Respiratory: Pathology - Pulmonary infections Flashcards
Five common and one uncommon causative pathogens in community-acquired pneumonia. Which is most common?
- Streptococcus pneumoniae (most common)
- Haemophilus influenzae
- Moraxella catarrhalis
- Staphylococcus aureus
- Legionella pneumophila
Rarely:
1. GNRs (e.g. Klebsiella, Pseudomonas)
Three causative pathogens of atypical community-acquired pneumonia. Which of these is most common?
- Mycoplasma pneumoniae (most common)
- Chlamydia species
- Coxiella burnetti (Q fever)
- Viruses
Which viruses can cause atypical pneumonia in children vs adults?
Children: RSV, parainfluenza
Adults: influenza A and B, adenovirus, SARS
Three groups of typical causative organisms in hospital-acquired pneumonia
- Enterobacteriaceae (Serratia marscenens, Klebsiella, E. coli)
- Other GNRs including Pseudomonas
- Staphylococcus aureus (usually penicillin-resistant)
Typical causative organisms in aspiration pneumonia
Anaerobic oral flora (Bacteroides, Prevotella, Fusobacterium) mixed with aerobic species (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenza, Pseudomonas)
Three typical causative organisms in chronic pneumonia
- Nocardia
- Actinomyces
- Granulomatous (e.g. due to TB and atypical mycobacteria, Histoplasma capsulatum)
Typical organism mix in necrotising pneumonia and lung abscess
Anaerobic +/- aerobic mixed infection
Six causative organisms of pneumonia in immunocompromise
- CMV
- Pneumocystis jiroveci
- Mycobacterium avium complex
- Invasive aspergillosis
- Invasive candidiasis
- Other “typical” organisms
Give two examples of causative organisms in pneumonia for which there is a vaccine available
Streptococcus pneumoniae
Haemophilus influenza
What type of bacteria (Gram stain, morphology) is Haemophilus influenza?
Gram negative coccobacillus
What is the most common bacterial cause of COPD exacerbation?
Haemophilus influenzae
What other (important to recognise) condition does Haemophilus influenzae cause?
Life-threatening meningitis in young children
What other condition may be caused by Moraxella catarrhalsi?
Otitis media in children
Which organism confers a higher risk of complications of pneumonia?
Staphylococcus aureus
Which causative organism more commonly causes pneumonia in IVDU?
Staphylococcus aureus
What is the fatality rate of Legionella pneumonia?
Up to 50%
What are the two patterns of distribution seen in pneumonia? Give a brief description of each
Bronchopneumonia: patchy exudative consolidation of lung parenchyma
Lobar pneumonia: fibrinosuppurative consolidation of large portion or entire lobe
Describe the macroscopic and histological changes seen in bronchopneumonia?
Patchy exudative consolidation of lung parenchyma:
- Multiple consolidated areas of acute suppurative inflammation, with neutrophil-rich exudate filling airspaces
- Frequent bilateral and basal (due to effect of gravity on secretions)
- Lesions appear slightly elevated, dry, granular, grey-red to yellow in colour, and are poorly demarcated
What organisms most commonly cause bronchopneumonia?
Staphylococci
Streptococci
Pneumococci
H. influenzae
Pseudomonas aeruginosa
Coliform bacteria
Describe the four stages of response seen in lobar pneumonia
- Congestion: vascular engorgement and accumulation of intra-alveolar fluid occurring within first 24hrs
- Red hepatisation: massive confluent exudation (with neutrophils, RBCs and fibrin) produces gross appearance that is liver-like (hence “hepatisation”; looks red, firm, airless)
- Grey hepatisation: RBCs disintegrate but fibrinosuppurative exudate persists, gross appearance is now greyish-brown and dry
- Resolution: enzymatic and cellular degradation and clearance of exudate occurs; lung may undergo organisation leaving fibrosis and adhesions
What organisms most commonly cause lobar pneumonia?
Pneumococci
Occasionally caused by Klebsiella, staphylococci, streptococci, Haemophilus)
Four complications of pneumonia (more commonly lobar)
- Lung abscess
- Empyema
- Organisation of exudate into fibrotic scar tissue
- Bacteraemia, sepsis, and haematogenous seeding of other organs (including heart valves, pericardium, brain, kidney, spleen, joints)
What was the term “atypical pneumonia” initially used to describe?
Acute febrile respiratory disease characterised by patchy inflammatory changes in lungs, largely confined to alveolar septa and pulmonary interstitium
Why is “atypical” pneumonia so-called?
Produces atypical clinical presentation, with:
- Moderate sputum production
- No physical findings of consolidation
- Moderated WCC elevation
- Lack of alveolar exudate
Describe the pathogenesis of atypical pneumonia
Organisms attach to upper respiratory tract epithelium and cause cell necrosis and inflammatory response
When process extends to alveoli, there is predominantly interstitial inflammation - however outpouring of fluid into alveolar spaces may produce CXR changes which mimic bacterial (lobar) pneumonia
What has a higher mortality rate: typical or atypical pneumonia?
Typical
Three morphological features of atypical pneumonia
- Predominance of interstitial pneumonitis: widened oedematous alveolar walls with mononuclear infiltrate
- Formation of hyaline membranes (reflects diffuse alveolar damage)
- Frequent superimposed bacterial infection
What kind of virus is influenza?
Single-stranded RNA
Describe the structure of influenzavirus and how this determines its classification and subtype
Single-stranded RNA virus, with RNA bound by nucleoprotein (determines classification: A, B or C)
Lipid bilayer envelope contains haemagglutinin and neuraminidase (determines subtype: H1-3, N1-2)
Two mechanisms of infection clearance in influenza
- Cytotoxic T cells
- Intracellular anti-influenza protein (Mx1) induced in macrophages by IFN-a and IFN-B
What phenomenon typically causes influenza epidemics vs pandemics?
Epidemics due to antigenic drift (mutations of haemagglutinin and neuraminidase)
Pandemics due to antigenic shift (haemagglutinin and neuraminidase replaced by recombination of RNA segments with those of animal viruses)
What is a lung abscess?
Local suppurative process in lung charactered by necrosis of lung tissue
In what % of lung abscesses is an oral anaerobe exclusively isolated?
60% (remainder are mixed infections)
Five mechanisms whereby bacteria causing lung abscesses are introduced
- Aspiration (including dental sepsis, anaesthesia)
- Antecedent primary lung infection (usually Staphylococcus aureus, Klebsiella, or type 3 pneumococcus)
- Septic embolism
- Neoplasia (causing obstruction)
- Other (including direct trauma penetration, haematogenous spread, and spread from neighbouring organs)
