Bacterial and Viral Diseases of the Respiratory System

Anatomy and Structural Components of the Upper Respiratory System

The upper respiratory system comprises several anatomical structures including the nasal cavity, the pharynx (commonly known as the throat), the larynx (or voice box), the trachea, and the bronchi. Within the nasal cavity, coarse hairs serve the vital function of filtering large particulate matter from the air as it enters the respiratory tract. The larynx contains the vocal cords and is protected by the epiglottis, which is a specialized flap of tissue designed to prevent fluids or food from entering the airway during swallowing. Further down, the trachea, also known as the windpipe, branches into the primary bronchi.

Detailed anatomical views identify various specific regions including the sinus, oral cavity, tongue, middle ear, and the auditory (Eustachian) tube with its corresponding opening. Other notable structures include the tonsils, the esophagus, and the spine. The tracheobronchial tree further bifurcates into the right and left primary bronchi, which then divide into secondary and tertiary bronchi, eventually leading to the terminal bronchioles. The carina marks the point of tracheal bifurcation. Surrounding the lungs are pleurae including the visceral pleura, the parietal pleura, and the pleural cavity. The diaphragm sits at the base of the thoracic cavity, facilitating the mechanics of breathing.

Anatomy and Function of the Lower Respiratory System

The lower respiratory system is primarily composed of the lungs. The bronchial tree is formed as secondary bronchi divide into progressively smaller bronchioles, a structure that significantly increases the surface area exposed to oxygen. The functional units for gas exchange are the alveoli. During this process, oxygen ($O_2$) diffuses into the blood while carbon dioxide ($CO_2$) diffuses from the blood into the alveoli. Alveolar structures include alveolar ducts and alveolar sacs, which are supported by elastic connective tissue and serviced by pulmonary arterioles, pulmonary venules, and lymphatic vessels.

While the nasal cavity and throat contain normal microbiota—which can sometimes include pathogenic microorganisms—the lower respiratory system is typically sterile. This sterility is maintained by the action of the mucociliary escalator, which works to remove inhaled microbes. The efficiency of this clearing mechanism is notably decreased by smoking and the inhalation of toxic fumes, which leads to a failure in clearing out microbes and increases the risk of infection.

Bacterial Diseases of the Upper Respiratory Tract

Pharyngitis, or a sore throat, is an infection of the pharynx that can be caused by bacteria, viruses, or a combination of both. A common bacterial form is Streptococcal pharyngitis (strep throat), which is caused by group A $\beta$-hemolytic Streptococcus pyogenes. This infection most commonly affects children between the ages of $5-15\,\text{years}$. Symptoms include an inflamed throat, swollen adenoids and lymph nodes in the neck, and tonsils that become tender and develop white, pus-filled lesions. Clinical presentation often involves chills, fever, headache, acute throat soreness, nausea, and vomiting. Notably, the absence of a cough and nasal discharge helps distinguish strep throat from the common cold. Diagnosis is achieved via throat swab, positive throat culture, or a rapid enzyme-labeled antibody screening test. Treatment requires immediate administration of penicillin; if delayed, S. pyogenes can trigger an immune response resulting in rheumatic fever in approximately $3\%$ of untreated cases.

Laryngitis involves an infection of the larynx leading to the loss of voice, frequently caused by Haemophilus influenzae, Streptococcus pneumoniae, or viruses individually or in combination. Epiglottitis is a more severe infection of the epiglottis caused by H. influenzae which can lead to airway closure and suffocation. Croup is a viral infection in children affecting the larynx and epiglottis. If infections spread further, they can manifest as sinusitis (sinuses), bronchitis (bronchi), tonsillitis (tonsils), or pneumonia (lungs).

Sinusitis, often referred to as Resdung, is caused in more than half of cases by Streptococcus pneumoniae, Moraxella catarrhalis, H. influenzae, Staphylococcus aureus, or Streptococcus pyogenes. Allergic rhinitis is a related diagnosis involving symptoms affecting the nose triggered by breathing in allergens like dust, animal dander, or pollen; it is typically treated with antihistamines or steroids. Bronchitis affects about $15\%$ of the general population in its chronic form, often linked to smoking, air pollution, or heredity. Causative agents include Streptococcus pneumoniae and Mycoplasma pneumoniae. These infections have the potential to spread to the alveoli, resulting in pneumonia.

Diphtheria is caused by Corynebacterium diphtheriae and is transmitted via droplets of respiratory secretions. The bacteria are infected with a prophage carrying an exotoxin-producing gene. The disease usually begins in the pharynx $2-4\,\text{days}$ after exposure. It causes damage to epithelial cells, where fibrins and blood cells combine to form a pseudomembrane that can block the airway passage. Although dangerous, it is easily prevented by immunization. Treatment involves antitoxins or antibiotics such as erythromycin or clindamycin.

Viral Diseases of the Upper Respiratory System

The common cold, or coryza, can be caused by any of approximately $200$ different viruses. Rhinoviruses are responsible for about $50\%$ of all colds and grow best at temperatures slightly below body temperature ($33-34\,^\circ\text{C}$). The incubation period is typically $2-4\,\text{days}$, and the second most common cause is coronaviruses. Symptoms include sneezing, nasal secretions, headache, cough, and congestion, typically lasting about one week. Transmission usually occurs through indirect contact, and treatments include antihistamines and interferon.

Parainfluenza is caused by the Paramyxovirus, leading to rhinitis (nasal inflammation), pharyngitis, bronchitis, and occasionally pneumonia. Clinical signs include harsh breathing sounds, a red throat, and a distinct barking cough. Recovery is usually rapid, occurring within a few days, and is mediated by the body's production of antibodies.

Bacterial Diseases of the Lower Respiratory Tract: Pertussis

Whooping cough, or Pertussis, is caused by Bordetella pertussis, an aerobic, Gram-negative, encapsulated coccobacillus that only infects humans. Mild forms can be caused by Bordetella parapertussis or B. bronchiseptica. The disease progresses through three stages: the catarrhal stage (resembling a cold with fever, sneezing, vomiting, and a dry cough), the paroxysmal stage (occurring a week later, characterized by mucus buildup in the trachea and bronchi, airway obstruction, and cyanosis or "bluing of the skin" due to low oxygen levels), and the convalescence stage (which can last months with a mild cough).

Bordetella pertussis does not invade tissues or enter the blood directly. Instead, it produces several virulence factors: endotoxins, exotoxins, and hemagglutinins. Hemagglutinins are surface antigens that allow the bacteria to attach to the cilia of epithelial cells in the upper respiratory tract. Prevention is achieved through the DTaP vaccine, which utilizes acellular pertussis cell fragments. Diagnosis is based on isolating the bacteria from the posterior nasal passage on blood agar, followed by serological tests. Treatment involves early administration of antitoxin, ampicillin, and supportive care such as suctioning, oxygen therapy, rehydration, and electrolyte balance.

Pneumonia and Legionnaires' Disease

Pneumonia is an inflammation of lung tissue that can be caused by bacteria, viruses, fungi, helminths, chemical substances, radiation, or allergies. Major bacterial causes include Streptococcus pneumoniae (Gram-positive encapsulated diplococci), Staphylococcus aureus, Klebsiella pneumoniae, and Mycoplasma pneumoniae, the latter of which creates exotoxins. Diagnosis is performed by culturing the bacteria. The primary drugs of choice for treatment are penicillin, azithromycin, or fluoroquinolones.

Legionnaires' Disease is caused by Legionella pneumophila, a Gram-negative, strict aerobe. It is transmitted when the organism, which grows in soil or water, becomes airborne and is inhaled as an aerosol. There is no person-to-person transmission. After an incubation period of $2-10\,\text{days}$, the disease presents with fever, chills, headache, diarrhea, vomiting, chest and abdominal pain, profuse sweating, mental disorders, and fluid in the lungs. Severe cases can lead to kidney failure and death. A milder, nonpneumonic form (Pontiac fever) presents with flu-like symptoms after a $48\,\text{h}$ incubation. Treatment includes azithromycin, fluoroquinolones, or erythromycin.

Tuberculosis (TB)

Tuberculosis is caused by Mycobacterium tuberculosis, which is an acid-fast, aerobic rod with a slow generation time of $12-18\,\text{h}$. It is transmitted through the inhalation of droplets from human to human. The bacteria are engulfed by white blood cells, where they develop slowly. Symptoms include liquid accumulation in the lungs, mirroring pneumonia. In patients with late-stage HIV infection, the M. avium-intracellulare complex is a common infecting agent. Treatment for TB is prolonged, typically requiring $3$ or $4$ drugs taken for at least $6\,\text{months}$. Multidrug-resistant (MDR) strains are increasingly common. Prevention in some regions is through the BCG vaccine (Bacillus of Calmette and Guerin), which uses a live, avirulent culture of Mycobacterium bovis.

Diagnosis of Tuberculosis involves several steps. Initial screening is performed with the Tuberculin skin test; a positive reaction indicates either a current or previous infection. This is followed by diagnostic imaging such as X-rays or CT scans, acid-fast staining of sputum, and bacterial cultures. Cultures must be maintained for at least $8\,\text{weeks}$ before a sample can be declared negative. Microscopic examination of M. tuberculosis may reveal characteristic corded growth patterns under a light microscope (LM) at a scale of $5\,\mu\text{m}$.