Legionnaires' disease is caused by the Legionella bacteria mainly exist in the water. Hot water supply systems, the cooling towers of air-conditioning system and steam condensing equipment may contain this bacterium. In fact, it has been isolated from cold water, hot water and shower water, as well as their source streams, ponds and even soil. The bacteria can survive in tap water or distilled water for several months.
Legionnaires' disease isn’t transmitted from human to human. It is mainly caused by inhalation or choking of the mist or water containing Legionella.
Legionnaires' disease belongs to opportunistic infections. Although everyone can be infected, people with a poor immune system constitute a high-risk group, For example, smokers, diabetics, individuals with chronic lung disease, kidney disease or cancer, and people with a compromised immune system, especially those receiving corticosteroid treatments or having had an organ transplantation, are more susceptible to Legionnaires' disease. The severity of the disease tends to increase with age, and most of the patients are over 50 years old.
The incubation period of Legionnaires' disease ranges from 2 to 10 days, and it is usually 5-6 days. After the onset of disease, the patient will first have symptoms such as aversion to food, uncomfortable feeling, muscle pain and headache. Also, the disease will usually progress to high fever within a day (the body temperature usually goes up to 39.0-40.5 °C), accompanied by chills, dry cough, abdominal pain and diarrhea. The patient's chest X-ray will show pulmonary interstitial and may progress to bilateral pneumonia, and this may even lead to respiratory failure. The death rate may be as high as 15%. Among patients with a weakened immune system, the death rate is even higher.
In recent years, the annual number of confirmed Legionnaires’ disease cases is about 40 to110 in Taiwan. Legionnaires' disease epidemic also occurred in hospitals, resorts, hotels and gyms.
The monthly distribution of Legionnaires' disease cases shows that the number of cases peaks in the summer. This pattern is the same as other countries, but Taiwan’s curve isn’t as evident as other countries.
Regarding the age distribution of Legionnaires' disease cases, Taiwan has the same trend with other countries, with the incidence increasing with age. The elders are at a higher risk of Legionnaires' disease, and this will be a main target of disease prevention.
Legionnaires' disease Surveillance in Taiwan
Prevention and Control
The Legionella spp. bacteria often multiply in natural water, groundwater, artificial water, and even in the soil, and do not cause illness to healthy people. Hence the purpose of prevention and control is not to completely wipe out Legionella spp., but to minimize risk. The current concept of Legionnaires’ disease prevention and control in Taiwan is to remove the required growth conditions in water for Legionella spp..
Under favorable conditions, the Legionella spp. in the environment may cause epidemic in communities. First, pathogens must exist in the environment. L. pneumophila is the major pathogen, and other bacterial species of the same genus has relatively lower impact to human health. Then, the Legionella spp. in the environment needs to enter the respiratory tracts of human body through the mist. Finally, when the elderly or immunocompromised susceptible host were exposed, they may be infected and fall ill.
Therefore, in terms of control, special attention should be paid to the high risk environment in a public place where the Legionella spp. can get access to human body, including the locations or facilities prone to mist, e.g., cooling tower, fountain, sprinkler system, spa water storage tank, hot tub, drinking fountain, water outlet as in shower and faucet, or other environmental equipment contaminated by Legionella spp., especially when water temperature is between 35°C to 45°C that suitable for the growth of Legionella spp., is more risky. In addition, the location of obstructed water flow or prone to generate biofilm are also good environment for the multiplication of Legionella spp.. With regard to environmental investigation of an epidemic, the locations of water supply systems where the cases were exposed frequently before their onset may be the likely infection sources that need particular attention. For the aforementioned high risk environment, especially public places that high-risk groups like elderly or immunocompromised people often access such as hospitals or nursing homes, need to be intensively controlled.
In Legionnaires’ disease epidemic investigation, if any suspected or confirmed case had been found within six months at the same place where the current confirmed case might have exposed, it might be a suspected cluster and further control measures should be in placed in accordance with “Guidelines for prevention on suspected Legionnaires' disease cluster events in public places”.