Microbial Diseases of the Respiratory System

Microbial 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 structures and associated infections. Understanding these divisions is crucial for identifying the origin and progression of respiratory diseases.

• Upper Respiratory Tract: Includes the nose, sinuses, pharynx, auditory tubes, middle ear, tonsils, adenoids, larynx, and lacrimal apparatus.

• Lower Respiratory Tract: Comprises the trachea, bronchial tree, and alveoli.

Diseases of the Upper Respiratory Tract

Bacterial Infections

The upper respiratory tract is a common site for infections, many of which are caused by bacteria. These infections can range from mild to life-threatening if not properly managed.

• Epiglottitis: Caused by Haemophilus influenzae type B. Presents with fever, hypoxia, tachycardia, sore throat, dysphagia, and sometimes a muffled voice. Rapid progression can lead to airway obstruction. The Hib vaccine has made this disease rare.

• Streptococcal Pharyngitis (Strep Throat): Caused by Streptococcus pyogenes (Group A, beta-hemolytic). Symptoms include whitish exudate on tonsils, sore throat, inflammation, and fever. Rapid strep tests aid diagnosis. Complications include otitis media, endocarditis, and scarlet fever.

• Scarlet Fever: Also caused by S. pyogenes, which produces an erythrogenic toxin. Characterized by a pinkish-red skin rash, high fever, and "strawberry tongue." Can lead to rheumatic fever.

• Diphtheria: Caused by Corynebacterium diphtheriae. Produces a potent toxin, transmitted via droplets. Symptoms include sore throat, fever, malaise, and a grey pseudomembrane in the throat that may obstruct the airway. Prevented by the DTaP vaccine.

• Otitis Media: Middle ear infection, often a complication of upper respiratory infections. Commonly caused by Streptococcus pneumoniae but can also be viral. Symptoms include ear pain, inflammation, and pus in the middle ear, which can cause a bulging tympanic membrane.

Viral Infections

• Common Cold: Caused by over 200 viruses, primarily rhinoviruses and coronaviruses. Symptoms include sneezing, nasal congestion, and secretion, but usually no fever. Transmission is via respiratory droplets and contaminated surfaces. Can lead to complications such as otitis media.

Diseases of the Lower Respiratory Tract

Lower respiratory tract infections are generally more severe and less common than those of the upper tract. They can be caused by bacteria, viruses, or fungi.

Bacterial Diseases

• Pertussis (Whooping Cough): Caused by Bordetella pertussis. Destroys ciliated cells in the trachea, leading to severe coughing fits. Has three stages: catarrhal (cold-like), paroxysmal (severe coughing), and convalescence. Prevented by vaccination.

• Tuberculosis (TB): Caused by Mycobacterium tuberculosis. Characterized by a waxy cell wall rich in mycolic acid, making it resistant to many treatments. Transmission is primarily airborne. Diagnosis involves the Mantoux skin test, and treatment requires prolonged multi-drug therapy. The BCG vaccine is used in many countries.

• Pneumonia: A general term for infections of the lungs, classified by the affected area (lobar, bronchopneumonia, interstitial). Most commonly caused by bacteria such as Streptococcus pneumoniae (typical pneumonia), Klebsiella pneumoniae, Mycoplasma pneumoniae, and Haemophilus influenzae. Symptoms include fever, chest pain, difficulty breathing, and characteristic sputum.

• Legionellosis (Legionnaire's Disease): Caused by Legionella pneumophila. Found in water systems, multiplies in macrophages, and is resistant to chlorine. Symptoms are similar to other pneumonias but more common in older men and those with chronic illnesses.

Viral Diseases

• Influenza (Flu): Caused by Influenzavirus (types A, B, C). Characterized by sudden fever, malaise, muscle aches, and cough. The virus mutates rapidly (antigenic drift and shift), requiring annual vaccine updates. Secondary bacterial infections are a major complication.

• Viral Pneumonia: Can be a complication of influenza, chickenpox, measles, or caused by emerging coronaviruses (SARS, MERS, COVID-19). Symptoms and severity vary widely.

• Respiratory Syncytial Virus (RSV): The most common cause of viral respiratory disease in infants. Nearly all children are infected by age 2. Can cause severe pneumonia in infants and older adults.

Fungal Diseases

• Histoplasmosis: Caused by Histoplasma capsulatum. Grows in macrophages and is associated with bat and bird droppings. Endemic in the Midwest and South of the US. Usually mild but can be severe in immunocompromised individuals.

• Coccidioidomycosis (Valley Fever): Caused by Coccidioides immitis. Found in dry, alkaline soils of the southwestern US. Symptoms include chest pain, fever, and cough. Severe cases resemble TB.

• Pneumocystis Pneumonia: Caused by Pneumocystis jirovecii. Common in the lungs but only causes disease in immunocompromised individuals, especially those with AIDS. Untreated, it is nearly always fatal.

• Blastomycosis: Caused by Blastomyces dermatitidis. Dimorphic fungus endemic to the Mississippi and Ohio River valleys. Symptoms resemble bacterial pneumonia, and abscesses may form. Most infections are asymptomatic.

Summary Table: Key Respiratory Pathogens and Diseases

Pathogen	Disease	Main Symptoms	Prevention
Haemophilus influenzae	Epiglottitis, Pneumonia	Sore throat, fever, airway obstruction	Hib vaccine
Streptococcus pyogenes	Strep throat, Scarlet fever	Sore throat, fever, rash	Hygiene, treat carriers
Corynebacterium diphtheriae	Diphtheria	Sore throat, pseudomembrane	DTaP vaccine
Streptococcus pneumoniae	Pneumonia, Otitis media	Fever, chest pain, rust sputum	Pneumococcal vaccine
Mycobacterium tuberculosis	Tuberculosis	Chronic cough, weight loss	BCG vaccine (not in US)
Influenzavirus	Influenza	Fever, malaise, cough	Annual flu vaccine
Histoplasma capsulatum	Histoplasmosis	TB-like symptoms	Avoid exposure
Pneumocystis jirovecii	Pneumocystis pneumonia	Pneumonia in immunocompromised	Prophylaxis in HIV/AIDS