The risk does exist

Sir, I read with interest your editor's summary in the recent issue (BDJ 2010; 209: 354–355). You wrote 'I am not aware of large scale or indeed local incidents of illness stemming from dental practices due to DUWLs' and that 'in an evidence-based world some answers would be reassuring'. You correctly highlighted that there may be differences in the hazards arising in DUWLs in dental hospital compared to dental practice. Dental hospitals do differ from smaller dental premises. The former have more complex plumbing systems, served by large water storage tanks with multiple dead legs on the system, which provide suitable habitats for Legionella proliferation.¹ Furthermore, it is well recognised that clinical members of the dental team have a greater exposure to contaminated DUWLs and are therefore more likely than patients to demonstrate evidence of disease associated with the DUWL exposure.²

HTM 01-05 guidance on managing DUWLs highlights Legionella as a marker organism for poor quality water. Legionellosis can take two forms, either Legionnaire's disease, a pneumonia (with a mortality rate of approximately 12%) or a milder flu-like illness Pontiac fever. In the 1990s, before biocidal treatment was introduced, three of the London dental schools reported on legionellae contamination of their DUWLs.² Fortunately, exposed staff at the schools were not shown to have significantly raised antibody levels to legionellae.² Although one dentist was diagnosed with Legionnaire's disease direct occupational exposure could not be proven. By contrast, in mainland Europe, in hospitals with Legionella contaminated DUWLs, dental clinic personnel have exhibited clinically significant Legionella antibody levels at concentration normally associated with legionellosis infection in the recent past. However, no clinical cases of Legionnaire's disease were diagnosed but flu-like Pontiac fever might have gone unrecognised. The authors suggested that long term low level exposure might result in protective antibodies against the more pathogenic L. pneumophila serogroup. In addition, most of the evaluated dental personnel did not fulfil the known risk criteria for Legionnaire's disease and were thereby less likely to develop disease.^2,3

If we turn now to data from dental practices in the UK. We undertook a large study with randomised enrolment of 270 dental practices in greater London and Northern Ireland. Legionellae were only isolated from 0.37% of the sampled DUWLs. Unsurprisingly, therefore, the Legionella antibody detection rate in these dentists did not exceed the background rate for UK blood donors.⁴ Although it should be noted that Legionella were recovered more frequently from the recruited dental practices' hot water supply, elimination of legionellae from the hot water was achieved once the practices raised the hot water running temperature to 60^oC as recommended in HTM 01-05. Although these results offer some reassurance, it should be remembered that Atlas et al.⁵ reported on the death of an American GDP from exposure to Legionella dumoffi found in his DUWLs. Similar to results described in the editor's summary the majority of the 270 practices surveyed demonstrated a DUWLs bacteria in excess of the permitted count of 100-200 cfu/ml. DUWL bacteria comprise mainly environmental species commonly isolated from drinking waters,⁵ but it should not be assumed that these species are necessarily benign. We found that 14% of the surveyed dentists had asthma, a figure considerably higher than the 5% average adult occupational asthma rate. Occupational asthma can be triggered by exposure to aerosolised bacteria and their endotoxins. We found in the subgroup of practitioners who developed asthma since becoming dentists there was a statistically significant association between the development of asthma and exposure to heavily contaminated waterlines in their own practice.⁶

Other bacteria considered to be of clinical significance in DUWLs are pseudomonads and non-tuberculous mycobacterium (NTM). Martin in 1987 reported on the infection of two dental patients with underlying malignancies who developed dental abscesses with the same pseudomonas species isolated from the DUWLs used in their treatment.⁷ Serious NTM infections linked to exposure to NTM contaminated DUWLs have also been reported.⁸ Therefore I would suggest that the risk from contaminated dental unit waterlines both to vulnerable patients and to the dental team, though small, does exist and should not be ignored.