OVERVIEW

What type of bacteria is Legionella?

Legionella, also known as Legionella pneumophila, is an intracellular parasitic bacterium. Therefore, only antimicrobial drugs capable of penetrating cells and maintaining their biological activity can inhibit or kill Legionella pneumophila.

Legionella is widely found in nature and can survive for long periods in soil and water, particularly favoring warm water environments between 30–40°C. Regular disinfection of air conditioning systems and large water supply systems can help prevent infections to some extent.

What is Legionella pneumonia?

Legionella pneumonia, also known as Legionnaires' disease or Legionella pneumonia syndrome, is a lung disease caused by infection with Legionella (Legionella pneumophila). It primarily presents with fever and respiratory symptoms and may also involve damage to multiple organ systems.

With timely and effective antibiotic treatment, most patients can fully recover. However, some high-risk patients may experience disease progression, with a fatality rate ranging from 1% to 10%.

Legionella has now been identified as a relatively common cause of both community-acquired pneumonia and hospital-acquired pneumonia.

Do Legionella pneumonia patients need to be isolated?

Legionella infection is not contagious, so patients with Legionella pneumonia do not require isolation or special precautions in daily interactions.

However, even a single case of Legionella pneumonia can indicate an outbreak, not due to person-to-person transmission but possibly due to shared exposure to Legionella-contaminated water sources.

For this reason, the World Health Organization has included it in the reporting scope for infectious diseases.

Is Legionella pneumonia common? Does it have a seasonal pattern?

Legionella pneumonia can occur year-round, with peaks in summer and autumn. Middle-aged and elderly individuals are more susceptible, and men are affected more frequently than women.

Are Legionella pneumonia and community-acquired pneumonia the same disease?

No, community-acquired pneumonia is caused by various microorganisms such as bacteria, viruses, chlamydia, and mycoplasma outside hospital settings. Among these cases, 1%–10% may be due to Legionella infection.

In terms of treatment, Legionella pneumonia is often resistant to antibiotics such as penicillin, cephalosporins, and aminoglycosides.

SYMPTOMS

What are the common manifestations of Legionella pneumonia?

The incubation period of Legionella pneumonia is 1–2 days. Initially, respiratory symptoms are not obvious. Patients typically experience chills and high fever (oral temperature usually exceeds 39°C), cough (dry or with a small amount of sputum), and gastrointestinal symptoms (diarrhea, vomiting).

Some patients may also develop muscle and joint pain, headache, chest pain, fatigue, ataxia, confusion, cognitive dysfunction, and other symptoms. Some may exhibit a relatively slower heart rate during fever.

How does Legionella pneumonia progress?

After Legionella invades the bronchioles and alveoli through inhalation, it primarily uses structures such as its flagella to adhere to lung tissue cells. It then releases toxins and enzymes, invades lung cells, replicates within them, and ultimately causes lung cell damage or death.

Therefore, the pathogenicity of Legionella is closely related to its flagella.

What complications may Legionella pneumonia cause?

Complications of Legionella pneumonia include respiratory failure, lung abscess, empyema, extrapulmonary infections, and death.

CAUSES

What are the common causes of Legionella pneumonia?

The cause of Legionella pneumonia is infection by Legionella bacteria, and common areas where Legionella can thrive include:

• Artificial water systems, particularly air conditioning systems, respiratory nebulizers, ultrasonic humidifiers, hospital faucets, water heaters, and showerheads, are the most likely places for Legionella to grow. This is why people worry about prolonged air conditioning use causing pneumonia in summer.

• Contaminated water sources.

Which groups are more susceptible to Legionella pneumonia?

Elderly individuals, smokers, those with weakened immune systems, and people with chronic lung diseases are more prone to Legionella pneumonia.

DIAGNOSIS

What is the diagnosis of Legionella pneumonia?

Doctors may suspect Legionella pneumonia when a patient presents with the following conditions:

• Fever exceeding 39°C;

• Gastrointestinal symptoms, particularly diarrhea;

• Neurological manifestations, especially confusion;

• Hematuria;

• Hyponatremia;

• Liver dysfunction;

• Gram staining of respiratory secretions shows numerous neutrophils but few microorganisms;

• No response to treatment with penicillin, cephalosporins, or aminoglycoside antibiotics.

What tests are needed to diagnose Legionella pneumonia?

Since the clinical manifestations of Legionnaires' disease are nonspecific, specialized diagnostic laboratory tests are crucial for diagnosis. When Legionnaires' disease is suspected, both urinary antigen testing and Legionella culture of respiratory samples should be performed.

• Legionella culture is the most important laboratory test. Given that Legionella is a common pathogen in both community-acquired and hospital-acquired pneumonia, all clinical microbiology laboratories should routinely perform this test.

• Urinary antigen testing is rapid, sensitive, specific, and inexpensive, but it is only useful for diagnosing infections caused by Legionella pneumophila serogroup 1 (which accounts for 90% of community-acquired Legionella infections in the U.S.).

• Serological tests are generally of little use in diagnosing individual patients.

• Although PCR-based detection methods exist, their sensitivity has not yet surpassed that of microbial culture.

Which diseases can Legionella pneumonia be easily confused with? How to differentiate them?

Legionella pneumonia can easily be confused with pneumonia caused by other pathogens. Since its clinical symptoms are nonspecific, bacterial culture is necessary for differentiation.

TREATMENT

Which department should I visit for Legionella pneumonia?

Respiratory department or emergency department.

Can Legionella pneumonia heal on its own?

Legionella pneumonia requires antibiotic treatment to recover. Without treatment, the disease may progress and become life-threatening.

How should Legionella pneumonia be treated?

• Treatment depends on the severity of pneumonia, the patient's coexisting conditions, and local bacterial prevalence:

• Empirical treatment usually starts with levofloxacin or azithromycin;

• For confirmed Legionella pneumonia, levofloxacin or azithromycin is preferred; alternatives include ciprofloxacin, moxifloxacin, clarithromycin, roxithromycin, and doxycycline.

• For patients with extrapulmonary infections (e.g., cellulitis, skin abscesses, septic arthritis, or endocarditis), levofloxacin is often the first choice.

• Adjunctive therapies may be needed occasionally, such as corticosteroids for severe cases.

• For severe acute respiratory failure with potential reversibility, extracorporeal membrane oxygenation (ECMO) may improve survival rates.

How long should antibiotic treatment last for Legionella pneumonia?

• Generally, at least 5 days until clinical stability and no fever for ≥48 hours. Severe cases or patients with chronic diseases often require 7–10 days.

• Transplant recipients and immunocompromised patients face higher risks of severe infection or relapse. They should receive levofloxacin or azithromycin for ≥14 days, with adjustments based on response.

Does Legionella pneumonia require hospitalization?

Mild cases can be treated at home with oral antibiotics, but severe cases require hospitalization.

Can Legionella pneumonia be cured?

Most patients recover within 2–5 days after treatment. However, high-risk patients (e.g., elderly, hospital-acquired infections, delayed treatment, immunosuppression, or chronic diseases) may experience progression, with a mortality rate of 1%–10%.

DIET & LIFESTYLE

What should patients with Legionella pneumonia pay attention to in daily life and diet?

• In daily life, regularly disinfect air conditioners, clean bathtubs and shower systems, and avoid using tap water that has been stagnant in pipes for too long.

• Maintain personal hygiene, and keep bathrooms, toilets, and kitchens clean and well-ventilated.

• Engage in appropriate physical exercise to strengthen immunity.

• In terms of diet, consume liquid or semi-liquid foods with sufficient calories, protein, and vitamins to compensate for nutrient loss caused by high fever.

• Encourage patients to drink plenty of water to ensure adequate intake and help thin phlegm.

PREVENTION

Can Legionnaires' Disease Be Prevented? How to Prevent It?

Legionnaires' disease can be prevented to some extent by taking the following measures:

• Control Legionella contamination in water supply systems by conducting regular inspections and disinfection to prevent bacterial growth. Disinfection methods include conventional chlorination, ozone treatment, and ultraviolet disinfection.

• Use running water.

• Maintain clean and well-ventilated living spaces (especially bathrooms, toilets, and kitchens), and regularly inspect and disinfect household appliances such as air conditioners and showerheads.

• Practice good personal hygiene, engage in moderate physical exercise, and strengthen the body's resistance.