Pathology of Pulmonary
Infections
Prof. Frank Carey
Topics..
 Pneumonia
infection – abscess/bronchiectasis
 Tuberculosis
 The immunocompromised host
 Chronic
Pneumonia
 Gr.
“disease of the lungs”
 Infection involving the distal airspaces
usually with inflammatory exudation
(“localised oedema”).
 Fluid filled spaces lead to consolidation
Classification of Pneumonia
 By
clinical setting (e.g. community
acquired pneumonia)
 By organism (mycoplasma, pneumococcal
etc)
 By morphology (lobar pneumonia,
bronchopneumonia)
Organisms
– influenza, parainfluenza, measles,
varicella-zoster, respiratory syncytial virus
(RSV). Common, often self limiting but can
be complicated
 Bacteria
 Chlamydia, mycoplasma
 Fungi
 Viruses
Lobar Pneumonia
 Confluent
consolidation involving a
complete lung lobe
 Most often due to Streptococcus
pneumoniae (pneumococcus)
 Can be seen with other organisms
(Klebsiella, Legionella)
Clinical Setting
 Usually
community acquired
 Classically in otherwise healthy young
adults
Pathology
 A classical




acute inflammatory response
Exudation of fibrin-rich fluid
Neutrophil infiltration
Macrophage infiltration
Resolution
 Immune
system plays a part antibodies lead
to opsonisation, phagocytosis of bacteria
Complications
 Organisation
(fibrous scarring)
 Abscess
 Bronchiectasis
 Empyema
Bronchopneumonia
 Infection
starting in airways and spreading
to adjacent alveolar lung
 Most often seen in the context of preexisting disease
Clinical Context
 COPD
 Cardiac
failure (elderly)
 Complication of viral infection (influenza)
 Aspiration of gastric contents
Organisms
varied – Strep. Pneumoniae,
Haemophilus influenza, Staphylococcus,
anaerobes, coliforms
 Clinical context may help.
Staph/anaerobes/coliforms seen in
aspiration
 More
Complications
 Organisation
 Abscess
 Bronchiectasis
 Empyema
Lung Abscess
 Localised
collection of pus
 Tumour-like
 Chronic malaise and fever
 Context - aspiration
Bronchiectasis
 Abnormal
fixed dilatation of the bronchi
 Usually due to fibrous scarring following
infection (pneumonia, tuberculosis, cystic
fibrosis)
 Also seen with chronic obstruction
(tumour)
 Dilated airways accumulate purulent
secretions
Tuberculosis
 Mycobacterial
infection
 Chronic infection described in many body
sites – lung, gut, kidneys, lymph nodes,
skin….
 Pathology characterised by delayed (type
IV) hypersensitivity (granulomas with
necrosis)
Organisms
 M.
tuberculosis/M.bovis main pathogens in
man
 Others cause atypical infection especially in
immunocompromised host. Pathogenicity
due to ability;


to avoid phagocytosis
to stimulate a host T-cell response
Immunity and Hypersensitivity
 T-cell
response to organism enhances
macrophage ability to kill mycobacteria

this ability constitutes immunity
 T-cell
response causes granulomatous
inflammation, tissue necrosis and scarring

this is hypersensitivity (type IV)
 Commonly
both processes occur together
Pathology of Tuberculosis (1)
 Primary TB


(1st exposure)
inhaled organism phagocytosed and carried to
hilar lymph nodes. Immune activation (few
weeks) leads to a granulomatous response in
nodes (and also in lung) usually with killing of
organism.
in a few cases infection is overwhelming and
spreads
Pathology of Tuberculosis (2)
 Secondary TB



reinfection or reactivation of disease in a
person with some immunity
disease tends initially to remain localised, often
in apices of lung.
can progress to spread by airways and/or
bloodstream
Tissue changes in TB
 Primary


Small focus (Ghon focus) in periphery of mid
zone of lung
Large hilar nodes (granulomatous)
 Secondary

Fibrosing and cavitating apical lesion (cancer
an important differential diagnosis
Why does disease reactivate?
 Decreased T-cell



function
age
coincident disease (HIV)
immunosuppressive therapy (steroids, cancer
chemotherapy)
 Reinfection
at high dose or with more
virulent organism
The immunocompromised host
 Virulent
infection with common organism
(e.g. TB)
 Infection with opportunistic pathogen




virus (cytomegalovirus - CMV)
bacteria (Mycobacterium avium intracellulare)
fungi (aspergillus, candida, pneumocystis)
protozoa (cryptosporidia, toxoplasma)
Diagnosis
 High
index of suspicion
 Teamwork (physician, microbiologist,
pathologist)
 Broncho-alveolar lavage
 Biopsy (with lots of special stains!)