Chapter 13 Viruses and Prions

The term “virus” is Latin for ______

poison

Why did it take so long for viruses to be discovered? What important piece of technology allowed for the discovery of viruses?

viruses are incredibly small; it took the invention of the electron microscope to be able to see them

Explain the following sentence: Viruses are obligate intracellular parasites

This means that viruses MUST be inside of a living cell to replicate themselves. They “hijack” the cell meaning that they use all of its enzymes, nucleotides and ribosomes to make copies of itself

If viruses aren’t cells, what term to we use to describe them?

infectious particles

If viruses aren’t cells, what term do we use to describe them?

viruses are either active or inactive/infectious or non-infectious

Host Range

the number of species the virus can infect

Cell specificity

the type of cell the virus infects, viruses always attack a certain type of cell

Capsid

the protein coat of a virus, the purpose of the capsid is to protect the nucleic acid inside

Capsomere

protein subunits that make up the capsid

Naked virus

a virus comprised only of a capsid and nucleic acid

Enveloped virus

a virus that has an outer lipid coating

Bacteriophage or phage

a virus that infects bacteria

What two structures do ALL viruses have?

Capsid

Nucleic acid (either DNA or RNA)

What are two additional structures that some viruses MAY have?

protein spikes

envelope

What is the purpose of protein spikes that may be present on a virus?

They allow specific attachment to the host cell via a “lock and key” mechanism. They fit into certain receptors on the host cell’s surface

What are two purposes of the viral envelope?

1. Disguise - It makes the virus look like one of its host cell so it takes longer for the immune system to detect it

2. The envelope can fuse directly with the cell membrane of the host cell and this allows the virus to gain entry to the cell easily

What type of nucleic acid may viruses carry?

either DNA or RNA never both

Relatively speaking how large is a viral genome compared to a bacterial genome?

very very tiny; the viral genome only had a few genes at most

Hepatitis A

acute hepatits

Hepatitis B

chronic hepatitis eventually cirrhosis liver cancer

Hepatitis C

Chronic hepatitis and eventually cirrhosis/liver cancer

Varicella zoster

chicken pox and later possibly shingles

Herpes simplex 1 

weeping lesions (pox) above the diaphragm such as cold sores

Herpes simplex 2

weeping lesions (pox) below the diaphragm such as genital herpes

Adenovirus

upper respiratory infections

Reovirus

lower respiratory infection

Picornavirus

polio

Rhinovirus

common cold

Influenza

influenza

Epstein -Barr

infectious mononucleosis

HIV

AIDs

Which of the hepatitis viruses cause an acute illness but does not leave long term damage

Hepatitis A

We have a vaccine against which hepatitis virus

Hepatitis B

What is retrovirus

it contains RNA as it genetic material. It uses the enzyme reverse transcriptase to convert RNA into DNA which can then enter the lytic or latent cycle

What is the function of the enzyme “reverse transcriptase”? What drug is effective at inactivating reverse transcriptase?

it catalyzes the reverse transcription of the viral RNA to DNA

5 steps of viral multiplication and explain what is happening at each step

1. Adsorption - when the virus specifically attaches to its host cell

2. Penetration - when the virus enter the cell via fusing with the cell membrane or by endocytosis

3. Synthesis - this is where all of the machinery to the host cell is hijacked by the virus, RNA polymerase is used to produce viral mRNAs and host ribosomes are used to produce viral proteins

4. Assembly - where all of the newly mod viral components are assembled into complete viral particles

5. Plelease - where the host cell lysis and releases all of the newly made virus particles (up to 100,000 of them)

What are the two ways a virus can penetrate a host cell?

Endocytosis

Direct fusion of the viral envelope with the cell membrane

What is the time range of the typical viral replication cycle

anywhere from 8-36 hours

What are three ways viruses are released from host cell?

1. Budding off and taking some of the cell membrane with them

2. Exocytosis

3. Lysing the cell due to the sheer number of new viruses

What is CPE (the cytopathic effect)

virus - infected cell looks clumpy under a microscope helping pathologists identify viral infection

What is the difference between a lytic and a latent virus?

A lytic virus replicates right after infection while a latent virus hides its DNA in the host’s chromosome and may later reactivate causing outbreaks

Give four examples of lytic viruses

1. hepatitis A

2. rhinovirus

3. influenza

4. adenovirus

Give six examples of latent viruses

1. HIV 

2. varicella zoster

3. herpes simpler I and II

4. hepatitis B and C

5. epstein - barr

6. HPV

Why is it very difficult culture viruses in a laboratory?

Because viruses can only replicate WITHIN living cells a cell culture facility is needed to replicate viruses. This type of facility is very expensive

What are viral plaques?

Viral plaques are clear zones on an agar plate where viruses have killed bacteria showing areas of viral replication

List three statements that summarize the medical importance of implication of viruese:

1. most viruses cause acute illnesses that are mild in nature and do NOT require hospitalization

2. those that DO require hospitalization have high mortality rates especially in the already immnocompromised

3. some viruses may leave the patient with long term disability (such as polio)

What are oncoviruses

These are latent viruses where the viral DNA preferentially inserts into genes that control cell division. When these genes are disrupted, they can cause the cell to replicate out of control. This can produce a tumor which can be cancerous.

List three oncoviruses and the cancers to which they are linked

1. Human papillomavirus (HPV): has been linked to cervical cancer
2. Epstein-Barr virus: can cause Burkitt's lymphoma
3. Hepatitis B and C are linked to liver cancer

Are oncoviruses lytic or latent viruses? How do we know this?

They are latent. This is shown by the fact that they have to insert their DNA into the DNA of the host to have the cancer causing effect.

What are prions and what is the category of diseases they cause?

Prions are the smallest infectious particles discovered to date. They cause a category of diseases called spongiform encephalopathies.

What is especially dangerous about prions when considering patient contact and infection control?

prions cannot be destroyed by sterilization

List four prion diseases

1. Creutzfled-Jacob Disease
2. Kuru
3. (FFI) Fatal Familial Insomnia
4. Mad Cow Disease

What is the difference between the following types of ways patients can acquire prion diseases? Rank them in order of most commonly seen to least commonly seen.

1. Sporadic: most common; no known cause
2. Hereditary: second most common; inherited from family and due to a defective gene
3. Acquired: rarest; person is infected by prion from an outside source

What are three types of prion diseases that can occur in animals

1. Mad Cow Disease-bovine spongiform encephalopathy
2. Mad Cat Disease-feline spongiform encephalopathy
3. Scrapie-can occur in sheep and goats

Are antibiotics effective against the treatment of viral diseases? Why or why not?

No - only effective against bacterial infections

What is the MOST EFFECTIVE way to treat viral disease

vaccinated - stop virus BEFORE it can infect cells

Name two other ways that viral disease/symptoms may be treated

1. With an anti-viral drug such as Acyclovir or Tamiflu
2. With synthetic interferon
Most effective is preventative vaccination.