Respiratory System Infections Overview

Respiratory System Infections

• Respiratory infections are more common than infections of other organs.
• They are the leading cause of lost workdays.
• Most respiratory infections affect the upper respiratory tract, and are usually caused by viruses.
• Upper respiratory tract infections involve the nose, larynx, trachea, and main bronchi.
• Lower respiratory tract infections involve the bronchi, lungs, and terminal bronchi.
• These infections can range from trivial and mild to serious conditions with high morbidity and mortality rates.

Pathology of Pulmonary Sepsis (Upper Respiratory Tract)

• Common symptoms are time off work, trivial or mild, and transient.
• This is primarily a viral infection.
• Structures involved include the nose, sinuses, larynx, and trachea.

Pathology of Pulmonary Sepsis (Lower Respiratory Tract)

• Lower respiratory tract infections can be serious.
• Factors causing lower respiratory infections include bacteria, viruses, atypical organisms, and fungi.
• These infections can also be caused by irritants.
• Structures involved include the bronchi, terminal bronchi, and lung parenchyma.

Respiratory Tract Defenses

• Pneumonia occurs when the respiratory tract defenses are impaired.
• Potential causes include a weakened cough reflex (coma, anesthesia, drugs), aspiration, impaired nasal hair function, inhibited cilia function, compromised secretion of IgA antibodies, reduced alveolar macrophage activity (tobacco smoke, alcohol, excess oxygen), and accumulation of secretions (cystic fibrosis, bronchial obstruction).
• The alveoli can also be affected by fluid buildup (surfactant, immunoglobulins, complement).

Types of Infections

• Infections can be primary (viral, bacterial, mycoplasma, fungal), secondary bacterial (following a viral infection), or secondary to irritants.
• Upper respiratory tract infections (like the common cold) commonly involve different rhinovirus serotypes causing inflammation of the eyes and throat, and often watery discharge. Secondary infections can result in a purulent green discharge. Other infections in this region include viral sore throat (adenovirus), pharyngitis, and conjunctivitis.
• Bacterial infections of the upper respiratory tract are less common in developed countries and are often secondary to conditions that depress resistance (viral infections, chronic bronchitis, and bronchiectasis). They can also occur in healthy individuals as a result of bacteria colonizing sensitive areas like the nose and throat (streptococcus pyogenes). Acute laryngitis or epiglottitis can result from Haemophilus influenzae type B or streptococcus pyogenes, causing swelling and breathing difficulties.

Lower Respiratory Tract Infections

• Pneumonia is the infection of alveolar spaces. The host reaction causes exudate and results in consolidation.
• Morphology types include bronchopneumonia and lobar pneumonia, with many organisms present. The pattern can be radiologically confluent, indicating either bronchopneumonia or lobar infection.

Bronchopneumonia

• Etiology: Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, Staphylococcus pneumoniae, Klebsiella, Pseudomonas aeruginosa, and coliform bacteria are common causative agents.
• Inflammation begins in the bronchi. Polymorphs and fibrin spread to adjacent alveoli, leading to patchy consolidation, often widespread and bilateral. It rarely resolves without fibrosis.

Lobar Pneumonia

• Etiology: Streptococcus pneumoniae is the primary cause in 90-95% of cases.
• Inflammation begins in the alveoli. Exudate flows to bronchioles and alveoli, spread luminally, containing fibrin, edema fluid, and polymorphs. It affects the entire lobe, and typically resolves.

Pneumonia X-rays

• X-rays showcase characteristic changes associated with pneumonia.

Importance of diagnosis

• Identifying the causative organism (culture and sensitivity) is critical for appropriate treatment of pneumonia.

Aetiology of Pneumonia.

• Aetiology can be bacterial, fungal, viral (see below), aspiration, radiation or allergic mechanisms.

Pneumonia: Clinical Setting

• Several factors determine the clinical setting of pneumonia (e.g., community-acquired, nosocomial, gram-negative rods, Staph. Aureus.).
• The presence of risk factors (immunocompromised hosts) can lead to serious complications. Invasive procedures increase infection risk.

Community-Acquired Pneumonia

• Typically follows a viral infection.
• Symptoms include abrupt onset with high fever, chills, pleurisy, and mucopurulent cough.
• Chronic diseases (heart failure, COPD, diabetes) and immunodeficiency increase the risk.
• Lobar or bronchopneumonia presentations are common.
• Sputa may exhibit numerous neutrophils and/or gram-positive diplococci.
• Blood cultures may show bacteria in 20-30% of cases.
• Pneumonia responds to penicillin, although resistance is increasing.

Complications of Pneumonia

• Common complications include complete resolution with correct antibiotics, pleural adhesions, lung abscess, or sepsis.

Pleuracy

• Acute pleurisy can be a complication of lobar pneumonia, with the consolidation of lobes causing exudate on the pleural surfaces.

Viral Pneumonia

• Viral pneumonia (interstitial type) is usually milder and shows interstitial infiltrate, histiocytes, and lymphocytes but no alveolar exudate.
• It can be severe in immunocompromised individuals. Common viruses include Influenza, RSV, adenovirus, rhinovirus and SARS-CoV.

Pulmonary Tuberculosis (TB)

• Causative agent is Mycobacterium tuberculosis.

• Infection occurs via droplets.

• Primary TB (rare in modern times) initially forms a Ghon focus below the pleura, which triggers an immune response, leading to the formation of tubercles (epithelioid granulomas with caseation).

• Bacteria spreads to hilar lymph nodes. (Primary complex)

• Secondary TB is an reactivation of an existing primary infection (often associated with compromised immunity).

• Symptoms include persistent cough, fever, anorexia, weight loss, and possible haemoptysis.

• Bacteria can spread throughout the lungs or the body (miliary TB).

Study Guide:

• Review defense mechanisms of the respiratory system.
• Analyze the pathogenesis and morphology of bronchopneumonia and lobar pneumonia.
• Compare and contrast viral and bacterial pneumonia.
• Summarize the aspects of pulmonary tuberculosis.

This quiz delves into respiratory system infections, focusing on both upper and lower respiratory tract infections. It highlights their causes, typical symptoms, and the impact of these infections on health and work productivity. Gain a deeper understanding of how these infections can vary from mild to serious conditions.