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22.1 Anatomy and Normal Microbiota of the Respiratory Tract

  • The respiratory tract is divided into upper and lower regions at the epiglottis.
  • Air enters the upper respiratory tract through the nasal cavity and mouth, which both lead to the pharynx. The lower respiratory tract extends from the larynx into the trachea before branching into the bronchi, which divide further to form the bronchioles, which terminate in alveoli, where gas exchange occurs.
  • The upper respiratory tract is colonized by an extensive and diverse normal microbiota, many of which are potential pathogens. Few microbial inhabitants have been found in the lower respiratory tract, and these may be transients.
  • Members of the normal microbiota may cause opportunistic infections, using a variety of strategies to overcome the innate nonspecific defenses (including the mucociliary escalator) and adaptive specific defenses of the respiratory system.
  • Effective vaccines are available for many common respiratory pathogens, both bacterial and viral.
  • Most respiratory infections result in inflammation of the infected tissues; these conditions are given names ending in -itis, such as rhinitis, sinusitis, otitis, pharyngitis, and bronchitis.

22.2 Bacterial Infections of the Respiratory Tract

  • A wide variety of bacteria can cause respiratory diseases; most are treatable with antibiotics or preventable with vaccines.
  • Streptococcus pyogenes causes strep throat, an infection of the pharynx that also causes high fever and can lead to scarlet fever, acute rheumatic fever, and acute glomerulonephritis.
  • Acute otitis media is an infection of the middle ear that may be caused by several bacteria, including Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis. The infection can block the eustachian tubes, leading to otitis media with effusion.
  • Diphtheria, caused by Corynebacterium diphtheriae, is now a rare disease because of widespread vaccination. The bacteria produce exotoxins that kill cells in the pharynx, leading to the formation of a pseudomembrane; and damage other parts of the body.
  • Bacterial pneumonia results from infections that cause inflammation and fluid accumulation in the alveoli. It is most commonly caused by S. pneumoniae or H. influenzae. The former is commonly multidrug resistant.
  • Mycoplasma pneumonia results from infection by Mycoplasma pneumoniae; it can spread quickly, but the disease is mild and self-limiting.
  • Chlamydial pneumonia can be caused by three pathogens that are obligate intracellular parasites. Chlamydophila pneumoniae is typically transmitted from an infected person, whereas C. psittaci is typically transmitted from an infected bird. Chlamydia trachomatis, may cause pneumonia in infants.
  • Several other bacteria can cause pneumonia in immunocompromised individuals and those with cystic fibrosis.
  • Tuberculosis is caused by Mycobacterium tuberculosis. Infection leads to the production of protective tubercles in the alveoli and calcified Ghon complexes that can harbor the bacteria for a long time. Antibiotic-resistant forms are common and treatment is typically long term.
  • Pertussis is caused by Bordetella pertussis. Mucus accumulation in the lungs leads to prolonged severe coughing episodes (whooping cough) that facilitate transmission. Despite an available vaccine, outbreaks are still common.
  • Legionnaires disease is caused by infection from environmental reservoirs of the Legionella pneumophila bacterium. The bacterium is endocytic within macrophages and infection can lead to pneumonia, particularly among immunocompromised individuals.
  • Q fever is caused by Coxiella burnetii, whose primary hosts are domesticated mammals (zoonotic disease). It causes pneumonia primarily in farm workers and can lead to serious complications, such as endocarditis.

22.3 Viral Infections of the Respiratory Tract

  • Viruses cause respiratory tract infections more frequently than bacteria, and most viral infections lead to mild symptoms.
  • The common cold can be caused by more than 200 viruses, typically rhinoviruses, coronaviruses, and adenoviruses, transmitted by direct contact, aerosols, or environmental surfaces.
  • Due to its ability to rapidly mutate through antigenic drift and antigenic shift, influenza remains an important threat to human health. Two new influenza vaccines are developed annually.
  • Several viral infections, including respiratory syncytial virus infections, which frequently occur in the very young, can begin with mild symptoms before progressing to viral pneumonia.
  • SARS and MERS are acute respiratory infections caused by coronaviruses, and both appear to originate in animals. SARS has not been seen in the human population since 2004 but had a high mortality rate during its outbreak. MERS also has a high mortality rate and continues to appear in human populations.
  • Measles, rubella, and chickenpox are highly contagious, systemic infections that gain entry through the respiratory system and cause rashes and fevers. Vaccines are available for all three. Measles is the most severe of the three and is responsible for significant mortality around the world. Chickenpox typically causes mild infections in children but the virus can reactivate to cause painful cases of shingles later in life.

22.4 Respiratory Mycoses

  • Fungal pathogens rarely cause respiratory disease in healthy individuals, but inhalation of fungal spores can cause severe pneumonia and systemic infections in immunocompromised patients.
  • Antifungal drugs like amphotericin B can control most fungal respiratory infections.
  • Histoplasmosis is caused by a mold that grows in soil rich in bird or bat droppings. Few exposed individuals become sick, but vulnerable individuals are susceptible. The yeast-like infectious cells grow inside phagocytes.
  • Coccidioidomycosis is also acquired from soil and, in some individuals, will cause lesions on the face. Extreme cases may infect other organs, causing death.
  • Blastomycosis, a rare disease caused by a soil fungus, typically produces a mild lung infection but can become disseminated in the immunocompromised. Systemic cases are fatal if untreated.
  • Mucormycosis is a rare disease, caused by fungi of the order Mucorales. It primarily affects immunocompromised people. Infection involves growth of the hyphae into infected tissues and can lead to death in some cases.
  • Aspergillosis, caused by the common soil fungus Aspergillus, infects immunocompromised people. Hyphal balls may impede lung function and hyphal growth into tissues can cause damage. Disseminated forms can lead to death.
  • Pneumocystis pneumonia is caused by the fungus P. jirovecii. The disease is found in patients with AIDS and other immunocompromised individuals. Sulfa drug treatments have side effects, but untreated cases may be fatal.
  • Cryptococcosis is caused by Cryptococcus neoformans. Lung infections may move to the brain, causing meningitis, which can be fatal.
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