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Pathology
Lecture 21 Pathology of Infectious Diseases
1) To outline a learning strategy for the study of infectious diseases in both the
pathology and microbiology courses. Identify the infectious agent and describe the
mode of action. Then describe the gross and histological changes that occur as a
result of infection. Present this information in a table for memorization.
2) To define the general pathologic characteristics of injury caused by the major
infectious agents.
General pathologic characteristics of injury
Release of chemoattractants for PMNs (e.g. peptides, cytokines,
complement components). Inflammation can harm the host by
interfering with the function of the involved organ or by the inability of
an enclosed space to accommodate it.
Infiltrates involve mainly macrophages, lymphocytes, and plasma cells.
Granuloma may form from aggregates of altered macrophages, giant
cells, and other mononuclear cells around a necrotic focus. Pathogens
multiply inside macrophages. CMI involving cytotoxicity is required.
Focal cellular destruction with little or no inflammatory response except
that secondary to the death of the cells. Cells may fuse to form giant
cells or alter their growth to form aggregates or neoplasms.
Necrotizing lesions are produced by direct invasion of host cells
followed by multiplication and destruction or through the production of
toxins, which may directly injure the membrane or organelles.
Chronic inflammation and scarring relative to the degree of necrotizing
injury or inflammatory response. Some tissues repair and regenerate.
Infectous agent Type
Pyogenic Bacteria
Suppurative
(polymorphonuclear)
Inflammation
Viruses, intracellular bacteria,
and intracellular parasites
Mononuclar and
Granulomatous Inflammation
Viral Infection
Cytopathic-Cytoproliferative
Inflammation
Many infectious agents
Necrotizing Inflammation
Specific pathologic characteristics of injury
Gross: large area of lung consolidated
Histo: alveoli filled with acute exudates evolving to chronic with notable
lack of destruction. Alveolar septa remain intact.
Gross: brain swollen with obvious exudates over the surfaces, meningeal
vessels engoged, crebellar herniation possible.
Histo: purulent exudates fills subarachnoid space, surrounds
leptomeninges, and blood vessels.
Gross: Multiple localized areas of caseous necrosis in the lung.
Histo: granulomas alternate with necrosis, chronic inflammation and
fibrosis acid.
Gross: brain shows focal, hemorrhagic lesion in the temporal lobe.
Histo: Neuronal and glial cell death with perivascular inflammation,
some cells show viral intranuclear inclusions.
Gross: Skin furuncle (boil). Microscopic lesions in the lung, kidney, or
bone walled off as a firm tender nodule.
Histo: microabcesses show focal destruction, purulent exudates, and
when well developed, fibrous organization at the periphery.
Gross: several small ulcers on cecum and colon. Liver abcess.
Histo: flask-shaped ulcers scattered between near normal mucosa.
Gross: lung is congested with many foci of hemorrhage, necrosis, and
purulent exudates.
Histo: acute formation of microabcesses with destruction of alveolar
septa. Chronically organization and scarring of injured areas occurs.
Infectious agent
Streptococcus pneumoniae –
Lobar pneumonia
Many infectious agents
Chronic inflammation+ scarring
S. pneumoniae, Neisseria
meningitides, or Haemophilus
influenzae – Acute purpulent
meningitis
Tuberculosis
Herpes encephalitis
Staphylococcus aureus –
Sthaphylococcal abcesses
Entamoeba histolytica –
amoebic colitis and liver abcess
Necrotizing Bronchopneumonia
3) To relate the pathologic findings to the presence of specific infectious agents.
Pathologic Findings
Obligate intracellular parasites of prokaryotic or eukaryotic cells, composed
of DNA or RNA and a protein coat, use cellular machinery to replicate, and
are submicroscopic.
Single-celled prokaryotic organisms containing DNA, RNA, ribosomes, and
protein synthesis machinery, no organelles, most free living, and visible
with a light microscope and stain.
Single or multicellular eukaryotic organisms with a rigid cell wall with a
unique molecular composition, mostly free living, and although some are
microscopic (silver stain) some can be seen with the naked eye.
Single or multicellular eukaryotic organisms ranging from simple amoebas
to complex cysts, cuticles, and other structures, free living and obligate
species exist, and have complex life cycles with different forms.
Infectious agent
Viruses
Bacteria
Fungi
Parasites
4) To use pseudomembranous colitis as an example of the process involved in the
discovery of a “new” infectious disease.
a. Clinical – define a new set of clinical and pathological findings: acute diarrhea,
abdominal pain, fever, leukocytosis, and occasionally megacolon (hospital).
b. Epidemiologic – define a unique set of circumstances including transmission:
syndrome recognized in the 50’s in relation to surgery and staph infection.
Increase in the 60’s and 70’s associated with antibiotic therapy (clindamycin).
c. Diagnostic – isolation of agent for subclinical detection in the population: studies
show association with toxin produced by Clostridium difficile and culture with
toxin detection allows differential diagnosis of pseudomembranous colitis.
d. Pathogenesis – systemic evaluation (experimental) to understand the progress of
the disease: Two toxins identified (A & B) one enterotoxic and the other
cytotoxic. Spore formation allows pathogen to resist antibacterial treatment.
e. Treatment – isolate etiologic agent and test for susceptibility to antimicrobics:
susceptible to oral vancomycin.