Legionnaires Disease

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Diagnosis and reasoning

This patient has presented with fever, cough and dyspnea, while examination reveals crackles in the both lower zones; i.e. this appears to be a community acquired pneumonia (CAP). The first priority should be assessment, resuscitation and stabilization; in this respect, note the presence of marked dyspnea in association with a low oxygen saturation. Supplemental oxygen should be commenced and monitoring started. Following stabilization, the next step should be a detailed history and examination, with the aim of determining whether the underlying agent is typical or atypical in origin. In this regard, note that the cough is non productive; note also the presence of marked extrapulmonary manifestations; this is much more in favor of an atypical agent. Atypical pneumonias are caused by a specific group of organisms: Chlamydia psittaci, Francisella tularensis, and Coxiella burnetii, which are zoonotic pathogens; and Chlamydia pneumoniae, Mycoplasma pneumoniae and Legionella pneumophila, which are non-zoonotic. One interesting characteristic of the above is that each different agents tends to manifest different patterns of clinical symptoms; thus, it is frequently possible to arrive at a probable etiological diagnosis via analysis of clinical findings alone. In this respect, note the prominent gastrointestinal symptoms such as abdominal pain and diarrhea, as well as the presence of relative bradycardia, despite the high fever. These features are most suggestive of Legionnaires disease. However, note that this patient does not have a history suggestive of exposure to Legionella (for example, inhalation of droplets from a hot tub or air-conditioning unit in a hotel), although this does not exclude this diagnosis. His chest x-ray reveals infiltrates in both lower zones - confirming the presence of a pneumonia, while the full blood count shows a significant leukocytosis, which is non-specific. However, note the presence of hyponatremia and hypophosphatemia in the electrolyte assay; while hyponatremia can occur in almost any pneumonia (due to the syndrome of inappropriate antidiuretic hormone secretion - SIADH), hypophosphatemia is a less common finding which is more specific to legionellosis. Urinary antigen testing (UAT) for Legionella is sensitive, specific, non-invasive, and rapid; it is thus a good initial diagnostic test. The positive result seen here clinches the diagnosis. While bronchoalveolar lavage (BAL) is capable of providing a microbiological diagnosis, it is far more invasive; given the high accuracy of UAT, it is probably not indicated right now. However, if his symptoms do not improve, or an alternate diagnosis is suspected, BAL might be required down the line. Fluoroquinolones are the drugs of choice in patients with Legionnaire's disease; thus, Levofloxacin is a good choice in this patient. Cefoperazone is a third generation cephalosporin; while active against most typical organisms causing pneumonia, it has limited efficacy against Legionella. The Infectious Diseases Society of America (IDSA) guidelines on the management of CAP in adults define a set of criteria which should be fulfilled in order to justify ICU admission; this patient does not meet these criteria, and thus does not require ICU admission.


Discussion

Legionnaires' disease (Legionella pneumonia) accounts for between 1% to 15% of all community acquired pneumonias (CAP) worldwide. The causative organism is Legionella pneumophila, an obligate aerobic, poorly gram-staining organism. Legionella proliferates in stagnant water, from whence it is transmitted to humans via inhaled aerosol particles or droplets, such as those produced by air conditioners, spas, nebulizers, humidifiers and water fountains. Advanced age, chronic respiratory disease, smoking and an immunocompromised state are known risk factors for the development of the disease; it is a well known cause of severe CAP. Similar to other atypical pneumonias, patients typically manifest a non-productive cough; arthralgia and myalgia may also be prominent. And as with other atypical agents, Legionella also tends to give rise to a specific pattern of extrapulmonary manifestations; most typically, gastrointestinal symptoms such as abdominal pain and watery diarrhea, CNS symptoms such as confusion and headache, and relative bradycardia. Considering basic investigations, the full blood count (FBC) typically shows a leukocytosis, although lymphopenia may sometimes be seen; minor disturbances in the renal and hepatic profile, and electrolyte disturbances such as hyponatremia and hypophosphatemia are also frequently present. Chest radiographs generally fail to demonstrate specific features; note however that cavitation and abscess formation are rare in Legionella infection, and should prompt consideration of an alternate etiology. The specific diagnosis of Legionnaires' disease depends upon culture of specimens, urinary antigen testing (UAT) and detection of antibodies. Culture of sputum, pleural fluid, or bronchoalveolar lavage specimens requires special media and takes 3 to 5 days for results to become available; this is as opposed to UAT, which is also highly sensitive and specific, with results usually obtainable in 24 hours. The direct fluorescent antibody test also gives rapid results, although availability is often limited. Note that the Infectious Disease Division of the Winthrop-University Hospital has devised a scoring system for the diagnosis of Legionella pneumonia; this includes clinical features as well as laboratory findings. Individuals with a score of 15 or more are classified as falling into the category of 'Legionnaires' disease very likely'. The management of Legionella pneumonia depends on early identification and initiation of specific antibiotic therapy as well as supportive treatment according to the severity of the illness. Fluoroquinolones such as Levofloxacin or Moxifloxacin are recommended as first-line agents; new generation macrolides such as azithromycin, and doxycycline are also effective. Note that current guidelines recommend antibiotic therapy for a duration of 2 weeks. Supportive care may include antipyretics, analgesics, oral or intravenous fluid resuscitation and in severe cases, ICU care. The prognosis of Legionella pneumonia is largely dependant on the health status of the individual; persons with cardiopulmonary disease and a poor immune status generally have a worse prognosis. In addition, early initiation of antibiotic therapy may significantly improve the ultimate prognosis.


Take home messages

  1. The presence of extrapulmonary features in a patient with pneumonia strongly suggests infection with an atypical pathogen.
  2. Legionnaires disease is a well known cause of severe community acquired pneumonia.
  3. Fluoroquinolones, new generation macrolides and doxycycline are effective in the treatment of Legionella pneumonia.

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