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, sweating 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.
No response to common antibiotics such as sulfonamide and beta-lactams like penicillin.
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 pneumonia.
Absence of leukocytosis.
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 symptoms and problems with the upper respiratory tract (such as high fever, headache, a dry irritating cough followed later by a productive cough with radiographs showing consolidation), there are in general few physical signs. The patient looks better than the symptoms suggest.
Chlamydia psittaci: Causes psittacosis also known as 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 other species of bird. The incidence of infection in canaries and finches is believed to be lower than in psittacine birds.
Coxiella burnetii: Q fever (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. The infection results from inhalation of a spore-like small-cell variant, and from contact with the milk, urine, feces, vaginal mucus, or semen of infected animals. Rarely, the disease is tick-borne. The incubation period is 9-40 days. Humans are vulnerable to Q fever, and infection can result from even a few organisms. The bacterium is an obligate intracellular pathogenic parasite).
Francisella tularensis: Causes tularemia which is a serious infectious disease caused by the intracellular bacterium Francisella tularensis. A Gram-negative, nonmotile, pleomorphic coccobacillus, the bacterium has several subspecies with varying degrees of virulence. The most important of those is F. tularensis tularensis (Type A), which is found in lagomorphs (rabbits, hares and pikas) in North America, and it is highly virulent in humans and domestic rabbits. F. tularensis palaearctica (Type B) occurs mainly in aquatic rodents (beavers, muskrats) in North America and in hares and small rodents in northern Eurasia. It is less virulent for humans and rabbits. The primary vectors are ticks and deer flies, but the disease can also be spread through other arthropods. The disease is named after Tulare County, California.
Legionella pneumophila: Causes a severe form of pneumonia with a relatively high mortality rate, known as legionellosis or Legionnaires’ disease
Mycoplasma pneumonia: 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.
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) and measles.
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.