78. Infectious disorders of the lower airways

Page created on May 12, 2019. Not updated since.

Page created on May 12, 2019. Not updated since.

Lower respiratory tract infections

Pneumonia (the most common lower respiratory tract infection) is an acute inflammation of the lung parenchyme, with involvement of the bronchioli and alveoli, most often caused by microbes. These are very common diseases and very common causes of death. They may be bacterial (most common), viral, fungal, due to aspiration, etc.

Pneumonia is common because the lung is exposed to many litres of microbe-contaminated air every minute, and that smoking-related lung disorders predispose the lungs to get infected.

We differentiate between four types of lower respiratory tract infections:

  • Bronchitis, inflammation of the bronchi. Commonly associated with coughing up mucus
  • Bronchiolitis, inflammation of the bronchioli.
  • Pneumonia, where there’s exudate in the alveoli
  • Pneumonitis, where the inflammation is of the lung interstitium, the alveolar septa


Pneumonia is an acute inflammation of the lung parenchyme, with involvement of the bronchioli and alveoli, most often caused by microbes. It can have significant mortality. It’s a very common condition. 

Community-acquired pneumonia affects 0,1 – 1% of the adult population each year. It’s more common in young children and older adults, especially those with comorbidities. It’s more common in the winter months. The mortality is higher in the elderly than children.

Hospital-acquired pneumonia can affect anyone who is admitted to the hospital. It’s a severe condition which causes significant mortality.


According to the place where the patient acquired the infection:

  • Community-acquired pneumonia (CAP)
    • Absence of prior hospital admission in the last 7 days
  • Nosocomial pneumonia
    • Hospital-acquired pneumonia (HAP)
      • Acquired > 48 hours after hospital admission
    • Ventilator-associated pneumonia (VAP)
      • Acquired > 48 hours after endotracheal intubation
    • (Healthcare-associated pneumonia (HCAP))
      • Pneumonia in health care facilities other than hospital
      • Nowadays considered a type of CAP

This classification is important because the causative agents and therefore the management are different for each type.

We distinguish three types of pneumonia according to the pathology:

  • Bronchopneumonia, where the intraalveolar exudate associated with the inflammation has a patchy distribution along the whole lung
  • Lobar pneumonia, where the exudate distribution is limited at a lobe-by-lobe basis, so that one lobe is affected but the others may not be. It’s almost always caused by streptococcus pneumoniae. The affected lobe undergoes hepatization, where the consistency changes to be similar to that of the liver.
  • Interstitial pneumonia, or pneumonitis, where the alveolar septa are inflamed. It’s often caused by other pathogens than for the other two types.

Special types:

  • Atypical pneumonia
    • Caused by atypical bacteria
  • Aspiration pneumonia
    • Caused by aspiration of gastric fluids
  • Chemical pneumonitis


Microbial agents

  • Community-acquired pneumonia
    • Streptococcus pneumoniae (most cases)
  • Nosocomial pneumonia
    • Gram negative enterobacteria (E. coli, Klebsiella)
    • MSSA and MRSA
    • Pseudomonas aeruginosa
  • Atypical pneumonia
    • Chlamydia pneumoniae
    • Mycoplasma pneumoniae
  • Aspiration pneumonia
    • Anaerob bacteria

Risk factors

  • Children < 5 years
  • Adults > 40 years
  • Comorbidities (COPD, bronchiectasis, heart disease, renal disease)
  • Swallowing impairment (for aspiration pneumonia)
  • Decreased mucociliary clearance (occurs in smokers and in immobilisation)
  • Immunosuppression
  • Endotracheal intubation (risk is proportional to duration of intubation)

Viral infections

Viral lower respiratory tract infection are more common in children than in adults. They can be caused by:

  • Rhinovirus
  • Parainfluenza virus
  • Influenza virus
  • Respiratory syncytial virus
  • Coronavirus
  • Cytomegalovirus (CMV)

The viruses invade and kill the infected cells. This alveolar epithelial necrosis can lead to diffuse alveolar damage (DAD), which may cause hyaline membrane formation.

Viral infections can often depress the immune system so that secondary infections can also infect the lung, causing a bacterial superinfection of the lower respiratory tract.

Special forms of infectious pneumonia

Legionnaire disease

Legionnaire disease is a form of bacterial pneumonia caused by legionella pneumophila. It’s usually found in stale waters. Legionnaire pneumonia is usually quite severe and requires hospitalization.


The influenza virus is the most frequent cause of viral lower respiratory tract infection (bronchitis) in adults. This infection is often called the “flu” and occurs most commonly during the winter. The biggest problem with influenza is that the virus changes its surface antigens constantly, meaning that the body can never develop immunity against all types, because new types occur all the time. This also necessitates a new influenza vaccine every year.

The influenza virus causes degenerative changes in the bronchial epithelium and mucosa, destroying them and predisposing to secondary bacterial infection. The virus most commonly affects infects the pharynx and tracheobronchial tree, and only rarely causes outright pneumonia.


Severe acute respiratory distress syndrome (SARS) is a relatively recently discovered airborne disease. It was discovered in 2002, when it had an outbreak in China and many people died. It’s caused by a coronavirus called SARS-CoV-1.


COVID-19 is a systemic disease caused by the coronavirus SARS-CoV-2. It’s the causative agent of the COVID-19 pandemic, which is still ongoing at the time of writing. It was discovered in the end of 2019.

The main symptoms of COVID-19 are fever, cough, fatigue, and dyspnoea, but it has a relatively high mortality rate (2 – 5%) in those with underlying comorbidities and those who are unvaccinated.

It uses the ACE2 receptor to enter cells of the respiratory tract. The pathomechanism involves not only direct cytotoxic injury from the virus but also from immune system overactivation and systemic inflammation. Inflammation of the olfactory nerves cause loss of smell.

Vaccines which significantly reduced risk of infection and severe disease were rapidly developed.

CMV pneumonitis

CMV pneumonitis occurs in immunosuppressed people, often in people who have received transplants or have AIDS. Recipients of organs like the heart, liver and kidney may receive the virus from the donor organ, as the virus can ride along with the organ, they may experience reactivation of a latent infection, or they may be infected de novo. The disease mainly causes pneumonitis and colitis.

For histology of cytomegalovirus infection, see the slide.

Fungal pneumonia

The most important forms of fungal pneumonia are aspergillosis and PCP. Fungal infections in general mostly affect immunocompromised people.

Aspergillosis is a disease caused by the fungus aspergillus that causes necrotizing pneumonia. It infects immunocompromised people. An aspergilloma, also known as a fungus ball, is a clump of aspergilloma that exist in a body cavity, like bronchi, cavities after tuberculosis or abscesses. It rarely invades the lung parenchyme.

Pneumocystis jirovecii is a fungus that causes pneumocystis pneumonia (PCP). It too affects immunocompromised people, especially people with AIDS.


See its own topic.

Aspiration pneumonia

Aspiration pneumonia affect people with decreased gag and swallowing reflexes, like debilitated patients, drunk people and stroke patients. They can the aspire gastric contents, which will irritate the alveoli. Bacteria from the oropharynx infect the lung, causing necrotizing pneumonia.

Lung abscess

Lung abscess is a potential complication of several conditions:

  • Aspiration of infective material, from carious teeth or such
  • Aspiration of gastric content
  • In necrotizing pneumonias
  • In bronchial obstruction
  • Septic embolism, an embolism that carries bacteria, like from an infective endocarditis of the heart

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