Introduction
Background
Mycoplasma pneumoniae is a common cause of community-acquired pneumonia, and, usually, the disease has a prolonged, gradual onset. M pneumoniae was first isolated in cattle with pleuropneumonia in 1898.
In 1938, Reimann described the first cases of mycoplasmal pneumonia in man. Reimann coined the term "primary atypical pneumonia" after observing 7 patients in Philadelphia with marked constitutional symptoms, upper and lower respiratory tract symptoms, and a protracted course with gradual resolution. Peterson discovered the phenomenon of cold agglutinin in 1943, and high titers of cold agglutinins in patients with this type of pneumonia were discovered accidentally. In 1944, Eaton was credited with discovering a specific agent, coined Eaton’s agent, as the principal cause of primary atypical pneumonia. First thought to be a virus, Eaton’s agent was proved to be a Mycoplasma species in 1961. The general characteristics of Mycoplasma species are presented in the image below.
General characteristics of Mycoplasma species.
Pathophysiology
The responsible organism, M pneumoniae, is a pleomorphic organism that, unlike bacteria, lacks a cell wall, and unlike viruses do not need a host cell for replication. The prolonged paroxysmal cough seen in this disease is thought to be due to the inhibition of ciliary movement. The organism has a remarkable gliding motility and specialized filamentous tips end that allows it to burrow between cilia within the respiratory epithelium, eventually causing sloughing of the respiratory epithelial cells.
The organism has two properties that seem to correlate well with its pathogenicity in humans. The first is a selective affinity for respiratory epithelial cells, and the second is the ability to produce hydrogen peroxide, which is thought to be responsible for much of the initial cell disruption in the respiratory tract and for damage to erythrocyte membranes.
The pathogenicity of M pneumoniae has been linked to the activation of inflammatory mediators, including cytokines. A recent study reported on a recent emergence of drug-resistant M pneumoniae infection; however, the study concluded that host immune maturity and not the virulence factor of the organism is a major determinant factor of disease severity.1
Frequency
United States
M pneumoniae is now recognized as one of the most common causes of community-acquired pneumonia in otherwise healthy patients younger than 40 years, with the highest rate in individuals aged 5-20 years. M pneumoniae causes upper and lower respiratory illness in all age groups, particularly in temperate climates, and in summer, may cause as many as 50% of all pneumonias.
Mycoplasmal pneumonia can occur at any time of the year, but large outbreaks tend to occur in the late summer and fall. The incubation period tends to be smoldering and averages 3 weeks, in contrast to that of influenza and other viral pneumonias, which is generally a few days. Epidemics of mycoplasmal pneumonia tend to occur every 4-8 years in the general population and tend to be more frequent within closed populations, such as in military and prison populations. Although M pneumoniae is a common cause of pneumonia, only 5-10% of infected patients actually develop pneumonia.
Mortality/Morbidity
In almost all patients, the pneumonia resolves without any serious complications. M pneumoniae can cause severe pneumonia in children and has recently been associated with acute chest syndrome in patients with sickle cell anemia.
Race
No racial predilections are observed in mycoplasmal diseases.
Sex
No difference in disease frequency is observed between males and females, but illnesses are somewhat more severe in males.
Age
Mycoplasmal pneumonia is common in all age groups; however, it is most common in the first 2 decades of life and is rare in children younger than 5 years.
Clinical
History
Mycoplasmal pneumonia is a disease of gradual and insidious onset of several days to weeks. The patient's history may include the following:
- Fever
- Malaise
- Persistent, slowly worsening dry cough; absence of cough makes the diagnosis of M pneumoniae unlikely
- Headache
- Chills, not rigors
- Scratchy sore throat
- Sore chest and tracheal tenderness (result of the protracted cough)
- Pleuritic chest pain (rare)
Physical
Most cases of pneumonia due to M pneumoniae resolve after several weeks, although a dry cough can be present for as long as a month; some patients can have a protracted illness lasting as long as 6 weeks. Other findings may also include the following:
- A nontoxic general appearance
- Erythematous tympanic membranes or bullous myringitis in patients older than age 2 years, an uncommon but unique sign
- Mild pharyngeal injection with minimal or no cervical adenopathy, but no exudate
- Normal lung findings with early infection but rhonchi, rales, and/or wheezes several days later
- Various exanthems including erythema multiforme and Stevens-Johnson syndrome
Causes
- The causative agent is M pneumoniae, a bacterium lacking a cell wall, which belongs to the class Mollicutes, the smallest known free-living microorganisms.
- Because the organism can be excreted from the respiratory tract for several weeks after the acute infection, isolation of the organism may not indicate acute infection.
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References
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Further Reading
Keywords
mycoplasma pneumonia, mycoplasmal pneumonia, Mycoplasma pneumoniae, M pneumoniae, CAP, community-acquired pneumonia, atypical pneumonia, sore chest, tracheal tenderness, dry cough, bullous myringitis, sore throat, treatment, diagnosis, symptoms
