Legionnaires’ Disease: A Clinical Overview

Legionella bacteria cause Legionnaires' disease and Pontiac fever, together called legionellosis; more rarely they cause infections outside the lungs. There are at least 60 species, any of which could cause disease, but most illness comes from Legionella pneumophila, especially serogroup 1.

Who's at risk

Age and health

• age 50 or older
• chronic lung disease, such as emphysema or COPD
• weakened immune system from disease or medicine
• smoking, now or in the past
• systemic cancer
• underlying illness such as diabetes, kidney failure or liver failure

Settings and exposures

• hot tubs
• recent care at a health care facility
• recent travel with an overnight stay away from home
• use of respiratory therapy equipment

Where it comes from

Legionella lives naturally in fresh water, but usually not in numbers large enough to cause disease there. In human-made water systems it can multiply and spread to susceptible people in aerosols — mist from showers or hot-tub jets. Less often, people get sick by aspirating drinking water that contains it.

Systems that can pose a risk:

• large, complex plumbing systems
• showerheads and faucets
• hot tubs
• decorative fountains and water features
• cooling towers — structures holding water and a fan, part of centralized air cooling for buildings or industry

Home and car air conditioners — including hotel window units — don't use water to cool the air, so they aren't a risk.

How it causes disease

In water, Legionella grows inside amoebae and ciliated protozoa, single-celled organisms that give it nutrients and shelter from extreme temperatures and chemicals such as chlorine.

• Legionnaires' disease: in the lungs, human immune cells called alveolar macrophages look much like protozoa. Legionella invades and grows inside them, mistaking them for its natural host.
• Pontiac fever may result from an inflammatory reaction to Legionella endotoxin rather than true infection.

The two illnesses differ in their clinical features, symptoms and complications.

Diagnosis and reporting

• CDC recommends a Legionella urinary antigen test paired with lower respiratory testing by culture or molecular methods.
• Report cases promptly to state, territorial or local health departments, so officials can link cases, spot clusters and outbreaks and find the source.
• Reported Legionnaires' disease cases have generally been rising since the early 2000s. The Council of State and Territorial Epidemiologists issued its latest case definition in 2020.

Prevention

The key is limiting Legionella growth in complex building water systems and devices. CDC recommends that health care facilities build and carry out comprehensive water management programs.

Sources

Based on "Clinical Overview of Legionnaires' Disease," Centers for Disease Control and Prevention; a work of the United States government in the public domain.