Pathology
Lecture 36 Pulmonary Infections
1) Know the general types of pneumonia.
Community acquired pneumonias - 95% are due to viral, mycoplasma,
pneumococcal, or Legionella infections.
Nosocomial (hospital acquired) pneumonias - 1% of hospitalized patients, highest
risk is ICU, and 60% are due to gram-negative bacilli.
Aspiration pneumonia – breathing in infective material (frequently anaerobic
organisms) and/or gastric contents resulting in chemical pneumonitis, necrotizing
pneumonia, lung abscess, or empyema.
Pneumonia in immunocompromised hosts - HIV, leukemia, lymphoma,
chemotherapy, will marrow transplant, or solid organ transplant (opportunistic bugs).
2) Understand the pathogenesis and mechanisms involved in acquiring pneumonia.
Loss of Host Defense Mechanisms and Other Factors
Mechanism
Inhibition of cough reflex
Impaired Mucociliary
apparatus
Reduced phagocytosis and
bactericidal action
Bronchial obstruction
Decreased immunity
Artificial introduction
Secondary infection
Drug-resistant bacteria
Pathogenesis
Neuromuscular disease, drug overdose, incubation, coma, etc., aspiration.
Viral destruction of ciliated epithelium, smoking, genetic disease (immotile
cilia syndrome) preventing clearance of inhaled particles/microorganisms.
Alcohol, tobacco smoke, or anorexia interferes with alveolar macrophages,
impairing clearance of particles/organisms in alveoli.
Neoplasm, mucous plugging, etc. creates physical barrier preventing
clearance of microorganisms.
Immunocompromised hosts cannot fight off infection.
Intubation or contaminated respiratory care equipment.
Hematogenous spread from other sites.
Common in hospital environments.
3) Understand the differences between lobular and lobar pneumonia.
Lobular Pneumonia - Gross: patchy consolidation associated with airways, lesions
are elevated, dry, granular, and are firm to the touch. Micro: alveolar spaces are filled
by an exudate composed of neutrophils, fibrin, edema fluid, RBCs, and macrophages.
Lobar Pneumonia - Gross/micro: consolidation by fibrinopurulent material is
widespread involving entire lung lobes (rarely seen). Four stages: 1) congestion, 2)
red hepatization, 3) gray hepatization, and 4) resolution.
4) Know the common bacterial pathogens and the complications.
Common bacterial pathogens
a) Streptococcus pneumoniae (pneumococcus) most common in ambulatory patients.
b) Nosocomial infections - gram-negative bacilli are most common (Pseudomonas,
Klebsiella, Proteus, E. coli)
c) Staphylococcal and Haemophilus follow upper respiratory viral infections.
d) Legionella pneumophila is associated with aerosols and cooling systems.
Complications
a) Abscess - destruction of lung tissue and accumulation of neutrophils.
Pseudomonas aeruginosa, Staphylococcus aureus, and anaerobes contain enzymes
which liquefied lung tissue.
b) Empyema - purulent inflammation of the pleural space due to spread of infection.
c) Organization - formation of Masson bodies composed of fibroblasts, fibrin,
inflammatory cells, may mature leading scarring.
d) Bacteremic dissemination (sepsis) and spread to other organs.
5) Know the common viral pathogens and presentation. How does viral pneumonia
differ from bacterial pneumonia?
Common viral pathogens: influenza, adenovirus, measles, and varicella (most
common pneumonia in children, <10% in adults). Note: Mycoplasma and Chlamydia
are common in adults and clinically similar to viral pneumonia.
Presentation: fever, headaches, muscle aches, and dry, hacking nonproductive cough.
Mortality is <1% in most common competition is secondary bacterial pneumonia.
Differences from bacterial pneumonia: Gross: may be patchy or lobar, bilateral or
unilateral, congested, reddish blue color, and sometimes hemorrhagic. Micro:
mononuclear infiltrate, proteinaceous fluid in alveolar spaces, hyperplastic Type II
pneumocytes, hyaline membrane lined alveolar walls, sometimes necrosis of
bronchial alveolar epithelium (herpes, varicella, and adenovirus), and viral inclusions
(CMV, herpes, and measles).
6) Know the common fungal pathogens and their relevance and prognosis for the
patient.
Fungal pathogen
Coccidiodomycosis
Histoplasmosis
Tuberculosis
Relevance
>80% persons in endemic areas have a
positive skin test. Primarily granulomatous
inflammation, possibly purulent reaction.
Identified using silver stain as small (2-5 µ)
dark oval shaped forms producing epithelioid
cell granulomas and coagulative necrosis
Increased incidence due to AIDS epidemic and
emergence of drug-resistant strains. Caseating
granulomas with primary or secondary
complex, miliary spread, and
bronchopneumonia
Prognosis
Most primary infections are
asymptomatic, 10% have lung
lesions, pleuritic pain, and cough
Primary infection is usually
innocuous unless patient is
immunocompromised
Primary infection asymptomatic
or flu-like illness. Secondary
infection more severe, erosion
producing hemoptysis.
7) Understand the disease states associated with opportunistic infections and their
implications for the patient.
Disease
Pneumocystis carinii
Aspergillus species
Zygomycetes (Mucor
and Rhizopus)
Cryptococcus
Candida and
Torulopsis species
CMV/Herpes Simplex
Actinomyces and
Nocardia
Implications
Produces an alveolar infiltrate of foamy material and mononuclear cells.
Diagnosis is based on identifying the grouped, 4-5 µ, cup-shaped organisms.
Ubiquitous organism found in soil and commonly inhaled into the lungs. May
grow in old scars or invade parenchyma producing a necrotizing pneumonia.
Commonly invades veins and arteries producing hemorrhagic infarcts.
Like Aspergillus, it frequently invades arteries and veins. Hyphae are non-septate
and branch at 90°
Inhaled encapsulated yeast causing mild pulmonary symptoms and often
spreading to the CNS in immunocompromised patients.
Yeasts that are part of the normal flora but can produce bronchitis,
bronchopneumonia, hemorrhagic pneumonia, and acute abscess.
Can produce hemorrhagic interstitial pneumonias.
Filamentous, branching bacterium which can produce acute pneumonias with
rapid progression to abscesses.