| disease | Legionella Pneumonia |
| alias | Legionaires Pneumonia |
Legionnaires' pneumonia is a systemic disease primarily characterized by pneumonia, caused by Legionella pneumophila. In July 1976, during the American Legion convention in Philadelphia, an outbreak of pneumonia of unknown origin occurred, which was named Legionnaires' disease. In January of the following year, a unique bacterium was isolated from the lung tissue of patients who died from the disease, later identified as Legionella pneumophila. Subsequently, cases have been reported in many countries. Currently, outbreaks have occurred in some regions of China, predominantly in autumn and affecting mostly middle-aged and elderly individuals.
bubble_chart Etiology
Legionella pneumophila is a long-chain polymorphic short bacillus, which under electron microscopy exhibits the typical structure of Gram-negative bacilli. It cannot grow on conventional culture media and requires agar media containing L-cysteine ferrous salt, yeast extract, and activated charcoal for growth. The bacterium is found in water and soil and often causes respiratory infections through water supply systems, air conditioning, and aerosol inhalation therapy, mostly in epidemics but also sporadically. The elderly, individuals with chronic diseases, hematologic disorders, malignancies, Acquired Immune Deficiency Syndrome, or those receiving immunosuppressive therapy leading to immunodeficiency are susceptible to this disease. In some cases, it can cause mixed infections with large intestine bacilli, Klebsiella pneumoniae, Pseudomonas aeruginosa, fungi, or Pneumocystis carinii, complicating treatment and increasing mortality.
This disease is a multisystem disorder, with the main pathological changes occurring in the lungs, including acute fibrinous purulent bronchopneumonia, lobar consolidation with small abscess formation, diffuse alveolar damage, hyaline membrane formation, inflammatory cell infiltration in the alveoli, and interstitial mononuclear cell infiltration. The pathogenic bacteria can invade the pleural membrane, lymph nodes, liver, kidneys, brain, and bone marrow via the lymphatic-hematogenous route.
bubble_chart Clinical ManifestationsThe onset is usually gradual, but it can also occur suddenly after an incubation period of 2 to 10 days.
Systemic symptoms include high fever, which may be accompanied by shivering, headache, fatigue, and generalized myalgia. Abdominal pain, vomiting, and diarrhea are common, and the patient may rapidly progress to exhaustion. In severe cases, there may be delayed reactions, confusion, delirium, and other psychiatric or neurological symptoms, as well as signs of peripheral circulatory failure.
Respiratory manifestations include cough, chest pain, and a small amount of sticky sputum, sometimes mixed with blood, but usually without pus. Severe cases may present with dyspnea, cyanosis, or even respiratory failure. Most patients appear acutely ill, often with rales in the lungs. A few may exhibit signs of pulmonary consolidation and pleural friction rubs, and approximately 20% have a relative bradycardia.
[Laboratory and Auxiliary Examinations]
Peripheral blood leukocyte count (10–20×109/L) and neutrophil percentage are elevated, with increased erythrocyte sedimentation rate. Hyponatremia and hypophosphatemia are common, and microscopic hematuria or abnormal liver and kidney function may occur. Chest X-rays typically show unilateral or bilateral scattered alveolar infiltrates or round opacities with blurred edges, often in the lower lobes. These can rapidly progress to segmental or lobar consolidation, sometimes with small abscess formation, and may involve the pleura with pleural effusion.
Special tests include: direct fluorescent antibody staining of sputum, bronchoalveolar lavage fluid, or lung tissue to identify the pathogen; radioimmunoassay or enzyme-linked immunosorbent assay (ELISA) to detect soluble bacterial antigens in urine; indirect fluorescent antibody testing for serum antibodies, which is most commonly used—a fourfold increase in antibody titers between the acute and convalescent stages or a single convalescent-stage titer of ≥1:256 is diagnostic. Isolation of Legionella pneumophila from sputum, pleural fluid, or lung tissue via specialized culture confirms the diagnosis.
bubble_chart Diagnosis
A preliminary clinical diagnosis can be made based on epidemiological data and clinical manifestations, but confirmation requires serological or bacteriological evidence. Legionnaires' disease can be classified into two types based on clinical features: pneumonia type and extrapulmonary type. The former is characterized by significant respiratory symptoms and pulmonary inflammatory changes, while the latter primarily presents with systemic manifestations such as high fever, headache, abdominal pain, vomiting, diarrhea, and neurological symptoms, with mild respiratory symptoms and inconspicuous pulmonary lesions on X-ray. This classification holds certain clinical significance.
bubble_chart Treatment Measures
General treatment includes symptomatic management, supportive therapy, and intensive care similar to other bacterial pneumonias. The preferred antimicrobial therapy is erythromycin, typically 1.2–1.5 g/day, divided into four oral doses for mild cases or administered intravenously in divided doses for more severe cases, continued for 2–3 weeks. Rifampin at 10 mg/kg/day, administered at draught, or doxycycline at 200 mg/day, administered at draught, may be combined. To prevent relapse, the treatment course should extend beyond 3 weeks. Aminoglycosides, penicillin, and cephalosporins are ineffective against this condition.
Most patients can recover after 3-4 weeks of effective treatment with a good prognosis. However, patients with the aforementioned predisposing factors often experience more severe conditions, face greater treatment difficulties, and have a higher mortality rate.
Before obtaining definitive laboratory data, this disease should be differentiated from infectious pneumonia and systemic infections caused by other pathogens.
