Infections of the Respiratory System: Bacterial, Viral, and Fungal Pathogens

Infections of the Respiratory System

Criteria for Microbial Infection of the Respiratory Tract

For a microbe to successfully cause an infection in the respiratory system, several criteria must be met:

• Sufficient Infectious Dose: A large enough number of infectious agents must be inhaled to overcome host defenses.

• Airborne Transmission: The agents must be airborne or contained in droplet nuclei to reach the respiratory tract.

• Viability: The organism must remain alive and viable while suspended in the air.

• Susceptible Host Tissue: The microbe must find suitable tissue in the host for attachment and growth.

• Colonization: The organism must colonize respiratory surfaces before causing disease.

Bacterial Infections of the Respiratory System

Streptococcal Infections

• Streptococcal Pharyngitis (Strep Throat): Caused by Streptococcus pyogenes (Group A Streptococcus). Most common in children, transmitted via person-to-person contact.

• Scarlet Fever: Also caused by Group A beta-hemolytic streptococci. Characterized by fever, sore throat, chills, vomiting, abdominal pain, and a 'strawberry tongue' due to exotoxins. Treated with antibiotics.

• Streptococcus pneumoniae: Gram-positive, encapsulated, alpha-hemolytic diplococcus. Common inhabitant of the nasopharynx but can cause pneumonia, sinusitis, and other diseases when spreading to other areas. Risk is highest in young children, elderly, and immunocompromised individuals. Vaccination is available.

• Drug-Resistant Streptococcus pneumoniae (DRSP): Causes pneumonia, bacteremia, otitis media, meningitis, peritonitis, and sinusitis. Resistance is due to overuse of antimicrobials. Outbreaks occur in institutions. Pneumococcal conjugate vaccine is available.

Other Common Bacterial Infections

• Mycoplasma pneumoniae: Causes primary atypical pneumonia, usually mild and affecting people under 40. Transmitted via respiratory droplets, especially in crowded areas.

• Chlamydophila pneumoniae: Major cause of pneumonia or bronchitis, especially in school-age children. Transmission is by respiratory secretions.

• Pertussis (Whooping Cough): Caused by Bordetella pertussis, a small, aerobic, gram-negative coccobacillus. Highly contagious, transmitted by airborne droplets. Vaccine is available.

• Tuberculosis (TB): Caused by Mycobacterium tuberculosis. Transmitted through the air. May be latent or active. Symptoms include fever, night sweats, and loss of appetite. Treated with antibiotics; vaccines are available.

• Sinusitis: Caused by viruses, bacteria, or fungi, often secondary to a cold or allergy. Symptoms include congestion, fever, and sore throat. Treated with decongestants, analgesics, and antibiotics.

Rare and Opportunistic Bacterial Infections

• Staphylococcal pneumonia: Rare in healthy adults, common in immunocompromised individuals. Often hospital-acquired. Treated with penicillin or vancomycin for resistant strains.

• Haemophilus influenzae: Gram-negative coccobacillus, part of normal flora. Causes bronchitis and sometimes meningitis.

• Klebsiella pneumoniae: Gram-negative, encapsulated bacillus. Hospital-acquired, high fatality rate (25-50%).

• Diphtheria: Caused by Corynebacterium diphtheriae. Forms pseudomembrane in throat, can cause life-threatening obstruction and systemic toxin effects. Vaccine available.

• Legionellosis: Caused by Legionella pneumophila. Found in water sources, transmitted by inhalation of contaminated mist.

• Psittacosis: Caused by Chlamydia psittaci, found in bird droppings. Transmitted from birds to humans, causes fever, chills, and pneumonia. Treated with antibiotics.

• Inhalation Anthrax: Caused by Bacillus anthracis, a spore-forming, gram-positive bacillus. Infection via inhalation, skin, or ingestion. Releases toxins causing internal bleeding and necrosis.

• Q Fever: Caused by Coxiella burnetii, a spore-forming coccobacillus. Zoonotic, transmitted by inhalation of contaminated dust or fluids from animals. Symptoms include fever, headache, muscle pain, and jaundice. Early antibiotic treatment is most effective.

Viral Infections of the Respiratory System

• General: Viruses cause about 90% of acute respiratory and 50% of lower respiratory infections.

• Common Cold: Caused by over 200 different viruses. Symptoms include rhinitis, runny nose, congestion, and sneezing. Highly contagious, no specific treatment.

• Influenza (Flu): Caused by orthomyxoviruses. Symptoms include fever, headache, fatigue, dry cough, and muscle aches. Vaccination is available. Antigenic variation leads to new strains and epidemics.

• Viral Pneumonia: Caused by various viruses (e.g., influenza, adenovirus, RSV). Symptoms include fever, nonproductive cough, and myalgia.

• Hantavirus Pulmonary Syndrome (HPS): Caused by hantaviruses, transmitted by rodents. High fatality rate (30-40%).

• Severe Acute Respiratory Syndrome (SARS): Caused by SARS coronavirus. Person-to-person transmission, can be life-threatening. Symptoms include high fever, headache, and pneumonia.

• COVID-19: Caused by SARS-CoV-2. Highly contagious, pandemic from 2019-2021. Severe cases more common in elderly and those with comorbidities. mRNA vaccines available.

Fungal Infections of the Respiratory System

• Histoplasmosis: Caused by Histoplasma capsulatum. Transmitted by inhalation of airborne conidia from soil contaminated with bird or bat droppings. Most cases are asymptomatic; severe disease in immunocompromised individuals.

• Coccidioidomycosis: Caused by Coccidioides immitis. Found in desert soils. Transmitted by inhalation of arthroconidia. Symptoms range from none to severe respiratory illness.

• Blastomycosis: Caused by Blastomyces dermatitidis. Transmitted by inhalation, primarily affects lungs but can disseminate. Reservoir is wood and soil.

• Pulmonary Aspergillosis: Caused by Aspergillus species. Spores are inhaled from decaying plant material, compost, or dust. May colonize upper or lower respiratory tract. Treated with antifungal drugs.

Comparison Table: Selected Respiratory Pathogens

Pathogen	Type	Transmission	Main Symptoms	Prevention/Treatment
Streptococcus pyogenes	Bacteria	Droplet, person-to-person	Sore throat, fever, rash (scarlet fever)	Antibiotics, hygiene
Streptococcus pneumoniae	Bacteria	Droplet, person-to-person	Pneumonia, sinusitis, otitis media	Antibiotics, vaccine
Bordetella pertussis	Bacteria	Droplet	Whooping cough	Vaccine, antibiotics
Influenza virus	Virus	Droplet, fomites	Fever, cough, myalgia	Vaccine, antivirals
SARS-CoV-2	Virus	Droplet, airborne	Fever, cough, respiratory distress	mRNA vaccine, supportive care
Histoplasma capsulatum	Fungus	Inhalation of spores	Fever, cough, headache	Antifungals (if needed)

Summary of Key Points

• Respiratory infections can be caused by bacteria, viruses, or fungi, each with distinct transmission routes, symptoms, and treatments.

• Vaccination is a critical preventive measure for several respiratory pathogens (e.g., influenza, pertussis, pneumococcus, diphtheria, COVID-19).

• Antibiotic resistance is an emerging concern, especially for Streptococcus pneumoniae and Staphylococcus aureus.

• Opportunistic infections are more common in immunocompromised individuals and often require specialized treatment.

• Fungal respiratory infections are less common but can be severe in certain populations.

Additional info: This summary integrates and expands upon the provided lecture notes, adding definitions, examples, and a comparison table for clarity and exam preparation.