Legionellosis

Overview

Legionellosis refers to the infection caused by Legionella bacteria which can occur in the pneumonic and non-pneumonic forms. The pneumonic form (Legionnaires’ disease) can range from a mild cough to fatal pneumonia while the non-pneumonic form (Pontiac disease) is typically a self-limiting influenza-like illness.

Disease epidemiology

Legionella pneumophila, the most common cause of cases, are environmental organisms found in water. It can be found in natural freshwater environments (e.g.  lakes, rivers, creeks and hot springs) or in man-made aquatic reservoirs (e.g. cooling towers, water storage tanks, hot water systems, spas, evaporative condensers, decorative fountains and other water-features). Other species, including Legionella longbeachae, can be found in potting mixes.

Legionnaires’ disease outbreaks have occurred worldwide, with single and sporadic infections being reported in Singapore. Seroprevalence studies suggest that asymptomatic or subclinical infection can also occur.

Pathogen(s)

There are at least 60 species of Legionella bacteria, most of which are pathogenic. Legionella pneumophila serogroup 1 accounts for most human infections.

Transmission

Legionellosis is transmitted via the inhalation of contaminated aerosols, and aspiration of contaminated water.

Incubation period:

• Legionnaires’ disease: Typically 5 to 6 days; range is 2 to 16 days

• Pontiac fever: Typically 24 to 48 hours; range is 5 to 72 hours

Infectious period:

• No direct human-to-human transmission

Clinical features

There are 2 distinct clinical manifestations of legionellosis:

• Legionnaires’ disease: Characterised by pneumonia and a dry cough. Clinical presentation is similar to other types of pneumonia. Severity of the disease is variable and ranges from mild to severe.

• Pontiac fever: A self-limiting flu-like illness that does not progress to pneumonia. Predominant symptoms are malaise, myalgia, fever, chills and headache. Chest X-ray remains clear. Only symptomatic treatment is required. Recovery occurs within one week.

Risk factors

Risk factors include:

• Increasing age (most cases aged above 50 years)

• Cigarette smoking

• Diabetes mellitus

• Chronic lung disease

• Renal disease

• Malignancy

• Compromised immunity, particularly organ transplant recipients and patients receiving corticosteroids

Diagnosis

Legionellosis is diagnosed when laboratory tests detect Legionella bacteria in respiratory samples via culture, immunofluorescence or polymerase chain reaction, in serum via Legionella serology or in urine via Legionella antigen.

Treatment and management

Legionnaires’ disease: Antibiotics are recommended for treatment. Treatment should be started in a timely manner as delay is associated with increased mortality. Severe cases will require hospitalisation.

Pontiac fever: Mild and self-limiting; does not require antibiotic therapy.

Precaution, prevention, and control

Standard precautions apply in healthcare settings.

Epidemiological investigations should be conducted to identify the source of infection and the mode of transmission. This includes sampling of water and sludge from cooling towers of air-conditioned premises for the isolation of Legionella.

There should be regular mechanical cleaning of the cooling towers and routine treatment with biocides to inhibit organic growth within the air-conditioning system. A corrosion and scale inhibitor should be used. Routine environmental monitoring for Legionella should be performed in hospitals, given the risk for Legionnaires’ disease.

Notification

Legionellosis is a notifiable disease under the Infectious Diseases Act.

Who should notify:

• Laboratories

When to notify:

• On laboratory confirmation

How to notify:

• Please refer to the Infectious Disease Notification for more information.

Notification timeline:

• Within 72 hours

Resources

Please refer to the Weekly Infectious Diseases Bulletin for the numbers of confirmed Legionellosis cases in Singapore.

For more information on Legionellosis, please refer to the World Health Organization and Centres for Disease Control and Prevention websites. 

For general travel advisory, please refer to Health Advice for Travellers.