primary atypical pneumonia

Atypical pneumonia is a pneumonia not caused by one of the more traditional pathogens. It can be caused by a variety of microorganisms. When developed 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 occurring form of pneumonia.

Distinction between atypical and typical pneumonia, however, is medically insufficient. For the treatment of pneumonia it is important to know the exact causing organism. Moreover, S. pneumoniae has become a relatively lesser important cause.

Terminology

"Primary atypical pneumonia" is called primary to indicate, that it developed independently, not following another disease.
"Atypical pneumonia" is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis). ^[3] These atypical organisms include special bacteria, viruses, fungi and protozoa. ^{[4] [5]} At the time that atypical pneumonia was described first, organisms like Mycoplasma, Chlamydophila and Legionella still were not recognized as bacteria and instead considered as viruses. Hence "atypical pneumonia" was also called "non-bacterial". ^[6] In literature the term bacterial pneumonia contrasted with atypical pneumonia is, although actually incorrect, still in use. Meanwhile, many of 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.

Atypical symptoms

Usually the atypical causes also involve atypical symptoms:

• No responding on common antibiotics as sulfonamide^[7] and beta-lactams like penicillin.
• No signs and symptoms of lobar consolidation^[8][9], meaning that the infection is restricted to small areas, rather than involving a whole lobe. As the disease progresses, however, the look can tend to lobar pneumonia.
• Absence of leukocytosis.
• Extrapulmonary symptoms, related to the causing organism.^[5]
• Moderate amount of sputum.
• Lack of alveolar exudate.^[10]
• Despite general symptoms and problems with the upper respiratory tract like high fever, headache, a dry irritating cough, later a productive cough and radiographs, showing consolidation, there are in general few physical signs. The patient looks worse than the symptoms suggest.^[1][7]

Radiography

Chest radiographs (X-ray photographs) often show a pulmonary affection before physical signs of atypical pneumonia are observable at all.^[7] This is called occult pneumonia. In general, occult pneumonia is rather oft 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. ^{[11] [12]}

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. ^[7]

Occurrence

Mycoplasma is found more often in younger than in elder people. ^{[13] [14]} Elder people are more often infected by Legionella.^[14]

Causes of atypical pneumonia

Pneumonia
Infectious pneumonias
Bacterial pneumonia Viral pneumonia Fungal pneumonia Parasitic pneumonia Atypical pneumonia Community-acquired pneumonia Healthcare-associated pneumonia Hospital-acquired pneumonia Ventilator-associated pneumonia Severe acute respiratory syndrome
Pneumonias caused by infectious or noninfectious agents
Aspiration pneumonia Lipid pneumonia Eosinophilic pneumonia Bronchiolitis obliterans organizing pneumonia
Noninfectious pneumonia
Chemical pneumonia
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The most common causative organisms are (often intracellular living) bacteria: ^[15]

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^[16]

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.^{[17] [18]}
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.

Viral

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^{[citation needed]} and is contrasted with viral pneumonia.

Known viral causes of atypical pneumonia are severe acute respiratory syndrome (SARS) ^[19] and measles. ^[4]

External links

• Phyllis C. Braun, PhD, and John D. Zoidis, MD Atypical Pneumonia September 2004
• Norman L. Cressy, M.D. The Pneumonias (From: Preventive Medicine in World War II Series, Volume IV: Communicable Diseases: Transmitted Chiefly through Respiratory and Alimentary Tracts, Chapter XII)
• Atypical pneumonia in the Nordic countries: aetiology and clinical results of a trial comparing fleroxacin and doxycycline. pmid 9145823; journal=J. Antimicrob. Chemother. , volume=39, pages=499–508 year=1997
• The difference between M. pneumoniae and S. pneumoniae on high-resolution-CT BMC Med Imaging. 2009

References