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Cause

Legionellosis is a bacterial disease caused by Legionella. There are at least 60 different species of Legionella, any of which could be pathogenic. However, the majority of disease is caused by Legionella pneumophila, particularly serogroup 1.

Types

Legionella can cause Legionnaires' disease and Pontiac fever, collectively referred to as legionellosis. More rarely, these bacteria can cause extrapulmonary infections.

Risk factors

Risk factors for legionellosis include:

Age and medical conditions

• Age ≥50 years
• Chronic lung disease (such as emphysema or COPD)
• Immune system disorders due to disease or medication
• Smoking (current or historical)
• Systemic malignancy
• Underlying illness such as diabetes, renal failure, or hepatic failure

Settings or exposures

• Exposure to hot tubs
• Recent care at a healthcare facility
• Recent travel with an overnight stay outside of the home
• Use of respiratory therapy equipment

How it spreads

Sources

Legionella can be found in natural, freshwater environments, but generally isn't present in sufficient numbers to cause disease in that setting. In human-made water systems, Legionella can grow and be transmitted to susceptible people via aerosolization (such as from showers or hot tub jets). Less commonly, people can get sick by aspiration of drinking water containing Legionella.

Some human-made water systems that may pose risk include:

• Large, complex plumbing systems
• Showerheads and sink faucets
• Hot tubs
• Decorative fountains and water features
• Cooling towers

Cooling towers are structures that contain water and a fan as part of centralized air-cooling systems for buildings or industrial processes.

Home and car air-conditioning units don't use water to cool the air, so they aren't a risk for Legionella growth. This includes window units, such as the kind seen at some hotels.

Pathogenesis

In water, Legionella can grow and multiply within amoebae and ciliated protozoa, which are small one-celled organisms. Protozoa benefit Legionella by providing:

• Nutrients for replication
• A shelter from adverse environmental conditions

Adverse conditions can include extreme temperatures and chemicals like chlorine.

Legionnaires' disease

Human immune cells called alveolar macrophages look very similar to protozoa. When in human lungs, Legionella invades and grows within alveolar macrophages, mistaking them for their natural host and causing Legionnaires' disease.

Pontiac fever

It's possible that an inflammatory response to Legionella endotoxin, rather than true infection, results in Pontiac fever.

Clinical features

Legionnaires' disease and Pontiac fever present differently in terms of clinical features, symptoms, and complications.

Prevention

Reduce risk of Legionella

Minimizing Legionella growth in complex building water systems and devices is key to preventing infection.

obÌåÓý recommends that healthcare facilities develop and implement comprehensive water management programs (WMPs). There are some considerations that healthcare facility staff should be aware of when it comes to using these programs.

Report cases

Timely identification and reporting of legionellosis cases are important for allowing public health officials to act quickly. This way they can identify and stop potential clusters and outbreaks by linking new cases to previously reported ones which may help in identifying the source of exposure.

Disease trends

In general, reported cases of Legionnaires' disease have been increasing since the early 2000s.

Case definitions

The Council of State and Territorial Epidemiologists released the most recent case definition for legionellosis in 2020.

Resources

Fact sheet: What clinicians need to know about Legionnaires' disease