What is atypical pneumonia?

Atypical pneumonia, sometimes called walking pneumonia, is a bacterial infection of the upper and lower respiratory tract. Some people with walking pneumonia do not even realize they have it, because the symptoms are so mild they can go undetected. They may feel well enough to go about their daily routines. Usually, walking pneumonia does not require bed rest or hospitalization like other forms of pneumonia.

Most of the time, walking pneumonia is caused by tiny bacteria called mycoplasma pneumoniae, which can live and grow in the nose, throat, windpipe (trachea), and lungs. The mycoplasma bacteria do not have rigid cell walls. They can change their size and shape depending on their environment.

The term “atypical” was applied to mycoplasma pneumonia after it was discovered that the bacteria that cause it are resistant to penicillin and some other antimicrobial agents (beta-lactams) that are effective against pneumococcal pneumonia. This has to do with the absence of a rigid cell wall in the mycoplasma bacterium.

There are 3 types of atypical pneumonia. These include:

  • Mycoplasma pneumonia: This type of pneumonia is caused by tiny bacteria called Mycoplasma pneumoniae. It is generally milder than other types, such as pneumococcal pneumonia. Children and adults who are infected often show symptoms resembling those of a cold or flu, such as coughing, sneezing, and a mild fever. Generally, people with mycoplasma pneumonia do not have to be hospitalized. However, sometimes the infection can cause more serious complications that require treatment.
  • Chlamydophila pneumonia: This is a respiratory illness caused by Chlamydophila (or Chlamydia) pneumoniae bacteria. School-age children tend to be at greatest risk for this type.
  • Legionella pneumonia (Legionnaires’ disease): This is a respiratory infection caused by Legionella pneumophila bacteria. Unlike the other forms of atypical pneumonia, it is not spread through person-to-person contact. Legionnaires’ disease tends to be more serious than other types of atypical pneumonia. It can lead to respiratory failure and death in some cases.

How common is atypical pneumonia?

Walking pneumonia is quite common. About 2 million cases of mycoplasma pneumonia occur in the United States each year. Mycoplasma pneumonia accounts for 1 to 10 cases out of 50 cases of community-acquired pneumonia (pneumonia contracted outside of a health care setting).

An estimated 8,000-18,000 patients are hospitalized for Legionnaires’ disease in the United States each year.

What causes atypical pneumonia?

Mycoplasma pneumonia is spread through person-to-person contact when an infected person coughs or sneezes, releasing airborne water droplets containing the bacteria. Mycoplasma bacteria can infect the upper or lower respiratory tract.

The infection can be easily spread in crowded places such as schools, dormitories, and nursing homes. It tends to affect younger adults and school-aged children more than older adults.

Chlamydophila pneumonia is spread through person-to-person contact when an infected person sneezes or coughs. School-age children are at greatest risk of infection. But people of any age can become infected.

Legionella pneumonia (Legionnaire’s disease) has a different route of transmission, since it is not spread through person-to-person contact. Legionella bacteria live in the environment and can grow in warm water found in hot tubs, hot water tanks, fountains, or cooling towers. People may acquire Legionnaire’s disease when they inhale water vapor or mist containing the bacteria. Most healthy people do not develop the disease after exposure to the bacteria.

However, certain groups may be at increased risk of being infected. They include older adults (50 years old or older), current or former smokers, and people with chronic lung conditions (chronic pulmonary obstructive disorder or asthma). People whose immune systems are weakened due to diseases such as diabetes, cancer, or kidney failure, or those taking drugs that suppress the immune system are also at increased risk.

Legionnaires’ disease is also more likely to affect travelers who stay in hotels or aboard cruise ships.

What are the symptoms of atypical pneumonia?

Early symptoms may include:

  • Sore throat (pharyngitis)
  • Tracheobronchitis (inflammation of the trachea and bronchi of the lungs)
  • Low-grade fever
  • Persistent dry cough
  • Headache

The onset of mycoplasma pneumonia may be gradual, with an incubation period of 1 to 4 weeks following exposure. During the later stages of the illness, symptoms may worsen, the fever may become higher, and coughing may yield discolored phlegm (sputum).

Additional symptoms associated with Legionnaires’ disease might include

Symptoms usually occur within 2 weeks after exposure.

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