answer from the powerpoint

Chapter 5

Viruses

Public Health 4030

Ebola

Smallpox

Chikungunya 1Ch. 05 - Viruses

Zika

Viruses as Infectious Agents

• Viruses are obligate intracellular parasites

• Can cause infection in animals, plants, and microbes

• Small and simple

• 85% of human diseases are a result of viruses

• Singular purpose – replicate themselves as many times as possible • How is this different than a “computer virus”? • Destroy cells in its pathway when replicating

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Viruses are a concern of public health officials

in the United States and around the world

• Bird flu, West Nile virus, Ebola, H1N1, etc.

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Smallpox has a long and interesting history

• Described as long as 10,000 years ago

• Evidence from pox scars on mummy of Pharaoh

Ramses V from 1157 B.C.

• Early example of biowarfare, British troops in 1760s

gave smallpox contaminated blankets to Native

Americans

• Edward Jenner developed

vaccine in 1796

– “first vaccine”

• The only disease agent to be

eradicated

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The two largest known pandemics

were caused by viruses

• 1918, “Spanish flu” pandemic – Strain of influenza that killed

more than 25 million people worldwide in one year • (3.6% of the worlds population)

• If it happened today – 234 million

• HIV/AIDS – First recognized in United States in early 1980s

– Now worldwide pandemic

– Devastating Africa

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Viruses were first described in scientific terms

as “filterable agents”

• Russian Dmitri Ivanowski, 1892, passed diseased tobacco leaf

extract through filters that removed bacteria, filtrate caused

disease (tobacco mosaic virus)

– Term filterable was eventually dropped as way to describe virus

• In 1935 Wendell Stanley crystallized TMV – Electron

microscope invented about the same time.

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Virus Structure

• viral genomes RNA or DNA

4 kinds:

• double-stranded (ds)

DNA

• double-stranded (ds) RNA

• single-stranded (ss) DNA

• single-stranded (ss)

RNA • viral proteins + genome = nucleocapsid (proteins surround the genome)

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Virus Structure

• Nucleic Acids

slapped cheek disease

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Virus Structure (cont.)

• Protein Coat

– Capsid is term for protein coat, made of capsomere

subunits

– Nucleocapsid proteins surround the viral genome

– There are 3 capsid types (helical, polyhedral, and complex)

that make up a virion (virion = complete virus particle)

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Virus Structure (cont.)

• Viral Envelopes

– Non-Enveloped: “naked” have protein – capsid - surface

– Enveloped: plasma membrane, containing host and viral

protein, surrounds the virion as it exits the cell

• Spikes: surface viral proteins used in attachment

– WHICH TYPE IS MORE STABLE IN ENVIRONMENT?

TEM of poliovirus Enveloped? - No

Figure 05.03C: TEM, influenza virus.

ENVELOPED? – Yes. How can you tell?

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Viral Classification

• Host Cell: (microbe, plant, or animal) – Bacterial viruses termed bacteriophage or phage

– What issue might we have with this?

• Clinical: organ or organ system - What issue might we have with this?

• Structure: by type of nucleic acid and shape

- Current classification start with this…

- Also use replication cycle to assist with classification…

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Viral Replication • There are 5 stages in a viral replication cycle

*This is generalized – there is considerable variation in virus strategies for replication

WHY IS UNDERSTANDING THIS IMPORTANT? – Disease presentation and transmission….

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Viral Replication (cont.)

• Adsorption:

– Virus docks with specific cell surface receptors

– Recognition specificity establishes host range

Figure 05.06: Virus “docking” with receptor molecules.

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Viral Replication (cont.)

• Penetration:

– Viruses have evolved various strategies for

delivering the genome to the cytoplasm

• The nucleic acid alone enters the cell and the virion

remains outside the cell (e.g., phage, polio).

• For some enveloped viruses (e.g., measles, mumps) the

viral and cell membranes fuse to release nucleocapsid.

• For other enveloped viruses (e.g., poxviruses, influenza)

the entire virion is endocytosed. The vesicle and viral

membranes fuse to release the nucleocapsid).

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Fig. 7. Strategies of viral penetration of cells

DNA/RNA alone

(e.g. phage, polio)

enveloped viruses (e.g., measles,

mumps, cold viruses); fusion of

envelope/membranes to release

nucleocapsid.

enveloped viruses; the entire virion

is endocytosed (“engulfed”).

• Penetration:

Viral infection cycles

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Viral infection cycles (cont.)

– varies by genome type (DNA or RNA, ss or ds)

– viral genomes contain genetic information for

making all virus parts

– viral parts (protein, nucleic acid) are made using

the host cell functions. (Exceptions: ssRNA

viruses)

• Genome replication:

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ds DNA

dsDNA

(using host cell functions)

messenger RNA

virus proteins

virus assembly

dsDNA (many copies)

• Genome replication:

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ds RNA

one RNA strand (a messenger RNA)

virus proteins

virus assembly

dsRNA (many copies)

(no dsDNA involved) Rotavirus dsRNA (most common cause of diarrhea in children)

• Genome replication:

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messenger RNA

virus proteins

virus assembly

ssDNA (many copies)

ssDNA

dsDNA

• Genome replication:

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virus proteins

virus assembly

ssRNA (many copies)

ssRNA

(no dsDNA involved)

(= messenger RNA)

• Genome replication:

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Retroviruses – HIV, SIV, HTLV-1….

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Some bacteriophage can cause two types of

infection: lytic or lysogenic

Figure 05.05: Replication cycle of a bacterial virus.

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Viral Replication (cont.)

• Assembly:

– Once the parts are made (proteins, genome, etc.),

they “self-assemble,” in an assembly line fashion, to make new virions

Figure 05.08: Viral assembly.

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Viral Replication (cont.)

• Release: – New virions exit the cell by one of several virus-specific mechanisms

• Phage and some animal viruses rupture (lyse) the cell (the host cell

dies).

• Some enveloped viruses are released by budding, or “endocytosis in reverse.”

– budding is gradual and does not necessarily kill the cell

Figure 05.09: Lysis of bacterial host cell by phage.

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Budding of enveloped viruses

Figure 05.10A: Release by budding.

Bird flu (H7N9) emerging from cell.

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Diagnosis of Viral Infection

• Unlike bacteria, viruses are usually not cultured for diagnosing illness

– Too difficult, expensive, and time-consuming

• Diagnosis frequently based on signs and symptoms

• Telltale rashes are particularly useful (chickenpox, etc.)

• Definitive tests – presence of antibody

• Past or present infection

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Diagnosis of Viral Infection

• Molecular technology is available

in specialized viral diagnostic

labs.

• Enzyme-linked immunosorbent

assays (ELISAs)

• Some designed as rapid tests

– Rapid influenza A & B test

• Nucleic acid-based diagnostics

– Uses some form of

polymerase chain reaction

(PCR)

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Diagnosis of Viral Infection cont.

Table 05.04: Most useful diagnostic tests used for viral infections.

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