Atypical Pneumonias

See also: Managing Pneumonia in the Community, Pneumonia, Lower Respiratory Tract Infection in Children.

An atypical pneumonia tends to present and progress differently from the 'standard' pneumonias due to e.g. Streptococcus pneumoniae and Haemophilus influenzae. The 'classical' atypical pneumonias are due to pulmonary infection with:

• Mycoplasma pneumoniae
• Chlamydia pneumoniae
• Legionella pneumophila

Other microorganisms that cause similar patterns of presentation via pulmonary infection include:

• Chlamydophila psittaci (exposure to birds, particularly ill ones, is a useful clue in the history)
• Coxiella burnetii (presenting as Q fever)
• Viral pneumonias including Influenza A, Severe Acute Respiratory Syndrome (SARS), RSV, adenoviridae and pneumonitis due to Varicella (chickenpox pneumonitis)

• It is thought that the three main atypical organisms might be implicated in up to 40% of community acquired pneumonias.¹
• However, the precise incidence of these infections is not accurately known, as they are often not identified in clinical practice due to the lack of a readily available, reliable, standard test to confirm the diagnosis (with the possible exception of Legionella).²
• By the age of 20, 50% of people in the United States have detectable levels of antibody to Chlamydia pneumoniae.¹

Risk factors

• Mycoplasma and Chlamydia spread by person to person contact, and spread is most common in closed populations, e.g. schools, offices.
• Legionellae are found most commonly in freshwater and man-made water systems. Man-made systems for heating and cooling water can provide breeding grounds for Legionellae. Condensers, showers, whirlpools, cooling towers and respiratory equipment have all been implicated in outbreaks, and all such installations in public use should be inspected regularly.

Signs in atypical pneumonia

• Vital signs should be checked.
• Look for evidence of extra-thoracic involvement if an atypical pneumonia is suspected.
• On the whole, chest signs are not helpful. Indeed, it is often the discordance between the chest signs and the illness of the patient, or the floridity of initial CXR appearance, that raises the suspicion of an atypical pneumonia.
• Non-specific chest signs and evidence of consolidation may be found, but this is much less common than in the 'standard' pneumonias.
• There may be signs in other systems due to complications of the infection.

• 'Standard' bacterial pneumonia
• Viral pneumonia
• Fungal respiratory disease (infection or hypersensitivity)
• Extrinsic allergic alveolitis
• Acute bronchitis/tracheitis
• Chlamydophila psittaci infection
• Aspiration pneumonia
• Q fever
• Pulmonary tuberculosis
• Primary cardiopulmonary disease ± infection
• Costochondritis
• Bornholm disease
• Gastroenteritis
• Opportunistic infection due to HIV/AIDS or other causes of immunocompromise
• Meningitis
• Pleural effusion
• Empyema/lung abscess
• Adult respiratory distress syndrome (ARDS)
• Other causes of systemic sepsis

• Severe cases need to be managed in hospital with intravenous antibiotics, oxygen and possibly respiratory support.
• Atypical pneumonias are usually treated as for other community-acquired pneumonias, at least initially.
• There is no evidence that routinely giving antibiotics active against atypical organisms leads to better outcomes in non-severe community-based cases of pneumonia.⁶
• Macrolides, such as erythromycin, clarithromycin and azithromycin, have been shown to be effective in the treatment of all three infective organisms.
• Erythromycin tends to be less well tolerated, and although effective in the treatment of Mycoplasma and Chlamydia, there are few trials demonstrating its efficacy in the treatment of Legionella.¹
• Severe Legionella infections may require rifampicin as well as a macrolide.
• Tetracycline, doxycycline and fluoroquinolones are also effective against all three infective organisms.

• Mycoplasma pneumoniae:
  • Rashes such as erythema multiforme, erythema nodosum and urticaria
  • Neurological complications like Guillain-Barré syndrome, transverse myelitis, cerebellar ataxia and aseptic meningitis
  • Haematological complications such as cold agglutinin disease and haemolytic anaemia
  • Joint symptoms like arthralgia and arthritis
  • Cardiac complications such as pericarditis and myocarditis
  • Pancreatitis (rarely)

• Legionella pneumophila:
  • Gastrointestinal upset
  • There may be neurological complications such as confusion, disorientation and focal neurological deficit
  • Arthralgia and myalgia are often reported
  • Pancreatitis, peritonitis
  • Pericarditis, myocarditis, endocarditis
  • Glomerulonephritis

• Chlamydia pneumoniae:
  • Sinusitis, otitis media
  • Reactive arthritis, erythema nodosum-type vasculitis
  • Myocarditis, endocarditis
  • New-onset asthma may occur after the acute infection

• The prognosis for Mycoplasma pneumoniae is generally good with most patients making a full recovery. The mortality rate is approximately 1.4%, with the majority of deaths occurring in the elderly or patients with co-existing pathologies.¹
• Most cases of Chlamydial infection are mild. The mortality rate is approximately 9%, usually associated with secondary infection or underlying disease.¹
• Legionella has the most severe course, and may cause significant morbidity if not treated early. The mortality rate is 14%.¹

Adequate maintenance and microbiological testing of humidifier devices/cooling systems in buildings help to reduce the incidence of Legionella pneumophila.