Atypical pneumonia refers to pneumonia caused by certain bacteria -- namely, Legionella pneumophila, Mycoplasma pneumoniae, and Chlamydophila pneumoniae.

While atypical pneumonias are commonly associated with milder forms of pneumonia, pneumonia due to Legionella, in particular, can be quite severe and lead to high mortality rates.

Atypical pneumonia due to Mycoplasma and Chlamydophila usually cause milder forms of pneumonia and are characterized by a more drawn out course of symptoms unlike other forms of pneumonia which can come on more quickly with more severe early symptoms.

Mycoplasma pneumonia often affects younger people and may be associated with symptoms outside of the lungs (such as anemia and rashes), and neurological syndromes (such as meningitis, myelitis, and encephalitis). Severe forms of Mycoplasma pneumonia have been described in all age groups.

Chlamydophila pneumonia occurs year round and accounts for 5-15% of all pneumonias. It is usually mild with a low mortality rate. In contrast, atypical pneumonia due to Legionella accounts for 2-6% of pneumonias and has a higher mortality rate.

Elderly individuals, smokers, and people with chronic illnesses and weakened immune systems are at higher risk for this type of pneumonia. Contact with contaminated aerosol systems (like infected air conditioning systems) has also been associated with pneumonia due to Legionella.

• Chills
• Fevers
• Cough
• Headache
• Muscle stiffness and aching
• Rapid breathing
• Shortness of breath
• Loss of appetite
• Malaise
• Confusion (especially with Legionella)
• Rash (especially with Mycoplasma)
• Diarrhea (especially with Legionella)

People with suspected pneumonia should undergo a medical evaluation, including a thorough physical exam and a chest x-ray -- especially since the physical exam may not always distinguish pneumonia from acute bronchitis or other respiratory infections.

Depending on the severity of illness, additional studies, such as a complete blood count, blood cultures, and sputum cultures, may be obtained.

When certain forms of atypical pneumonia are suspected, tests of your urine or a throat swab may be ordered as well.

The mainstay of treatment for atypical pneumonia is antibiotic therapy. In mild cases, treatment with oral antibiotics at home may be appropriate. Severe cases (especially common with pneumonia caused by Legionella) may require intravenous antibiotics and oxygen supplementation.

Antibiotics with activity against these organisms include -- erythromycin, azithromycin, clarithromycin, fluoroquinolones and their derivatives (such as levofloxacin), and tetracyclines (such as doxycycline).

Most patients with pneumonia due to Mycoplasma or Chlamydophila do well with appropriate antibiotic therapy, although there is a small chance of relapse if antibiotics are used for too short a period of time (less than two weeks).

In the case of pneumonia due to Legionella, severe illness occurs particularly among the elderly and those with chronic diseases and weakened immune systems. It is associated with a higher mortality rate.

• Respiratory failure, mechanical ventilation -- especially in severe forms of pneumonia due to Legionella
• Rash and hemolytic anemia -- especially associated with pneumonia due to Mycoplasma

Seek medical evaluation if you develop fevers, cough, and/or shortness of breath. While there are numerous causes for these symptoms, you will need to be evaluated for pneumonia.

There are no proven methods for preventing atypical pneumonia, and no vaccinations are available at this time for atypical pneumonias.

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Mandell LA, Wunderink RG, Anzueto A, et al. Infectious Diseases Society of America/American Thoracic Society consensus guidelines on the management of community-acquired pneumonia in adults. Clin Infect Dis. 2007 Mar 1;44 Suppl 2:S27-72.

American Thoracic Society. Guidelines for the management of adults with hospital-acquired, ventilator-associated, and healthcare-associated pneumonia. Am J Respir Crit Care Med. 2005 Feb 15;171(4):388-416.