 7} In contrast, the number of exposed individuals is very high; 200 million people have dental visits in United States each year, ⁹ and the US dental workforce routinely and occupationally exposed to L. pneumophila comprises almost 200,000 dentists. ¹⁰ These data demonstrate that LD incidence, and therefore LD risk in dental healthcare settings, is limited. Such an inference, however, does not imply that infection control measures focusing on DUW are unnecessary, ¹¹ given the general high level of contamination, ⁸ but only that these measures are based on the Precautionary Principle. ¹²

In conclusion, the chicken–egg dilemma (ie, strain-typing matches of isolates from the environment and the patient do not demonstrate where the organism occurred first) regarding waterborne pathogens ¹³ may also apply to the present report. In addition, the scientific evidence for an active role of human carriers in LD transmission and L. pneumophila spread is increasing. This hypothesis is even more convincing than the hypothesis of the atmospheric dispersion of contaminated aerosols for more than 10Km, in explaining the long-distance LD outbreaks. ¹⁴