Pathology of Respiratory Tract Infection

Question 1

What is pneumonia and what occurs?

Answer 1

Disease of the lungs - infection involving distal airspaces, usually with inflammatory exudation (“localised oedema”)

Fluid-filled spaces lead to CONSOLIDATION

Question 2

Different methods of pneumonia classification?

Answer 2

By clinical setting, e.g: CAP and HAP

By organism, e.g: Mycoplasma, Pneumococcal, etc

By morphology - by parts of the lungs invovled; e.g: lobar pneumonia, bronchopneumonia)

Question 3

Different organisms that can cause pneumonia?

Answer 3

Viruses - common and often self-limiting but can be complicated, e.g: influenza, parainfluenza, measles, varicella-zoster (often in children), Respiratory Syncitial Virus (RSV)

Bacteria - Chlamydia, Mycoplasma, Fungi

Question 4

What is and what causes lobar pneumonia?

Answer 4

CONFLUENT CONSOLIDATION involving a COMPLETE LUNG LOBE (alveoli will have fluid in them)

Most often due to Streptococcus pneumoniae (pneumococcus) but also with other, e.g: Klebsiella, Legionella

Question 5

Describe the pathology of pneumonia

Answer 5

Acute inflammatory response:

Exudation of fibrin-rich fluid

Neutrophil infiltration

Macrophage infiltration (clear debris)

Resolution

In early phase, no antibodies produced but later on they are; antibodies cause OPSONISATION (stick to bacteria and then them susceptible to phagocytosis)

Question 6

Complications of pneumonia?

Answer 6

ORGANISATION (fibrous scarring):

Inflammatory exudate not fully removed

Granulation tissue forms, then scar tissue which can lead to BRONCHIECTASIS

ABSCESS can lead to EMPYEMA (pus in pleural space)

Question 7

What is bronchopneumonia and who gets it?

Answer 7

Infection starting in airways and spreading to adjacent alveolar lung

Question 8

Who gets bronchopneumonia?

Answer 8

Most often seen in context of pre-existing disease and can start with bronchitis, e.g: person may have COPD, CHD/heart failure (elderly), complication of a viral infection (influenza), ASPIRATION of gastric contents (alcoholic)

Question 9

Organisms causing bronchopneumonia?

Answer 9

More varied: Strep. pneumoniae Hameophilus influenzae Staphylococcus Anaerobes Coliforms

Staph, anaerobes and coliforms are seen in aspiration, so often treated with > 1 antibiotic

Question 10

Describe a lung abscess, what it is, symptoms and context

Answer 10

Localised collection of pus that is tumour-like/presents as a mass lesion in lungs

Can cause chronic malaise and fever

Context: is seen in aspiration

Question 11

What is and what causes bronchiestasis? Problems?

Answer 11

Abnormal FIXED DILATATION of bronchi (dilated airways accumulate purulent secretions; if pathogens are inhaled into this, can causes recurrent infections) Usually due to fibrous scarring following infection, like pneumonia, TB, CF - physiotherapist tries to prevent bronchiectasis in CF )

Also seen with chronic obstruction, like a tumour, as there is scarring behind

Question 12

Describe tuberculosis

Answer 12

Mycobacterial infection that is a chronic infection describe in many body sites, e.g: lungs, gut, kidneys, lymph nodes, skin

Question 13

Pathological characteristic of TB?

Answer 13

Delayed (type IV) HYPERSENSITIVITY (granulomas - accumulation of macrophages and giant cells - with necrosis) - immune system causes most of the damage in TB, due to T cell stimulation

Question 14

Main organisms causing TB?

Answer 14

In humans:

• M. tuberculosis*
• M. bovine*

Question 15

Pathology of TB caused by other organisms?

Answer 15

Others - cause atypical infection, esp. in immunocompromised host

Pathogenicity is due to ability:

To avoid phagocytosis

To stimulate a host T-cell response, instead of an acute inflammatory response (causing granuloma formation and necrosis)

Question 16

Explain immunity and hypersensitivity in TB?

Answer 16

Commonly, both processes occur together:

T-cell response to organism ENHANCES MACROPHAGE ABILITY to KILL mycobacteria - ability constitutes IMMUNITY

T-cell response causes GRANULOMATOUS INFLAMMATION, tissue NECROSIS and SCARRING - this is HYPERSENSITIVITY (type IV)

Balance of the two will decide the no. of organisms found

Question 17

Describe primary TB

Answer 17

1st exposure and up to 5 yrs after (when there is no immunity/BCG):

Inhaled organism phagocytosed and carries to HILAR LYMPH NODES (enlarge). Immune activation takes a few weeks and leads to granulomatous response in nodes + lung, usually killing organism

Sometimes, infection is overwhelming and spreads (if genetically, T cells are not effective enough or individual is malnourished)

Question 18

Describe secondary TB

Answer 18

Reinfection/reactivation of disease in a person with SOME immunity

Disease tends to initially remain localised, leading to apical lesions (assman focus) but can progress to spreading via airways and/or bloodstream

Question 19

Tissue changes in primary TB?

Answer 19

Gohn focus (small) is an area of infection and caseous necroses in the PERIPHERY of the MID ZONE of the lung, beneath the pleura; this is found in TB

Large hilar nodes (granulomatous)

May see caseous necrosis

Question 20

Method of narrowing down to TB?

Answer 20

Acid/alcohol fast bacilli stain

Question 21

Tissue changes in secondary TB?

Answer 21

Fibrosing and CAVITATING apical lesion (cancer is an important differential diagnosis)

Question 22

What is miliary TB?

Answer 22

Widespread dissemination of the organism (white foci will be seen spread out)

Question 23

What is galloping consumption?

Answer 23

TB bronchopneumonia speed of progression

Question 24

Why does TB reactivate?

Answer 24

Due to decreased T-cell function with:

Age

Coincident disease (HIV) I

mmunocompromisation (cancer)

Immunosuppressive therapy (steroids, chemotherapy)

OR

Due to reinfection at high dose/with more virulent organism

Question 25

Describe infection in the immunocompromised host and give examples

Answer 25

Virulent infection with common organism, e.g: TB

Infection with OPPORTUNISTIC pathogen, e.g: Virus - cytomegalovirus (CMV)

Bacteria - Mycobacterium avium intracellulare is common in HIV Fungi - aspergillus, canidida, pneumocystic (PCP) Protozoa - cryptosporidia, toxoplasma

Question 26

How to make an infection diagnosis?

Answer 26

High index of suspicion

Teamwork

Broncho-alveolar lavage (BAL) - bronchoscope is passed through the mouth/nose into the lungs and fluid is squirted into a small part of the lung and then collected for examination

Biopsy (with many stains)

Question 27

Problems with cavities?

Answer 27

May be from previous TB; can fill with, e.g; aspergillus and cause problems like fatal haemorrhage

Antibiotics may not be able to get into the cavity

Question 28

Differences between lobar pneumonia and bronchopneumonia

Answer 28

Lobar pneumonia - affects the lobe of a lung (complete consolidation); in 90% of cases, the cause is Strep. pneumoniae, with other potential causes being Staph. aureus and Klebsiella

Bronchopneumonia - affects the airways and then the alveoli; the organisms causing this are more varied, like H. influenzae, Strep anaerobes and Coliforms