Microbial Diseases of the Respiratory System

Chapter 24: Microbial Diseases of the Respiratory System

Instructor: Dr. Stuart

Learning Objectives

  • Name 2 fungal species that cause disease in the lower respiratory tract.
  • List the viral species that cause pneumonia.
  • Describe the stages of Whooping Cough and the bacteria responsible.
  • List the viruses that cause the common cold.
  • List the bacterial species that cause strep throat and diphtheria.
  • Differentiate pharyngitis, laryngitis, tonsillitis, sinusitis, and epiglottitis.
  • Discuss diseases of the upper and lower respiratory tract.
  • Characterize the normal microbiota of the upper and lower respiratory systems.
  • Describe how microorganisms are prevented from entering the respiratory system.

Structure and Function of the Respiratory System

  • Divisions of the respiratory system:
    • Upper respiratory system
    • Lower respiratory system
  • Commonality of Infections:
    • Infections of the upper respiratory system are the most common types of infection.
    • Pathogens that enter the respiratory system can infect other parts of the body.

Upper Respiratory System

  • Components:
    • Nasal cavity (nose)
    • Pharynx (throat)
    • Larynx (voice box)
  • Functions:
    • Nose hairs filter large particles from air entering the tract.
    • Associated structures include the middle ear and eustachian (auditory) tubes that empty into the upper throat, as well as ducts from sinuses and nasolacrimal which empty into the nasal cavity.

Lower Respiratory System

  • Components:
    • Trachea (windpipe), lungs, bronchial tubes, bronchioles, and alveoli.
  • Defense Mechanisms:
    • Ciliary escalator moves particles toward the throat via ciliary action to prevent lung infection.
    • Alveolar macrophages destroy microorganisms in the lungs.
    • Respiratory mucus protects mucosal surfaces, functioning as a physical/chemical barrier and containing IgA antibodies.

Normal Microbiota of the Respiratory System

Upper Respiratory System

  • Normal Flora:
    • Upper respiratory system (nasal cavity and throat) can include pathogenic microbes.
  • Role of Normal Microbiota:
    • Suppresses pathogens by competing for nutrients and producing inhibitory substances.

Lower Respiratory System

  • Sterility:
    • The lower respiratory system is almost sterile with no microbes typically found; however, Staphylococcus sp. may be present in the trachea.

Bacterial Diseases of the Upper Respiratory System

Categories of Infections

  • Areas of the upper respiratory tract can become infected, leading to various illnesses which are largely self-limiting (recovery occurs without medicines).
  • Infections can be caused by several bacteria, viruses, or both, leading to conditions such as:
    • Pharyngitis: Sore throat.
    • Laryngitis
    • Tonsillitis
    • Sinusitis
    • Epiglottitis:
    • Known as one of the most life-threatening diseases of the upper respiratory system; can result in death within a few hours.
    • Haemophilus influenzae type b (Hib) is a common causative agent.

Strep Throat

  • Definition: Streptococcal pharyngitis (strep throat) caused by group A streptococci (GAS).
  • Pathogenicity of Streptococcus pyogenes:
    • Resistance to phagocytosis.
    • Production of streptokinases that lyse clots.
    • Production of streptolysins that are cytotoxic.
  • Symptoms: Local inflammation, fever, tonsillitis, and enlarged lymph nodes.
  • Diagnosis: Rapid antigen detection tests.

Scarlet Fever

  • Definition: Occurs when S. pyogenes is lysogenized and secretes erythrogenic toxin.
  • Symptoms:
    • Strawberry tongue.
    • Red skin rash due to toxin presence.
    • High fever.

Diphtheria

  • Causative Agent: Corynebacterium diphtheriae.
  • Description of Illness:
    • Forms a tough grayish membrane in the throat, consisting of fibrin and dead tissue, which can block air passage to the lungs.
  • Diphtheria Toxin:
    • Secreted by lysogenized bacteria, inhibits protein synthesis and can circulate in the blood, damaging the heart and kidneys.
  • Cutaneous Diphtheria:
    • Results in skin ulcers that heal slowly.
  • Prevention: Through the DTaP vaccine, which includes toxoids for diphtheria, tetanus, and pertussis.

Viral Diseases of the Upper Respiratory System

The Common Cold

  • Causative Agents: Over 200 different viruses can cause the common cold.
    • Rhinoviruses: Account for 30-50% of cases; thrive in temperatures lower than body temperature.
    • Coronaviruses: Contribute 10-15%.
    • Enterovirus D68 (EV-D68) and 20-30% of viruses previously unknown also play a role.
  • Symptoms: Sneezing, nasal secretion, and congestion; common colds are often not accompanied by fever and can lead to laryngitis and otitis media.
  • Treatment: Antibiotics are ineffective; relief is provided via cough suppressants and antihistamines.

Bacterial Diseases of the Lower Respiratory System

Overview of LRS Infections

  • Many bacteria and viruses causing upper respiratory diseases can also lead to lower respiratory diseases.
  • Conditions include:
    • Bronchitis
    • Bronchiolitis
    • Pneumonia: A severe complication of bronchitis that implicates the pulmonary alveoli.

Pertussis (Whooping Cough)

  • Causative Agent: Bordetella pertussis, which is a Gram-negative coccobacillus.
  • Characteristics:
    • Produces a capsule that enhances its infectiousness and allows it to attach to ciliated cells in the trachea.
    • Depletes ciliated cells, shutting down the ciliary escalator, which is essential for clearing mucus and pathogens from the lungs.
  • Toxins Produced:
    • Tracheal cytotoxin damages ciliated cells.
    • Pertussis toxin enters the bloodstream, exacerbating the illness.

Stages of Pertussis

  • Stage 1: Catarrhal Stage
    • Symptoms resemble those of the common cold.
  • Stage 2: Paroxysmal Stage
    • Characterized by violent coughing fits, gasping for air, and accumulation of mucus in the bronchi, potentially resulting in broken ribs in small children.
  • Stage 3: Convalescence Stage
    • Can last for months.
  • Prevention: Through the DTaP vaccine.

Bacterial Pneumonias

  • Typical Pneumonia:
    • Primarily caused by Streptococcus pneumoniae (pneumococcal pneumonia) wherein infected alveoli fill with fluids and RBCs, impairing oxygen uptake.
  • Atypical Pneumonias: Caused by other microorganisms, including:
    • H. influenzae: Affects children under 5 and adults over 65.
    • Mycoplasma pneumoniae: Known as walking pneumonia.
    • Legionella pneumophila: Grows in water and A/C units; transmitted via inhalation.
    • Chlamydophila pneumoniae: Common in college students.
    • Coxiella burnetti: Causes Q fever, transmitted by tick bites.

Viral Diseases of the Lower Respiratory System

Overview of Viral Pneumonias

  • Association with Other Illnesses: Viral pneumonias often occur as a complication of influenza, measles, or chickenpox.
  • Diagnosis: Few labs can correctly test clinical samples for viruses; if no bacteria are found, viral etiology is assumed.
  • Emerging Coronavirus Diseases:
    • SARS–CoV-1 emerged in Asia in 2003, leading to over 900 deaths in Asia.
    • Middle East respiratory syndrome Coronavirus (MERS-CoV) reported in Saudi Arabia in 2012; there were 2 cases in the U.S.

SARS-CoV-2 Pandemic

  • Emergence:
    • SARS-CoV-2 emerged in Asia in 2019, resulting in a global pandemic.
  • Reservoirs and Transmission: Bats are natural hosts with no disease; infected wild animals such as camels transmit the virus to humans.

Case Statistics of SARS-CoV-2

  • Global Cases: 632,519,814
  • U.S. Total Cases: 97,501,412
  • Global Vaccination Doses: 12,911,295,057
  • U.S. Vaccination Doses: 640,913,400

Respiratory Syncytial Virus (RSV)

  • Significance: Most common viral respiratory disease in infants and can cause life-threatening pneumonia in older adults.
  • Epidemiology: Nearly all children become infected by age 2 and approximately 14,000 deaths occur annually, mainly among older adults.
  • Mechanism: Causes cell fusion (syncytium) in cell culture, leading to severe respiratory issues and prolonged coughing.

Fungal Diseases of the Lower Respiratory System

Fungal Infections Overview

  • Mechanism: Fungi produce spores that are easily inhaled, often resulting in infections of the lower respiratory tract.
  • Increase in Incidence: The incidence of fungal diseases is increasing, likely due to the rise in immunocompromised populations, primarily as a result of cancer treatments.

Pneumocystis Pneumonia

  • Causative Agent: Pneumocystis jirovecii, which is found in healthy human lungs.
  • Classification: There is ongoing debate over whether it is classified as a protozoan or fungus.
  • Epidemiological Significance: It serves as a primary indicator of AIDS.
  • Lifecycle:
    • Located in the lining of the alveoli, forms cysts that rupture to release trophozoites.

Blastomycosis

  • Causative Agent: Blastomyces dermatitidis, a dimorphic fungus that typically grows in soil.
  • Infection Process: Infection starts in the lungs and can spread throughout the body, presenting symptoms similar to bacterial pneumonia, along with cutaneous abscesses and extensive tissue damage.
  • Annual Reports: Approximately 100 cases reported annually.

Distinguishing Fungal Pneumonia from Bacterial Pneumonia

  • Key Differences:
    • Presentation of symptoms can vary, and distinct laboratory tests can differentiate between the two forms of pneumonia.