Atypical pneumonia

"Walking pneumonia" redirects here. For rocking pneumonia, see Rockin' Pneumonia and the Boogie Woogie Flu.
Atypical pneumonia
Classification and external resources
Specialty Infectious disease, pulmonology
ICD-10 J18.9
ICD-9-CM 483
DiseasesDB 1132
MedlinePlus 000079

Atypical pneumonia, also known as walking pneumonia, is the type of pneumonia not caused by one of the more traditional pathogens. Its clinical presentation contrasts to that of "typical" pneumonia. A variety of microorganisms can cause it. When it develops independently from another disease it is called primary atypical pneumonia (PAP).

The term was introduced in the 1930s[1][2] and was contrasted with the bacterial pneumonia caused by Streptococcus pneumoniae, at that time the best known and most commonly occurring form of pneumonia. The distinction was historically considered important, as it differentiated those more likely to present with "typical" respiratory symptoms and lobar pneumonia from those more likely to present with "atypical" generalized symptoms (such as fever, headache, sweating and myalgia) and bronchopneumonia.[3]

Distinction between atypical and typical pneumonia is, however, medically insufficient. For the treatment of pneumonia, it is important to know the exact causal organism.


"Primary atypical pneumonia" is called primary because it develops independently of other diseases.
"Atypical pneumonia" is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis).[4] These atypical organisms include special bacteria, viruses, fungi, and protozoa. In addition, this form of pneumonia is atypical in presentation with only moderate amounts of sputum, no consolidation, only small increases in white cell counts, and no alveolar exudate.[5][6] At the time that atypical pneumonia was first described, organisms like Mycoplasma, Chlamydophila, and Legionella were not yet recognized as bacteria and instead considered viruses. Hence "atypical pneumonia" was also called "non-bacterial".[7] In literature the term atypical pneumonia (contrasted with bacterial pneumonia) is still in use, though incorrect. Meanwhile, many such organisms are identified as bacteria, albeit unusual types (Mycoplasma is a type of bacteria without a cell wall and Chlamydias are intracellular parasites). As the conditions caused by these agents have different courses and respond to different treatments, the identification of the specific causative pathogen is important.

Signs and symptoms

Usually the atypical causes also involve atypical symptoms:


The most common causative organisms are (often intracellular living) bacteria:[6]

Chlamydophila pneumoniae
Mild form of pneumonia with relatively mild symptoms.
Chlamydophila psittaci
Causes psittacosis.
Coxiella burnetii
Causes Q fever.
Francisella tularensis
Causes tularemia.
Legionella pneumophila
Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires' disease.
Mycoplasma pneumoniae
[12] Usually occurs in younger age groups and may be associated with neurological and systemic (e.g. rashes) symptoms.

Atypical pneumonia can also have a fungal, protozoan or viral cause.[13][14]
In the past, most organisms were difficult to culture. However, newer techniques aid in the definitive identification of the pathogen, which may lead to more individualized treatment plans.


When comparing the bacterial-caused atypical pneumonias with these caused by real viruses (excluding bacteria that were wrongly considered as viruses), the term "atypical pneumonia" almost always implies a bacterial etiology and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia include respiratory syncytial virus (RSV), influenza A and B, parainfluenza, adenovirus, severe acute respiratory syndrome (SARS)[15] and measles.[5]


Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all.[8] This is called occult pneumonia. In general, occult pneumonia is rather often present in patients with pneumonia and can also be caused by Streptococcus pneumoniae, as the decrease of occult pneumonia after vaccination of children with a pneumococcal vaccine suggests.[16][17]

Infiltration commonly begins in the perihilar region (where the bronchus begins) and spreads in a wedge- or fan-shaped fashion toward the periphery of the lung field. The process most often involves the lower lobe, but may affect any lobe or combination of lobes.[8]


Mycoplasma is found more often in younger than in older people.[18][19] Older people are more often infected by Legionella.[19]


  1. 1 2 Walter C, McCoy MD (1946). "Primary atypical pneumonia: A report of 420 cases with one fatality during twenty-seven month at Station Hospital, Camp Rucker, Alabama". Southern Medical Journal. 39 (9): 696. doi:10.1097/00007611-194609000-00005.
  2. Pneumonia, Atypical Bacterial at eMedicine
  3. Pneumonia, Typical Bacterial at eMedicine
  4. Memish ZA, Ahmed QA, Arabi YM, Shibl AM, Niederman MS (October 2007). "Microbiology of community-acquired pneumonia in the Gulf Corporation Council states". Journal of Chemotherapy. 19 (Suppl 1): 17–23. PMID 18073166.
  5. 1 2 Diseases Database Causes of atypical pneumonia
  6. 1 2 3 Cunha BA (May 2006). "The atypical pneumonias: clinical diagnosis and importance". Clin. Microbiol. Infect. 12 (Suppl 3): 12–24. doi:10.1111/j.1469-0691.2006.01393.x. PMID 16669925.
  7. "Primary atypical pneumonia" at Dorland's Medical Dictionary
  8. 1 2 3 4 Commission on Acute Respiratory Diseases, Fort Bragg, North Carolina (April 1944). "Primary Atypical Pneumonia" (PDF). American Journal of Public Health and the Nations Health. 34 (4): 347–357. doi:10.2105/AJPH.34.4.347.
  9. Gouriet F, Drancourt M, Raoult D (October 2006). "Multiplexed serology in atypical bacterial pneumonia". Ann. N. Y. Acad. Sci. 1078: 530–40. doi:10.1196/annals.1374.104. PMID 17114771.
  10. Hindiyeh M, Carroll KC (June 2000). "Laboratory diagnosis of atypical pneumonia". Semin Respir Infect. 15 (2): 101–13. doi:10.1053/srin.2000.9592. PMID 10983928.
  11. p714, Robbins and Cotran Pathologic Basis of Disease 8th edition, Kumar et al., Philadelphia 2010
  12. MYCOPLASMA PNEUMONIAE at the US National Library of Medicine Medical Subject Headings (MeSH)
  13. Diseases Database
  14. Tang YW (December 2003). "Molecular diagnostics of atypical pneumonia" (PDF). Acta Pharmacol. Sin. 24 (12): 1308–13. PMID 14653964. Archived from the original (PDF) on 2011-07-08.
  15. "Severe Acute Respiratory Syndrome (SARS) — multi-country outbreak". Archived from the original on 7 December 2008. Retrieved 2008-12-21.
  16. Murphy CG, van de Pol AC, Harper MB, Bachur RG (March 2007). "Clinical predictors of occult pneumonia in the febrile child". Acad Emerg Med. 14 (3): 243–9. doi:10.1197/j.aem.2006.08.022. PMID 17242382.
  17. Rutman MS, Bachur R, Harper MB (January 2009). "Radiographic pneumonia in young, highly febrile children with leukocytosis before and after universal conjugate pneumococcal vaccination". Pediatric Emergency Care. 25 (1): 1–7. doi:10.1097/PEC.0b013e318191dab2. PMID 19116501.
  18. Schneeberger PM, Dorigo-Zetsma JW, van der Zee A, van Bon M, van Opstal JL (2004). "Diagnosis of atypical pathogens in patients hospitalized with community-acquired respiratory infection". Scandinavian Journal of Infectious Diseases. 36 (4): 269–73. doi:10.1080/00365540410020127. PMID 15198183.
  19. 1 2 National Heart, Lung, and Blood Institute, U.S.A. What Causes Pneumonia?
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