Respiratory: Pathology - Pulmonary infections

Question 1

Five common and one uncommon causative pathogens in community-acquired pneumonia. Which is most common?

Answer 1

1. Streptococcus pneumoniae (most common)
2. Haemophilus influenzae
3. Moraxella catarrhalis
4. Staphylococcus aureus
5. Legionella pneumophila

Rarely:
1. GNRs (e.g. Klebsiella, Pseudomonas)

Question 2

Three causative pathogens of atypical community-acquired pneumonia. Which of these is most common?

Answer 2

1. Mycoplasma pneumoniae (most common)
2. Chlamydia species
3. Coxiella burnetti (Q fever)
4. Viruses

Question 3

Which viruses can cause atypical pneumonia in children vs adults?

Answer 3

Children: RSV, parainfluenza
Adults: influenza A and B, adenovirus, SARS

Question 4

Three groups of typical causative organisms in hospital-acquired pneumonia

Answer 4

1. Enterobacteriaceae (Serratia marscenens, Klebsiella, E. coli)
2. Other GNRs including Pseudomonas
3. Staphylococcus aureus (usually penicillin-resistant)

Question 5

Typical causative organisms in aspiration pneumonia

Answer 5

Anaerobic oral flora (Bacteroides, Prevotella, Fusobacterium) mixed with aerobic species (Streptococcus pneumoniae, Staphylococcus aureus, Haemophilus influenza, Pseudomonas)

Question 6

Three typical causative organisms in chronic pneumonia

Answer 6

1. Nocardia
2. Actinomyces
3. Granulomatous (e.g. due to TB and atypical mycobacteria, Histoplasma capsulatum)

Question 7

Typical organism mix in necrotising pneumonia and lung abscess

Answer 7

Anaerobic +/- aerobic mixed infection

Question 8

Six causative organisms of pneumonia in immunocompromise

Answer 8

1. CMV
2. Pneumocystis jiroveci
3. Mycobacterium avium complex
4. Invasive aspergillosis
5. Invasive candidiasis
6. Other “typical” organisms

Question 9

Give two examples of causative organisms in pneumonia for which there is a vaccine available

Answer 9

Streptococcus pneumoniae
Haemophilus influenza

Question 10

What type of bacteria (Gram stain, morphology) is Haemophilus influenza?

Answer 10

Gram negative coccobacillus

Question 11

What is the most common bacterial cause of COPD exacerbation?

Answer 11

Haemophilus influenzae

Question 12

What other (important to recognise) condition does Haemophilus influenzae cause?

Answer 12

Life-threatening meningitis in young children

Question 13

What other condition may be caused by Moraxella catarrhalsi?

Answer 13

Otitis media in children

Question 14

Which organism confers a higher risk of complications of pneumonia?

Answer 14

Staphylococcus aureus

Question 15

Which causative organism more commonly causes pneumonia in IVDU?

Answer 15

Staphylococcus aureus

Question 16

What is the fatality rate of Legionella pneumonia?

Answer 16

Up to 50%

Question 17

What are the two patterns of distribution seen in pneumonia? Give a brief description of each

Answer 17

Bronchopneumonia: patchy exudative consolidation of lung parenchyma
Lobar pneumonia: fibrinosuppurative consolidation of large portion or entire lobe

Question 18

Describe the macroscopic and histological changes seen in bronchopneumonia?

Answer 18

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

Question 19

What organisms most commonly cause bronchopneumonia?

Answer 19

Staphylococci
Streptococci
Pneumococci
H. influenzae
Pseudomonas aeruginosa
Coliform bacteria

Question 20

Describe the four stages of response seen in lobar pneumonia

Answer 20

1. Congestion: vascular engorgement and accumulation of intra-alveolar fluid occurring within first 24hrs
2. Red hepatisation: massive confluent exudation (with neutrophils, RBCs and fibrin) produces gross appearance that is liver-like (hence “hepatisation”; looks red, firm, airless)
3. Grey hepatisation: RBCs disintegrate but fibrinosuppurative exudate persists, gross appearance is now greyish-brown and dry
4. Resolution: enzymatic and cellular degradation and clearance of exudate occurs; lung may undergo organisation leaving fibrosis and adhesions

Question 21

What organisms most commonly cause lobar pneumonia?

Answer 21

Pneumococci
Occasionally caused by Klebsiella, staphylococci, streptococci, Haemophilus)

Question 22

Four complications of pneumonia (more commonly lobar)

Answer 22

1. Lung abscess
2. Empyema
3. Organisation of exudate into fibrotic scar tissue
4. Bacteraemia, sepsis, and haematogenous seeding of other organs (including heart valves, pericardium, brain, kidney, spleen, joints)

Question 23

What was the term “atypical pneumonia” initially used to describe?

Answer 23

Acute febrile respiratory disease characterised by patchy inflammatory changes in lungs, largely confined to alveolar septa and pulmonary interstitium

Question 24

Why is “atypical” pneumonia so-called?

Answer 24

Produces atypical clinical presentation, with:
- Moderate sputum production
- No physical findings of consolidation
- Moderated WCC elevation
- Lack of alveolar exudate

Question 25

Describe the pathogenesis of atypical pneumonia

Answer 25

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

Question 26

What has a higher mortality rate: typical or atypical pneumonia?

Answer 26

Typical

Question 27

Three morphological features of atypical pneumonia

Answer 27

1. Predominance of interstitial pneumonitis: widened oedematous alveolar walls with mononuclear infiltrate 2. Formation of hyaline membranes (reflects diffuse alveolar damage) 3. Frequent superimposed bacterial infection

Question 28

What kind of virus is influenza?

Answer 28

Single-stranded RNA

Question 29

Describe the structure of influenzavirus and how this determines its classification and subtype

Answer 29

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)

Question 30

Two mechanisms of infection clearance in influenza

Answer 30

1. Cytotoxic T cells 2. Intracellular anti-influenza protein (Mx1) induced in macrophages by IFN-a and IFN-B

Question 31

What phenomenon typically causes influenza epidemics vs pandemics?

Answer 31

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)

Question 32

What is a lung abscess?

Answer 32

Local suppurative process in lung charactered by necrosis of lung tissue

Question 33

In what % of lung abscesses is an oral anaerobe exclusively isolated?

Answer 33

60% (remainder are mixed infections)

Question 34

Five mechanisms whereby bacteria causing lung abscesses are introduced

Answer 34

1. Aspiration (including dental sepsis, anaesthesia) 2. Antecedent primary lung infection (usually Staphylococcus aureus, Klebsiella, or type 3 pneumococcus) 3. Septic embolism 4. Neoplasia (causing obstruction) 5. Other (including direct trauma penetration, haematogenous spread, and spread from neighbouring organs)