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Forekomst af Legionella i varmvandssystem

Summary and conclusions

A total of 35 samples from hot water outlets (taps and showers) were collected from 22 buildings. Analysis for viable counts of Legionella showed Legionella pneumophila in 29 samples from 19 buildings. The concentration of Legionella in the positive samples ranged from 10¹ cfu/litre (the detection limit) to ³ 4.9 x 10⁶ cfu/litre. In 10 of 22 hot water systems concentrations of Legionella were 10³ cfu/litre or less. In three buildings Legionella was not detected in samples from the hot water systems and for two of these buildings the negative result was confirmed by repeated sampling. For 8 of the hot water systems the concentrations of Legionella were 1 x 10⁴ cfu/litre or more.

PCR analysis showed Legionella pneumophila in 21 of 22 systems. The results of the positive samples ranged from £ 10³ genomes/litre to ³ 10⁶ genomes/litre. The system with the negative PCR result was also negative by culture (viable counts). Samples from the two other installations that were found negative when cultured were found positive by the PCR analysis (³ 10⁴ genome/litre and 10³ – 10⁴ genomes/litre respectively). The PCR analysis also detects dead and viable non-culturable organisms. Consequently, the two methods – culture and PCR – cannot be expected to give completely similar results, nor is it possible to make a quantitative comparison as PCR is to be regarded as a semi-quantitative method. The culture method is currently recognised as the standard method for detection of Legionella.

Within the interval 39° C - 51° C this study demonstrated a significant correlation for hot water systems in 13 blocks of flats between the concentration of Legionella in water samples collected from outlets (immediately after being switched on) and the temperature measured at the same outlet when a stable temperature had been reached. No significant correlations were found between the concentration of Legionella at outlets and temperatures measured in hot water leaving the hot water tank or measured in circulation hot water returning to the tank. These observations indicate that conditions for growth exist in the pipework and that the temperature in the hot water tank is not the only factor of importance.

The study confirms the presence of moderate to high concentrations of Legionella pneuomophila in hot water systems (³ 5 x 10³ cfu/litre in 11 systems). A simple correlation between the level of the concentration of Legionella in hot water and the risk of infection has never been demonstrated. High concentrations may not necessarily cause disease whereas in other situations infections have occurred from sources from which water samples contained apparantly low concentrations of Legionella .This is probably due to the mode of transmission and to the facts that healthy people are less prone to infection and different strains of Legionella have different virulence. To our knowledge infections have not occurred in relation to hot water systems with concentrations below 10³ cfu/litre. On the other hand, concentrations in this range demonstrate that conditions for growth are present, and minor changes in the management of the installation may lead to high concentrations.

The serogroups most frequently isolated from community acquired cases of Legionnaires’ disease in Denmark are groups 1, 3, 4, and 6 (serogroups 1 and 3 accounting for 80% of all cases). These serogroups were all represented in the water samples from the hot water systems sampled in this study.

The findings of this study support the hypothesis, that ordinary hot water systems may be a source of community acquired Legionnaires' disease in the Danish population.