Atypical pneumonia
Encyclopedia
Atypical pneumonia aka "walking pneumonia" is a pneumonia
not caused by one of the more traditional pathogens, and with a clinical presentation inconsistent with typical pneumonia. It can be caused by a variety of microorganism
s. When developed independently from another disease it is called Primary Atypical Pneumonia (PAP).
The term was introduced in the 1930s
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
and myalgia
) and bronchopneumonia
.
Distinction between atypical and typical pneumonia, however, is medically insufficient. For the treatment of pneumonia it is important to know the exact causal organism. Moreover, S. pneumoniae has become a relatively lesser important cause.
"Atypical pneumonia" is atypical in that it is caused by atypical organisms (other than Streptococcus pneumoniae
, Haemophilus influenzae
and Moraxella catarrhalis
).
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, nil consolidation, only small increases in white cell counts and no alveolar exudate.
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".
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.
Chlamydia pneumoniae: Mild form of pneumonia with relatively mild symptoms.
Chlamydia 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
: 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.
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.
.
Known viral causes of atypical pneumonia include Respiratory Syncytial Virus
(RSV), Influenza
A and B, Parainfluenza, Adenovirus, severe acute respiratory syndrome
(SARS)
and measles
.
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.
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.
Older people are more often infected by Legionella.
Pneumonia
Pneumonia is an inflammatory condition of the lung—especially affecting the microscopic air sacs —associated with fever, chest symptoms, and a lack of air space on a chest X-ray. Pneumonia is typically caused by an infection but there are a number of other causes...
not caused by one of the more traditional pathogens, and with a clinical presentation inconsistent with typical pneumonia. It can be caused by a variety of microorganism
Microorganism
A microorganism or microbe is a microscopic organism that comprises either a single cell , cell clusters, or no cell at all...
s. When developed independently from another disease it is called Primary Atypical Pneumonia (PAP).
The term was introduced in the 1930s
and was contrasted with the bacterial pneumonia
Bacterial pneumonia
Bacterial pneumonia is a type of pneumonia caused by bacterial infection.-Sign and symptoms:*Fever*Rigors*Cough*Dyspnea*Chest pain*Pneumococcal pneumonia can cause Hemoptysis-Gram positive:...
caused by Streptococcus pneumoniae
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...
, 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
Lobar pneumonia
Lobar pneumonia is a form of pneumonia that affects a large and continuous area of the lobe of a lung.It is one of the two anatomic classifications of pneumonia .- Symptoms :...
from those more likely to present with "atypical" generalized symptoms (such as fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...
, headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
and myalgia
Myalgia
Myalgia means "muscle pain" and is a symptom of many diseases and disorders. The most common causes are the overuse or over-stretching of a muscle or group of muscles. Myalgia without a traumatic history is often due to viral infections...
) and bronchopneumonia
Bronchopneumonia
Bronchopneumonia or bronchial pneumonia or "Bronchogenic pneumonia" is the acute inflammation of the walls of the bronchioles...
.
Distinction between atypical and typical pneumonia, however, is medically insufficient. For the treatment of pneumonia it is important to know the exact causal 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
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...
, Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...
and Moraxella catarrhalis
Moraxella catarrhalis
Moraxella catarrhalis is a fastidious, nonmotile, Gram-negative, aerobic, oxidase-positive diplococcus that can cause infections of the respiratory system, middle ear, eye, central nervous system and joints of humans.-History:...
).
These atypical organisms include special bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...
, viruses, fungi and protozoa
Protozoa
Protozoa are a diverse group of single-cells eukaryotic organisms, many of which are motile. Throughout history, protozoa have been defined as single-cell protists with animal-like behavior, e.g., movement...
. In addition, this form of pneumonia is atypical in presentation, with only moderate amounts of sputum, nil consolidation, only small increases in white cell counts and no alveolar exudate.
At the time that atypical pneumonia was described first, organisms like Mycoplasma
Mycoplasma
Mycoplasma refers to a genus of bacteria that lack a cell wall. Without a cell wall, they are unaffected by many common antibiotics such as penicillin or other beta-lactam antibiotics that target cell wall synthesis. They can be parasitic or saprotrophic. Several species are pathogenic in humans,...
, Chlamydophila
Chlamydophila
Chlamydophila is a bacterial genus belonging to the family Chlamydiaceae, order Chlamydiales, class/phylum Chlamydiae.-Taxonomy:Chlamydophila was recognized in 1999, with six species in Chlamydophila and three in the original genus, Chlamydia...
and Legionella
Legionella
Legionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....
still were not recognized as bacteria and instead considered as viruses. Hence "atypical pneumonia" was also called "non-bacterial".
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
Cell wall
The cell wall is the tough, usually flexible but sometimes fairly rigid layer that surrounds some types of cells. It is located outside the cell membrane and provides these cells with structural support and protection, and also acts as a filtering mechanism. A major function of the cell wall is to...
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:- No responding on common antibiotics as sulfonamideSulfonamide (medicine)Sulfonamide or sulphonamide is the basis of several groups of drugs. The original antibacterial sulfonamides are synthetic antimicrobial agents that contain the sulfonamide group. Some sulfonamides are also devoid of antibacterial activity, e.g., the anticonvulsant sultiame...
and beta-lactamsBeta-lactam antibioticβ-Lactam antibiotics are a broad class of antibiotics, consisting of all antibiotic agents that contains a β-lactam nucleus in its molecular structure. This includes penicillin derivatives , cephalosporins , monobactams, and carbapenems...
like penicillinPenicillinPenicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....
. - No signs and symptoms of lobar consolidation, 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 pneumoniaLobar pneumoniaLobar pneumonia is a form of pneumonia that affects a large and continuous area of the lobe of a lung.It is one of the two anatomic classifications of pneumonia .- Symptoms :...
. - Absence of leukocytosisLeukocytosisLeukocytosis is a raised white blood cell count above the normal range in the blood. It is frequently a sign of an inflammatory response, most commonly the result of infection, and is observed in certain parasitic infections...
. - Extrapulmonary symptoms, related to the causing organism.
- Moderate amount of sputum, or no sputum at all (i.e. non-productive).
- Lack of alveolar exudate.
- Despite general symptomSymptomA symptom is a departure from normal function or feeling which is noticed by a patient, indicating the presence of disease or abnormality...
s and problems with the upper respiratory tractUpper respiratory tractThe upper respiratory tract or upper airway primarily refers to the parts of the respiratory system lying outside of the thorax or above the sternal angle. Another definition commomly used in medicine is the airway above the glottis or vocal cords...
like high fever, headache, a dry irritating cough, later a productive cough and radiographs, showing consolidationConsolidation (medicine)Consolidation is a clinical term for solidification into a firm, dense mass. It is more specifically used in reference to a region of lung tissue that, normally compressible, has filled with liquid, a condition marked by induration of a normally aerated lung...
, there are in general few physical signs. The patient looks better than the symptoms suggest.
Cause
The most common causative organisms are (often intracellular living) bacteria:Chlamydia pneumoniae: Mild form of pneumonia with relatively mild symptoms.
Chlamydia psittaci: Causes psittacosis
Psittacosis
In medicine , psittacosis — also known as parrot disease, parrot fever, and ornithosis — is a zoonotic infectious disease caused by a bacterium called Chlamydophila psittaci and contracted from parrots, such as macaws, cockatiels and budgerigars, and pigeons, sparrows, ducks, hens, gulls and many...
.
Coxiella burnetii
Coxiella burnetii
Coxiella burnetii is an obligate intracellular bacterial pathogen, and is the causative agent of Q fever. The genus Coxiella is morphologically similar to Rickettsia, but with a variety of genetic and physiological differences. C...
: Causes Q fever
Q fever
Q fever is a disease caused by infection with Coxiella burnetii, a bacterium that affects humans and other animals. This organism is uncommon but may be found in cattle, sheep, goats and other domestic mammals, including cats and dogs...
.
Francisella tularensis
Francisella tularensis
Francisella tularensis is a pathogenic species of gram-negative bacteria and the causative agent of tularemia or rabbit fever. It is a facultative intracellular bacterium....
: Causes tularemia
Tularemia
Tularemia is a serious infectious disease caused by the bacterium Francisella tularensis. A Gram-negative, nonmotile coccobacillus, the bacterium has several subspecies with varying degrees of virulence. The most important of those is F...
.
Legionella pneumophila
Legionella pneumophila
Legionella pneumophila is a thin, ærobic, pleomorphic, flagellated, non-spore forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis or Legionnaires' disease.-Characterization:L...
: Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis
Legionellosis
Legionellosis is a potentially fatal infectious disease caused by gram negative, aerobic bacteria belonging to the genus Legionella. Over 90% of legionellosis cases are caused by Legionella pneumophila, a ubiquitous aquatic organism that thrives in temperatures between , with an optimum temperature...
or Legionnaires' disease.
Mycoplasma pneumoniae
Mycoplasma pneumoniae
Mycoplasma pneumoniae is a very small bacterium in the class Mollicutes.It causes the disease mycoplasma pneumonia, a form of atypical bacterial pneumonia, and is related to cold agglutinin disease.-Cell wall/Treatment:...
: 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.
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 and is contrasted with viral pneumoniaViral pneumonia
Viral pneumonia is a pneumonia caused by a virus.Viruses are one of the two major causes of pneumonia, the other being bacteria; less common causes are fungi and parasites...
.
Known viral causes of atypical pneumonia include Respiratory Syncytial Virus
Human respiratory syncytial virus
Human respiratory syncytial virus is a virus that causes respiratory tract infections. It is the major cause of lower respiratory tract infections and hospital visits during infancy and childhood. A prophylactic medication exists for preterm birth infants and infants with a congenital heart...
(RSV), Influenza
Influenza
Influenza, commonly referred to as the flu, is an infectious disease caused by RNA viruses of the family Orthomyxoviridae , that affects birds and mammals...
A and B, Parainfluenza, Adenovirus, severe acute respiratory syndrome
Severe acute respiratory syndrome
Severe Acute Respiratory Syndrome is a respiratory disease in humans which is caused by the SARS coronavirus . Between November 2002 and July 2003 an outbreak of SARS in Hong Kong nearly became a pandemic, with 8,422 cases and 916 deaths worldwide according to the WHO...
(SARS)
and measles
Measles
Measles, also known as rubeola or morbilli, is an infection of the respiratory system caused by a virus, specifically a paramyxovirus of the genus Morbillivirus. Morbilliviruses, like other paramyxoviruses, are enveloped, single-stranded, negative-sense RNA viruses...
.
Diagnosis
Chest radiographs (X-ray photographs) often show a pulmonary infection before physical signs of atypical pneumonia are observable at all.This is called occult pneumonia
Occult pneumonia
Occult pneumonia is a pneumonia that is not observable directly by the eye, but can only be shown indirectly, especially by radiography. Occult pneumonia can be made visible by chest X-rays....
. 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.
Infiltration
Infiltration (medical)
Infiltration is the diffusion or accumulation of substances not normal to it or in amounts in excess of the normal. The material collected in those tissues or cells is called infiltrate.-Classification:...
commonly begins in the perihilar region (where the bronchus
Bronchus
A bronchus is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles....
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.
Epidemiology
Mycoplasma is found more often in younger than in older people.Older people are more often infected by Legionella.