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Legionellosis – Causes, Symptoms and Treatment

Legionellosis is a bacterial infection caused by Legionella bacteria, primarily affecting the lungs. It is transmitted by inhaling contaminated water droplets or aerosols.

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Things worth knowing about "Legionellosis"

Legionellosis is a bacterial infection caused by Legionella bacteria, primarily affecting the lungs. It is transmitted by inhaling contaminated water droplets or aerosols.

What is Legionellosis?

Legionellosis is a bacterial infectious disease caused by Legionella pneumophila and related Legionella species. It presents in two main forms: the severe Legionnaires disease, which manifests as pneumonia, and the milder Pontiac fever, which causes flu-like symptoms without pneumonia. The disease was first identified in 1976 following an outbreak among attendees of an American Legion convention in Philadelphia.

Causes and Transmission

Legionella bacteria occur naturally in freshwater environments and thrive in artificial water systems when conditions are favorable. The main sources of transmission include:

Air conditioning systems and cooling towers
Hot water systems in hotels, hospitals, and public buildings
Whirlpools and spas
Medical ventilators and nebulizers

Transmission occurs through inhalation of contaminated aerosols or water mist. Direct person-to-person transmission is extremely rare and not considered epidemiologically relevant.

Risk Factors

Not everyone who inhales Legionella bacteria becomes ill. Certain groups are at higher risk:

Older adults (over 50 years of age)
Current and former smokers
People with weakened immune systems (e.g., due to chemotherapy or HIV)
Individuals with chronic lung disease or diabetes
Transplant recipients

Symptoms

Legionnaires disease typically begins 2 to 10 days after infection and presents with:

High fever (above 39 °C / 102 °F) and chills
Cough, often with sputum production
Shortness of breath and chest pain
Headache and muscle aches
Nausea, vomiting, and diarrhea
Confusion and neurological symptoms in severe cases

Pontiac fever is considerably milder, presenting with flu-like symptoms such as fever, fatigue, and muscle pain, without pneumonia developing. It typically resolves within a few days without specific treatment.

Diagnosis

Legionellosis is diagnosed using several methods:

Urinary antigen test: A rapid test that detects Legionella antigens in urine; however, it only identifies L. pneumophila serogroup 1
Culture: Growing Legionella from respiratory secretions or bronchoalveolar lavage fluid (gold standard, but time-consuming)
PCR: Molecular detection of Legionella DNA in respiratory samples
Serology: Detection of antibodies in the blood (only becomes meaningful after several weeks)
Imaging studies such as chest X-ray or CT scan to assess the extent of pneumonia

Treatment

Legionellosis requires treatment with antibiotics. The preferred agents are:

Fluoroquinolones (e.g., levofloxacin, moxifloxacin) or macrolides (e.g., azithromycin, clarithromycin)
In severe cases, combination therapy may be required
Treatment duration is typically 7 to 14 days, and up to 21 days in immunocompromised patients

Severe cases require hospitalization, potentially in an intensive care unit with respiratory support. Without timely treatment, Legionnaires disease can be life-threatening, with a case fatality rate ranging from 5 to 30 % depending on the patient population.

Prevention and Reporting

Preventing Legionella infections relies on regular maintenance and hygiene measures in water systems:

Hot water should be maintained at a minimum of 60 °C (140 °F)
Regular flushing and disinfection of water pipes
Monitoring and maintenance of cooling towers and air conditioning systems

In many countries, including Germany and the United States, legionellosis is a notifiable disease. Laboratories and physicians are required to report confirmed cases to the relevant public health authority.

References

Robert Koch Institut (RKI): Legionellosis - RKI Advisory. Available at: https://www.rki.de (2023)
World Health Organization (WHO): Legionellosis Fact Sheet. Available at: https://www.who.int (2022)
European Centre for Disease Prevention and Control (ECDC): Legionnaires disease in Europe - Annual Epidemiological Report. Stockholm: ECDC, 2022.

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