• legionnaires’ disease

The recent advances in the rapid diagnosis of legionnaire’s disease and its increasing use by healthcare professionals,¹ together with the development of new methods of molecular typing of Legionella pneumophila strains,² are bringing about a whole new picture on the epidemiology of this disease. This not only includes a greater and quicker possibility of detecting outbreaks, but also enables a more specific approach to the investigation of the emission sources under suspicion. The fact that an increasing number of outbreaks are reported yearly,^3,4 and that they seem to yield a growing number of cases, is probably related to this new situation. Moreover, many of the causal relations regarding the sources of some of the earlier investigated outbreaks might not prove to hold any more on the grounds of the new molecular typing methods.

Che and colleagues report in this issue of the journal⁵ an ecological association at the small area level between the presence of industrial systems with aerosol generating devices and the incidence of legionnaires’ disease in France, based on the reporting of sporadic cases. Such results are overall consistent with what is increasingly known about potential sources of Legionella pneumophila, which include water cooled systems in industrial plants. Nevertheless, this finding underscores the need to examine in more depth the implications of the reporting of isolated cases of legionnaires’ disease. More specifically, should the reporting of a sporadic case be enough evidence to purport a more intense epidemiological investigation, as well as specific environmental actions to prevent the occurrence of more cases and perhaps avoid potential outbreaks?

Surveillance systems are generally advised to collect epidemiological data on potential exposures for every reported legionnaires’ disease case, even if sporadic. It is assumed that this information will provide clues to help in the identification of sources of contaminated water potentially involved in the occurrence of the reported case. Moreover, it is expected that the accumulation of such information throughout time can shed additional light on sources of exposure to Legionella pneumophila. Nevertheless, one must bear in mind that in most cases the identification of such sources has been made possible in outbreak episodes, while isolated cases hardly ever allow for the finding of implicated sources, especially when they occur in urban areas, with a high number of cooling towers and other aerosol producing systems. In fact, environmental investigations in response of the reporting of isolated cases often find sources of Legionella pneumophila for which laboratory results indicate that they are not causally related with the reported cases, reinforcing the saying that “whoever looks for Legionella, finds it”. Thus, public health departments tend to regard quite sceptically the real usefulness of the epidemiological investigation of sporadic cases, which may be seen more or less as trying to “find the needle in a haystack”. Besides, the increasing acknowledgement that “Legionella is here to stay” tends to mitigate large scale investigations of sporadic cases, while concentrating on outbreaks, which usually raise ample media and community concerns, and allow for a more feasible use of standard methods of epidemiological investigation.

Still, one could argue that the investigation of isolated cases of legionnaires’ disease can be useful if it helps identifying potentially contaminating sources of Legionella, prompting cleaning and maintance actions by the owners of the systems. Nevertheless, the introduction by an increasing number of countries of strict regulations regarding risk management of cooling towers and other aerosol producing devices, which include registration, periodic sampling and testing, water treatment and occupational and safety systems, somehow questions the usefulness of traditional public health efforts to attain an effective control of the risk of legionnaires’ disease through the epidemiological investigation of every reported case. Instead, it seems advisable that public health and environmental authorities reorient their efforts towards the surveillance of a strict adherence to risk management regulations by owners of water aerosol producing devices.

Although it is still early to evaluate the sustained effectiveness of such strategies, public health departments, rather than spending unfocused efforts investigating sporadic cases of legionnaires’ disease, should instead use this opportunity to reinforce their capabilities in the reporting, investigation and control of large outbreaks, especially as regards the timeliness in the availability of laboratory results, the implementation of effective coordination systems with environmental authorities, as well as the adoption of comprehensive risk communication strategies.

Now that we begin to understand that Legionella can hardly be wiped out of our environment, we need to concentrate on keeping it off. While there is no doubt that good epidemiological surveillance is still needed, especially to reduce the number of undetected or late detected outbreaks, it is time for environmental surveillance activities to take the lead as the key public health measure in the control of legionnaires’ disease.