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An Introduction to the Viruses
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Viral Components
• All viruses have capsids- protein coats that enclose & protect their nucleic acid
• Viruses may have a membrane envelope
• Nucleic acid genome: may be dsDNA, ssDNA, ssRNA, dsRNA
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Size Comparison of Viruses
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Viral Taxonomy• Main criteria presently used are structure,
chemical composition, and genetic makeup
• Currently recognized: 3 orders, 63 families, and 263 genera of viruses
• Family name ends in -viridae, i.e.,Herpesviridae
• Genus name ends in -virus, Simplexvirus• Herpes simplex virus I (HSV-I)
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Host range
• Spectrum of cells a virus can infect – cell must have a specific structure (receptor)
on its surface for viral attachment– cell must contain all of the enzymes and
materials needed to produce new virions
• May be one species or many
• May be one tissue or many within a host
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Animal Virus Replication
1. Adsorption
2. Penetration/uncoating of genome
3. Duplication/synthesis
4. Assembly
5. Release
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Medical Importance of Viruses
• Viruses are the most common cause of acute infections
• Several billion viral infections per year
• Some viruses have high mortality rates
• Possible connection of viruses to chronic afflictions of unknown cause
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Cytopathic effects: virus-induced damage to cells
1. Changes in size & shape
2. Cytoplasmic inclusion bodies
3. Nuclear inclusion bodies
4. Cells fuse to form multinucleated cells
5. Cell lysis
6. Alter DNA
7. Transform cells into cancerous cells
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Cytopathic Changes in Cells
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Other noncellular infectious agents
1. Prions - misfolded proteins, contain no nucleic acid– cause spongiform encephalopathies – holes in
the brain– common in animals
• scrapie in sheep & goats• bovine spongiform encephalopathies (BSE), aka “mad
cow disease”• humans – Creutzfeldt-Jakob Disease
2. Viroids - short pieces of RNA, no protein coat– only identified in plants, so far
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Diagnosis of viral diseases
• More difficult than other agents• Consider overall clinical picture• Take appropriate sample – Infect cell culture- look for characteristic
cytopathic effects– Screen for parts of the virus– Screen for immune response to virus
(antibodies)– PCR to detect presence of viral genome