Microbial Diseases of the Respiratory System: Structure, Pathogenesis, and Control

Diseases of the Respiratory System

Overview of the Respiratory System

The respiratory system is divided into the upper and lower tracts, each with distinct anatomical features and associated diseases. Understanding the structure and function of these regions is essential for recognizing how pathogens cause disease and how these diseases are managed.

• Upper Respiratory Tract: Includes the nose, nasal cavity, pharynx, and associated structures such as the epiglottis and middle ear.

• Lower Respiratory Tract: Comprises the larynx, trachea, bronchi, bronchioles, and lungs.

Key Differences: The upper tract is exposed to the external environment and is more frequently colonized by microbes, while the lower tract is typically sterile due to protective mechanisms such as the mucociliary escalator.

Upper Respiratory Tract Diseases

Bacterial Infections

• Epiglottitis: Inflammation of the epiglottis, often caused by Haemophilus influenzae type b. It is a medical emergency due to the risk of airway obstruction. Vaccination (Hib vaccine) is effective prevention.

• Streptococcal Pharyngitis (Strep Throat): Caused by Streptococcus pyogenes. Symptoms include sore throat, fever, and swollen lymph nodes. Diagnosis is by rapid antigen detection or throat culture. Treated with penicillin or related antibiotics.

• Scarlet Fever: A complication of strep throat caused by strains of S. pyogenes producing erythrogenic toxin. Characterized by a red rash, high fever, and 'strawberry' tongue.

• Diphtheria: Caused by Corynebacterium diphtheriae. Presents with sore throat, fever, and a pseudomembrane in the throat. The diphtheria toxin can cause systemic effects. Prevented by the DTaP vaccine.

• Otitis Media: Middle ear infection, commonly caused by Streptococcus pneumoniae, Haemophilus influenzae, or Moraxella catarrhalis. Symptoms include ear pain and fever. Treated with antibiotics; tympanostomy tubes may be used for recurrent cases.

Viral Infections

• Common Cold: Caused by rhinoviruses, coronaviruses, and others. Symptoms include runny nose, sneezing, and sore throat. Treatment is supportive; prevention focuses on hygiene.

Lower Respiratory Tract Diseases

Bacterial Infections

• Pertussis (Whooping Cough): Caused by Bordetella pertussis. Disease progresses through three stages:

  • Catarrhal Stage: Mild respiratory symptoms, highly contagious.

  • Paroxysmal Stage: Severe coughing fits with characteristic 'whoop' sound.

  • Convalescent Stage: Gradual recovery; cough decreases.

  Prevention: DTaP vaccine.

• Tuberculosis (TB): Caused by Mycobacterium tuberculosis.

  • Special Features: Cell wall contains mycolic acid, making it acid-fast and resistant to desiccation and disinfectants. Slow generation time (~20 hours).

  • Transmission: Airborne droplets from infected individuals.

  • Pathogenesis: Inhaled bacilli are phagocytosed by alveolar macrophages but survive and multiply, forming granulomas (tubercles). Disease may be latent or active.

  • Diagnosis: Tuberculin skin test (Mantoux test) detects delayed-type hypersensitivity to TB antigens. Positive test indicates exposure, not necessarily active disease.

  • Treatment Challenges: Requires prolonged multi-drug therapy due to slow growth and resistance. MDR-TB and XDR-TB are significant concerns.

  • BCG Vaccine: Live attenuated vaccine used in many countries but not routinely in the US due to variable efficacy and interference with skin testing.

• Pneumonia: Inflammation of the lung alveoli, caused by various bacteria, viruses, or fungi.

  • Pneumococcal Pneumonia: Caused by Streptococcus pneumoniae. Symptoms include fever, cough, and rust-colored sputum. Prevented by pneumococcal vaccines.

  • Klebsiella Pneumonia: Caused by Klebsiella pneumoniae. Often affects immunocompromised individuals; can cause necrotizing pneumonia.

  • Mycoplasmal Pneumonia: Caused by Mycoplasma pneumoniae. Also known as "walking pneumonia" due to milder symptoms. Lacks a cell wall; not susceptible to beta-lactam antibiotics.

  • Haemophilus influenzae Pneumonia: Caused by H. influenzae. More common in individuals with underlying conditions.

  • Legionellosis (Legionnaire’s Disease): Caused by Legionella pneumophila. Associated with contaminated water sources (e.g., air conditioning systems). Presents with high fever, cough, and sometimes gastrointestinal symptoms.

Viral Infections

• Influenza (Flu): Caused by influenza viruses (types A, B, C).

  • Viral Features: Enveloped RNA virus with two key surface proteins:

    • Hemagglutinin (HA): Facilitates viral entry into host cells.

    • Neuraminidase (NA): Assists in viral release from infected cells.

  • Vaccine: Annual vaccination is necessary due to antigenic drift (minor mutations) and antigenic shift (major genetic changes) in viral proteins. Research is ongoing for a universal flu vaccine.

• Viral Pneumonia: Caused by various viruses, including:

  • SARS (Severe Acute Respiratory Syndrome): Caused by SARS-CoV coronavirus.

  • MERS (Middle East Respiratory Syndrome): Caused by MERS-CoV coronavirus.

  • COVID-19: Caused by SARS-CoV-2 coronavirus. Symptoms range from mild respiratory illness to severe pneumonia and ARDS.

• Respiratory Syncytial Virus (RSV): Major cause of lower respiratory tract infections in infants and young children. Can cause bronchiolitis and pneumonia.

Fungal Infections

• Histoplasmosis: Caused by Histoplasma capsulatum. Found in soil contaminated with bird or bat droppings. Inhalation of spores leads to pulmonary infection; can disseminate in immunocompromised hosts.

• Coccidioidomycosis (Valley Fever): Caused by Coccidioides immitis. Endemic to southwestern US. Inhalation of arthroconidia causes flu-like symptoms; can become chronic or disseminated.

• Pneumocystis Pneumonia: Caused by Pneumocystis jirovecii. Opportunistic infection, especially in HIV/AIDS patients. Presents with nonproductive cough and hypoxia.

• Blastomycosis: Caused by Blastomyces dermatitidis. Found in moist soil; inhalation of spores leads to pulmonary and sometimes disseminated disease.

Summary Table: Major Respiratory Diseases

Disease	Pathogen	Key Symptoms	Prevention/Treatment
Streptococcal Pharyngitis	Streptococcus pyogenes	Sore throat, fever	Penicillin, hygiene
Diphtheria	Corynebacterium diphtheriae	Sore throat, pseudomembrane	DTaP vaccine, antitoxin
Pertussis	Bordetella pertussis	Severe cough, whoop	DTaP vaccine, macrolides
Tuberculosis	Mycobacterium tuberculosis	Chronic cough, weight loss	Multi-drug therapy, BCG vaccine
Pneumococcal Pneumonia	Streptococcus pneumoniae	Fever, productive cough	Pneumococcal vaccine, antibiotics
Influenza	Influenza virus	Fever, myalgia, cough	Annual vaccine, antivirals
COVID-19	SARS-CoV-2	Fever, cough, dyspnea	Vaccines, supportive care
Histoplasmosis	Histoplasma capsulatum	Flu-like, chronic cough	Antifungals

Additional info:

• Antigenic drift and shift in influenza are responsible for seasonal epidemics and occasional pandemics.

• Diagnosis of respiratory diseases often involves clinical assessment, laboratory testing (cultures, PCR), and imaging (chest X-ray).

• Prevention strategies include vaccination, hygiene, and public health measures such as isolation and contact tracing for highly contagious diseases.