List of Legionnaires' disease outbreaks
This is a list of Legionnaires' disease outbreaks; Legionnaire's is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. The first reported outbreak was in Philadelphia, Pennsylvania in 1976 during a Legionnaires Convention at the Bellevue-Stratford Hotel.
An outbreak is defined as two or more cases where the onset of illness is closely linked in time and in space, where there is suspicion of, or evidence of, a common source of infection, with or without microbiological support.
Worldwide listings by year
1970s
1980s
1990s
2000s
2010s
2020s
Governmental controls to prevent outbreaks
Regulations and ordinances
The guidance issued by the UK government's Health and Safety Executive now recommends that microbiological monitoring for wet cooling systems, using a dipslide, should be performed weekly. The guidance now also recommends that routine testing for legionella bacteria in wet cooling systems be carried out at least quarterly, and more frequently when a system is being commissioned, or if the bacteria have been identified on a previous occasion.Further non-statutory UK guidance from the Water Regulations Advisory Scheme now exists for pre-heating of water in applications such as solar water heating systems.
The City of Garland, Texas requires yearly testing for legionella bacteria at cooling towers at apartment buildings.
Malta requires twice yearly testing for Legionella bacteria at cooling towers and water fountains. Malta prohibits the installation of new cooling towers and evaporative condensers at health care facilities and schools.
The Texas Department of State Health Services has provided guidelines for hospitals to detect and prevent the spread of nosocomial infection due to legionella.
The European Working Group for Legionella Infections was established in 1986 within the European Union framework to share knowledge and experience about potential sources of Legionella and their control. This group has published guidelines about the actions to be taken to limit the number of colony forming units of micro-organisms per mL at 30 °C :
| Aerobic count | Legionella | Action required |
| 10,000 or less | 1,000 or less | System under control. |
| more than 10,000 up to 100,000 | more than 1,000 up to 10,000 | Review program operation. The count should be confirmed by immediate re-sampling. If a similar count is found again, a review of the control measures and risk assessment should be carried out to identify any remedial actions. |
| more than 100,000 | more than 10,000 | Implement corrective action. The system should immediately be re-sampled. It should then be 'shot dosed' with an appropriate biocide, as a precaution. The risk assessment and control measures should be reviewed to identify remedial actions. |
Almost all natural water sources contain Legionella and their presence should not be taken as an indication of a problem. The tabled figures are for total aerobic plate count, cfu/ml at 30 °C with colony count determined by the pour plate method according to ISO 6222 or spread plate method on yeast extract agar. Legionella isolation can be conducted using the method developed by the US Center for Disease Control using buffered charcoal yeast extract agar with antibiotics.
Many other governmental agencies, cooling tower manufacturers, and industrial trade organizations have developed design and maintenance guidelines for preventing or controlling the growth of Legionella in cooling towers. However, in the US, there are no regulations requiring testing or maintaining any specified levels in these facilities.
Eliminating breeding grounds
The bacteria grow best in warm water, like the kind found in hot tubs, cooling towers, hot water tanks, large plumbing systems, or parts of the air-conditioning systems of large buildings. Indoor ornamental fountains have been confirmed as a cause of Legionnaires' disease outbreaks, in which submerged lighting as a heat source was attributed to the outbreak in all documented cases. Controlling the growth of Legionella in ornamental fountains is touched on in many of the listed guidelines, especially for solar water heating systems.Adding an antibacterial agent to the automobiles' windshield system's reservoir is also recommended Legionellae have been discovered in up to 40% of freshwater environments and have been in up to 80% of freshwater sites by PCR hybridization assay.
Legionella bacteria themselves can be inactivated by UV light. However, Legionella bacteria that grow and reproduce in amoebae or that are sheltered in corrosion particles cannot be killed by UV light alone.
Legionella will grow in water at temperatures from. However, the bacteria reproduce at the greatest rate in stagnant water at temperatures of.
Copper-Silver ionization is an effective industrial control and prevention process to eradicate Legionella in potable water distribution systems and cooling towers found in health facilities, hotels, nursing homes and most large buildings. In 2003, ionization became the first such hospital disinfection process to have fulfilled a proposed four-step modality evaluation; by then it had been adopted by over 100 hospitals. Additional studies indicate ionization is superior to thermal eradication.
A 2011 study by Lin, Stout and Yu found Copper-Silver ionization to be the only Legionella control technology which has been validated through a 4-step scientific approach.
A recent research study provided evidence that Legionella pneumophila, the causative agent of Legionnaires' disease, can travel airborne at least 6 km from its source. In 2000, ASHRAE issued guidelines to maintain water systems and to decrease the chances of Legionnaires' disease transmission. The guidelines were not valued because legionella multiply in such temperatures. On the other hand, a lot of states had regulations that limited temperatures in health care facilities in order to reduce scalding injuries.
It was previously believed that transmission of the bacterium was restricted to much shorter distances. A team of French scientists reviewed the details of an epidemic of Legionnaires' disease that took place in Pas-de-Calais in northern France in 2003–2004. There were 86 confirmed cases during the outbreak, of whom 18 died. The source of infection was identified as a cooling tower in a petrochemical plant, and an analysis of those affected in the outbreak revealed that some infected people lived as far as 6–7 km from the plant.
A study of Legionnaires' disease cases in May 2005 in Sarpsborg, Norway concluded that: "The high velocity, large drift, and high humidity in the air scrubber may have contributed to the wide spread of Legionella species, probably for >10 km."
In 2010 a study by the UK Health Protection Agency reported that 20% of cases may be caused by infected windscreen washer systems filled with pure water. The finding came after researchers spotted that professional drivers are five times more likely to contract the disease. No cases of infected systems were found whenever a suitable washer fluid was used.
Temperature affects the survival of Legionella as follows:
- : Disinfection range
- At : Legionellae die within 2 minutes
- At : They die within 32 minutes
- At : They die within 5 to 6 hours
- Above : They can survive but do not multiply
- : Ideal growth range
- : Growth range
- Below : They can survive but are dormant
Removing slime, which can carry legionellae when airborne, may be an effective control process.