Respiratory infections
Pneumonia
- Pneumonia can be defined as any infection of the lung parenchyma
- Pneumonia can result whenever lung defense mechanisms are impaired or the resistance of the host in general is lowered
- May be caused by a bacterial infection that following an URT viral infection
- Factors that increase susceptibility include;
- Decreased cough reflex leading to aspiration (seen in coma, aspiration, drug effects)
- Injury to the mucocilliary apparatus eg via smoking or other inhalations
- Decreased phagocytic or bactericidal function of the alveolar macrophage by alcohol, tobacco or oxygen toxicity
- Oedema or congestion (congestive heart failure)
- Secretion accumulation e.g. in CF or bronchial obstruction
- Defects in innate or specific immunity
- Decreased cough reflex leading to aspiration (seen in coma, aspiration, drug effects)
- Pneumonia can arise in seven distinct clinical settings;
- Community acquired acute pneumonia;
- Streptococcus pneumoniae
- Haemophilus influenzae
- Moraxella cararrhalis
- Staph aureus
- Legionella
- Klebsiella
- Pseudomonas
- Streptococcus pneumoniae
- Community acquired atypical pneumonia ;
- Mycoplasma pneumoniae
- Chlamydia
- Coxiella burnetti
- Viruses – respiratory syncytial virus, influenza, adenovirus, SARS
- Nosocomial pneumonia
- Gram negative rods belonging to the enterobacteriaeae e.g. Klebsiella, E.coli
- Pseudomonas spp
- Staph aureus – usually MRSA
- Aspiration pneumonia
- Anaerobic oral flora e.g. bacteroides and fusobacterium along with aerobes e.g. strep pneumoniae, staph aureus, H. influenzae, Pseudomonas
- Chronic pneumonia
- Nocardia
- Actinomyces
- Granulomatous; TB, Histoplasma capsulatum. Coccidioides spp
- Necrotising Pneumonia and lung abscess
- Anaerobic bacteria with or without a mixed aerobic infection
- Staph aureus, Klebsiella, Strep pyogenes
- Pneumonia in a compromised host
- CMV
- PCP
- Mycobacterium avium-intracellulare
- Invasive aspergillosis
- Invasive candidasis
- CMV
- Community acquired acute pneumonia;
Morphology
- Bronchopneumonia
- Marked by patchy exudative consolidation of lung parenchyma
- Most common causative agents are; staphylococci, pneumococci, H. influenzae, Pseudomonas and coliforms
- Grossly the lung shows dispersed elevated areas of consolidation and suppuration
- Histologically there is an acute suppurative exudation filling the airways, usually around bronchi and bronchioles
- Resolution of the infiltrate normally results in restoration of normal lung structure but organization with fibrous scarring can occur or aggressive disease can result in abscess formation
- Marked by patchy exudative consolidation of lung parenchyma
- Lobar pneumonia
- Involves a large portion or an entire lobe of the lung
- Most commonly caused by pneumococcus
- 4 stages of inflammatory response have been described;
- Congestion predominates in the first 24 hours
- Red hepatisation (consolidation) describes lung tissue with confluent exudates of neutrophils and RBC, giving a red, firm, liver like appearance
- Grey hepatisation follows with progressive disintegration of red cells and the persistence of a fibrinosupparative exudates, giving a grey brown gross appearance
- Resolution is the final stage in which consolidated exudate undergoes enzymatic and cellular degradation and clearance and the normal structure is restored
- Complications of non- interstitial pneumonia are;
- Abscess formation
- Empyema – spread of infection to the pleural cavity
- Organization of the exudate into fibrotic scar tissue
- Bacteraemia and sepsis with infection of other organs
Interstitial pneumonia
- Associated with ‘atypical’ pneumonias
- Patchy or lobular areas of congestion see WITHOUT the consolidation of lobar or bronchopneumonias
- The disease is interstitial in nature with the inflammatory reaction being localized with in the walls of the alveoli
- The alveolar septa are oedematous and have a mononuclear infiltrate of lymphocytes, histiocytes and occasional plasma cells
- Alveolar walls may be lined with hyaline reflecting diffuse alveolar damage
- Certain viruses cause necrosis of the bronchial or alveolar epithelium in severe infections
- Complications of interstitial pneumonia;
- Superimposed bacterial infection on top of a viral infection
- ARDS
Lung abscesses
- A lung abscess is an infection marked by localized suppurative necrosis of the lung tissue
- Commonly due to staphylococci, streptococci, gram negative species and anaerobes
- Mixed infections are frequent, reflecting aspiration of oral contents as a common cause
- Abscesses can be due to;
- Aspiration of infected material e.g. oropharyneal surgical procedures or aspiration due to dimished consciousness due to coma, drugs, anaesthesia or seizures – most commonly in the right lung reflecting the more vertical right bronchus
- Preceding primary bacterial infections
- Septic emboli from infected thrombi or right sided infective endocarditis
- Obstructive tumours
- Direct traumatic punctures
- Spread of infection from adjacent organs
Morphology
- Vary in size and number
- They contain variable mixtures of pus and air, depending on available drainage though airways
- Superimposed saprophytic infections are prone to flourishing within the necrotic material of the abscess cavity
- In chronic cases the abscess may be surrounded by a reactive fibrotic wall
Complications;
- Extension into the pleural cavity
- Haemorrhage
- Septic embolisation (may cause meningitis)
- Secondary amyloidosis (type AA)
Clinical manifestations
- Cough, fever and copious amounts of foul smelling purulent sputum
- Fever, chest pain and weight loss
- Clubbing
- When an abscess is discovered it is important to rule out underlying carcinoma as this may be present in 10-15% of cases
Bronchitis    Â
- Acute infection of the brochus
- Generally caused by viral infections – adenovirus, rhinovirus, RSV (children) and influenza
- Acute bronchitis can result from breathing irritating fumes, e.g. tobacco smoke or polluted air
- Can lead to a state of airway hyperactivity and asthma
- May lead to pneumonia
- Symptoms include malaise, retrosternal soreness and a dry tickly cough. Mucus may be produced, at first mucoid, becoming purulent
- Chest sounds clear
- May be chronic, which is characterized by persistent production of excess bronchial mucus
Bronchiolitis
- Inflammation of the terminal bronchioles
- Acute viral bronchiolitis is common in infants and most likely caused by RSV
- Typically an infant under two develops cough, wheeze, and shortness of breath over one or two days
- May be due to smoking resulting in chronic inflammation
- Airways can become obstructed due to;
- Secretion of mucus
- Luminal encroachment due to mucosal gland hypertrophy and mucosal oedema
- Bronchoconstriction
- Fibrotic airway distortion
- Plugging of small airways may give rise to focal lung atelectasis
- Prolonged inflammation may cause organization and fibrosis resulting in obliterative bronchiolitis and permanent lung damage