Week 7 - Viruses

Question 1

What are Koch’s Postulates?

Answer 1

1. Suspected pathogens must be present in all cases of the disease and absent from all healthy animals
2. The suspected pathogen must be grown in pure culture
3. Cells from a pure culture of the suspected pathogen must cause disease in a healthy animal
4. The suspected pathogen must be reisolated and shown to be the same as the original

Question 2

What are viroids?

Answer 2

1. First infectious molecule to be discovered - much smaller than viruses
2. Viroids are circular RNA molecules with no protein coat
3. They infect cells and get host cells enzymes to make more copies of their RNA
4. Viroids mainly infect plants (can sometimes infect animals)
   a) They can disrupt growth - abnormal or stunted growth

Question 3

What are prions?

Answer 3

1. Prions are slow-acting, infectious proteins that cause brain disease in mammals
   a) They are virtually indestructible
2. Prions propagate by causing normal proteins to be converted into a prion shape which makes the normal protein insoluble
   a) If this occurs in brain it leads to degenerative diseases
3. Prions are:
   a) Extrememely small (smaller than viruses)
   b) Not damaged or mutated by radiation
   c) Are made entirely of protein (no DNA/RNA)
   d) Are transmissible through blood
   e) Destroy brain cells by appearing to develop holes in brain tissue
4. Prions are essentially a misfolded version of a normal brain protein
   a) When a prion contacts a normally folded protein it induces the normal protein to change its shape abnormally
   b) This leads to prion aggregation causing cellular malfunctions and eventual degeneration of the brain
5. Examples of diseases caused by prions:
   a) Scrapie in sheep
   b) Mad cow disease
   c) Human BSE
   d) Creutzfold-Jacob disease

Question 4

What is a virus?

Answer 4

1. An ultramicroscopic infectious agent that replicates itself only within cells of living hosts (cannot replicate on their own)
2. It is a piece of nucleic acid (DNA or RNA) wrapped in a thin protein coat
3. Viruses are not cells and are not living organisms

Question 5

What is are the capsids and envelopes?

Answer 5

1. A capsid is the protein shell that encloses the viral genome
   a) Capsids are built from protein subunits called capsomeres or protomers
2. A viral envelope is a membranous envelope that surrounds the capsids to help infect hosts
   a) Not all viruses have envelopes
   b) Viral envelopes are derived from the host cell’s membrane and contains a combination of viral and host cell molecules
   c) Some viruses that contain envelopes include; influenza, HIV, HCV, COVID19
   d) Viral glycoprotein’s on the envelope bind to specific receptor molecules on the surface of a host cell

Question 6

How do viruses reproduce?

Answer 6

1. Viruses are obligate intracellular parasites, means they can only reproduce within host cell
2. Viruses have a host range - means a limited number of host cells that they can infect
   a) e.g. Influenza can only infect cells of the respiratory system while HIV can only infect cells of the immune system

Question 7

What are the phases of viral replication?

Answer 7

1. Attachment (absorption) - virus attaches to susceptible host cell by viral proteins attaching to host cell receptors
2. Entry (penetration) - of the virion or its nucleic acid
3. Synthesis - of viral nucleic acid and protein by cell metabolism as redirected by virus
4. Assembly - of capsids and packaging of viral genomes into virions (maturation)
5. Release - of mature virions from host cell by a number of different ways such as budding or bursting cell

Question 8

How do phages reproduce?

Answer 8

1. Two reproductive mechanisms:
   a) The lytic cycle
   b) The lysogenic cycle
2. Bacteriophages are the viruses that infect bacteria
   a) They have the most complex capsids found among viruses
3. Phages have an elongated capsid head that encloses their DNA
4. A protein tail attaches the phage to the host and injects the phage DNA inside

Question 9

Describe the lytic cycle:

Answer 9

1. A phage reproductive cycle that culminates in the death of the host cell
2. The lytic cycle produces new phages and digests the host’s cell wall, releasing the progeny viruses
3. A phage that reproduces only by the lytic cycle is called a virulent phage
4. Bacteria have defences against phages, including restriction enzymes that recognise and cut up certain phage DNA

Question 10

Describe the lysogenic cycle:

Answer 10

1. The lysogenic cycle replicates the phage genome without destroying the host cell
2. The viral DNA molecule is incorporated into the host cell’s chromosome
   a) This integrated viral DNA is known as a pro-phage
3. Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
4. Some environmental signals can trigger the virus genome to exit bacterial chromosome and switch to the lytic mode
   a) Phages that use both lytic and lysogenic cycles are called temperate phages

Question 11

How are viruses that infect animals classifed?

Answer 11

1. DNA or RNA

2. Single stranded or double stranded

Question 12

What is the Baltimore classification? Make diagrams for this

Answer 12

1. All viruses must produce mRNA that can be translated by cellular ribosomes
2. In this classification system, viruses are classified based off their unique pathways from viral genomes to mRNA
   a) Specific virus classes are based off the nature and polarity of their genomes (e.g. symmetry of capsid, whether it is naked or enveloped, what type of nucleic acid)

Question 13

How are infections categorised?

Answer 13

1. Route of transmission (how did you get it?)
2. Site of infection
3. Type of pathogen
4. Pathogenesis (type of disease/illness

Question 14

Describe measles:

Answer 14

1. Route of transmission: virus enters nose and throat by airborne transmission
2. Site of infection: respiratory system
3. Type of pathogen: Paramyxovirus (negative-strand RNA virus)
4. Pathogenesis:
   a) Often affects susceptible children as an acute, highly infections
   b) Symptoms include rash and bumps

Question 15

Describe rubella:

Answer 15

1. Route of transmission: virus enters nose and throat by airborne transmission
2. Site of infection: respiratory system
3. Type of pathogen: Positive-strand RNA virus of the togavirus group
4. Pathogenesis:
   a) Disease symptoms resemble measles but generally milder and less contagious

Question 16

Describe mumps:

Answer 16

1. Route of transmission: Spread by airborne droplets
2. Site of infection: Respiratory system
3. Type of pathogen: Paramyxovirus
4. Pathogenesis:
   a) Highly infectious
   b) Characterised by inflammation of the salivary gland

Question 17

Describe chicken pox (varicella):

Answer 17

1. Route of transmission: Spread by airborne droplets
2. Site of infection: Respiratory system which migrates to the skin surface causing painful skin eruption
3. Type of pathogen: Varicella-zoster virus (a herpesvirus)
4. Pathogenesis:
   a) Characterised by a systemic papular rash
   b) Highly contagious
   c) VZV virus establishes a lifelong latent infection in nerve cells

Question 18

Describe the common cold:

Answer 18

1. Route of transmission: Airborne droplets
2. Site of infection: Respiratory system
3. Type of pathogen: Rhinoviruses (positive-sense, single-stranded RNA viruses)
4. Pathogenesis:
   a) Symptoms include rhinitis, nasal obstruction, watery nasal discharges, and malaise
   b) Normally infections are of short duration

Question 19

Describe influenza:

Answer 19

1. Route of transmission:
2. Site of infection: Respiratory system
3. Type of pathogen: RNA virus of the orthomyxovirus group
   a) Three different types that infect humans (A, B, C)
   b) Contains an envelope with two glycoproteins
4. Pathogenesis:

Question 20

How does influenza create new viral diseases via antigenic drift?

Answer 20

1. Influenza viral strains can jump from different species to exchange genetic information to create new strains (e.g. swine flu)
2. This is performed by antigenic drift (random point mutations in the genes encoding HA and NA glycoproteins)
   a) If these mutations result in the change of an amino acid in a major antigenic site of either HA or NA, then antibodies that were previously effective will no longer be able to bind to this site, rendering them useless against a new viral strain
   b) These new strains cause viral epidemics within a population

Question 21

How does influenza create new viral diseases via antigenic shift?

Answer 21

1. When various influenza strains from different species infect the one host, then the various strains together lead to genetic reassortment of these strains to make a brand new virus strain with parts of each strain
   a) e.g. A pig gets human influenza strain from farmer, and bird influenza strain from a rooster leading to reassortment of avian and human strains
2. These new strains cause global epidemics and pandemics

Question 22

What is the treatment and prevention of influenza?

Answer 22

1. Prevention:
   a) Immunisation
   b) Careful worldwide surveillance
2. Treatment:
   a) Use of various drugs
   b) Most effective when administered early - will not cure disease but will shorten the length of disease
   c) Aspirin should be avoided

Question 23

Describe hepatitis viruses:

Answer 23

1. Liver inflammation caused by viruses or bacteria
   a) Sometimes results in acute illness followed by destruction of liver anatomy and cells (cirrhosis)
2. Types:
   a) Hepatitis A (infectious hepatitis) - causes mild, or rarely severe cases of liver disease
   i) Faecal oral transmission
   b) Hepatitis B (serum hepatitis) - causes acute, often severe disease that can lead to liver failure and death
   i) Mostly serum transmitted, sometimes faecal oral
   c) Hepatitis D - a defective virus that cannot replicate and express a complete virus unless the cell is also infected with hepatitis D
   d) Hepatitis C - produces a mild disease initially, but most individuals develop chronic hepatitis that can lead to chronic liver disease
   i) Has two glycoproteins that are stuck together and need to be displayed together
   ii) Large amount of variation - 6 subtypes of Hep C (some more difficult to treat)
   e) Hepatitis Evirus - causes an acute, self-limiting hepatitis that varies in severity

Question 24

Describe herpes:

Answer 24

1. Route of transmission:
   a) HSV-1: Spread via direct contact or through saliva
   b) HSV-2: Sexually transmitted
2. Site of infection:
   a) HSV-1: Infects epithelial cells around mouth and lips
   b) HSV-2: Body fluids from genitourinary tract and anogenital region
3. Type of pathogen:
   a) Herpes simplex 1 virus (HSV-1)
   b) Herpes simplex 2 virus (HSV-2)
4. Pathogenesis:
   a) HSV-1: Causes cold sores and lesions that heal without treatment in 2-3 weeks
   b) HSV-2: Causes painful blisters on penis of male and cervix, vulva, or vagina of females
   i) Currently incurable

Question 25

Describe human papillomavirus:

Answer 25

1. Route of transmission: Sexually transmitted
2. Site of infection: Basal epithelial cells through a break in the skin
   a) Virus entry: binds receptors and enters by receptor-mediated endocytosis
   b) Virus replication: life cycle linked to epithelial cell differentiation
   c) Latent period can be between 10-30 years
3. Type of pathogen: Non-enveloped icosahedral virus, 50-55 nm, 8kb dsDNA (circular)
   a) Replicates in nucleus
   b) Contains lytic and latency genes
4. Pathogenesis:
   a) Produce warts
   b) Can cause cervical neoplasia and progress to cervical cancer

Question 26

How is HPV diagnosed?

Answer 26

1. Pap smear to detect abnormal cells

2. Visual cervical exam detects warts

Question 27

What is the vaccine for HPV?

Answer 27

1. Two vaccines used:
   a) Gardasil (HPV 6, 11, 16, 18)
   b) Cervarix (HPV 16, 18)
2. Vaccinations of no benefit to those already infected
3. Vaccination of boys would only reduce HPV transmission by ~2%

Question 28

Describe HIV:

Answer 28

1. A virus (Human immunodeficiency virus) that causes AIDS
2. Two types:
   a) HIV-1 = more virulent form
   b) HIV-2 = less virulent and causes milder AIDs-like disease
3. Opportunistic infections such as pneumonia are common in AIDS patients
4. HIV pathogenesis:
   a) HIV infects cells that contain the CD4 cell surface protein (most commonly found on macrophages and T-helper cells)
   b) HIV essentially infects the immune system
   c) Can cause latent infection
   d) Infection leads to progressive decline in CD4 cells = cytokine production falls leading to reduction of the immune response (immunodeficiency)
5. Treatment:
   a) Four classes of drugs delay symptoms of AIDS and prolong life but do not cure AIDS
   b) There is no effective vaccine for HIV
   c) Preventing the spread of HIV infection requires education and avoidance of high-risk behaviour

Question 29

Describe norovirus:

Answer 29

1. Route of transmission: Transmitted in food and water
2. Symptoms:
   a) Common cause of gastrointestinal illness (vomiting, diarrhoea, malaise)

Question 30

Describe foodborne infectious diseases:

Answer 30

1. Large number of viruses cause different foodborne diseases
   a) e.g. enteroviruses such as poliovirus, norovirus, and hepatitis A are shed into food or water from faeces
   b) Noroviruses are responsible for most infections

Question 31

Describe rabies virus:

Answer 31

1. Route of transmission: Animal-transmitted viral disease
   a) Primarily as epizootic disease in animals but can be spread as zoonotic disease to humans
2. Site of infection:
   a) Infects CNS of warm-blooded animals
3. Type of pathogen: Rhabdovirus
   a) A negative-strand RNA virus that causes rabies
4. Pathogenesis:
   a) Enters the body through a wound or bite
   b) Virus proliferates in the brain and leads to fever, excitation, dilation of the pupils, excessive salivation, anxiety, and fear of swallowing
   c) Leads to death if no treated
5. Diagnosis:
   a) Rabies is diagnosed using tissue samples
6. Treatment and prevention:
   a) Victim can be passively immunised with rabies immune globulin
   b) Rabies virus vaccine also provides immunisation
   c) Rabies spread is prevented largely through immunisation of domestic animals

Question 32

Describe Hantavirus:

Answer 32

1. Route of transmission: Animal-transmitted disease (hantavirus-infected rodents) - inhalation of contaminated rodent faeces
2. Site of infection:
   a) Multiple sites dependent on disease caused; Hantavirus pulmonary syndrome (HPS) and Haemorrhagic fever with renal syndrome (HFRS)
3. Type of pathogen:
   a) Enveloped, segmented, negative-strand RNA virus
4. Pathogenesis of HPS:
   a) Sudden onset of fever, myalgia, thrombocytopenia, leukocytosis, and pulmonary capillary leakage
   b) Death occurs within several days in 35% of cases
5. Treatment:
   a) No treatment or vaccine exists

Question 33

Describe Yellow Fever:

Answer 33

1. Route of transmission: Arthropod-transmission - Mosquito bites
2. Type of pathogen: Yellow fever virus
3. Treatment and prevention:
   a) No antiviral drugs are effective
   b) Preventable by vaccination

Question 34

Describe Dengue Fever:

Answer 34

1. Route of transmission: Arthropod -transmission - mosquito bites
2. Type of pathogen: Dengue virus
3. Treatment and prevention:
   a) No antiviral drugs are effective
   b) No vaccine for dengue
4. Signs and symptoms:
   a) Fever and headaches
   b) Muscle and joint pain
   c) Severe abdominal pain
   d) Persistent vomiting
   e) Difficulty breathing
   f) Diarrhoea

Question 35

Describe Nile Virus:

Answer 35

1. Route of transmission: Arthropod-transmitted - mosquito bites
   a) Birds are natural reservoirs that pass it on to mosquitos
2. Type of pathogen: West Nile Virus
3. Treatment and Prevention:
   a) No antiviral drugs are effective
   b) No vaccine for West Nile Fever

Question 36

What is an emerging virus?

Answer 36

1. The causative agent of a new or previously unrecognised infection in a population
2. Zoonoses are infections of humans by viruses that pre-exist in stable relationships with nonhuman hosts
   a) Most emerging viruses come from zoonotic infections
3. There are four types of interaction between a virus and its host
   a) Stable in which both participants survive and reproduce
   b) Evolving in which host-virus interaction is unstable and unpredictable
   c) Dead-end interaction where virus is not transmitted to other members of the new host species
   d) Resistant in which the host blocks infection completely

Question 37

What are coronaviruses?

Answer 37

1. Enveloped RNA viruses that infect mammals and birds
   a) They have the largest RNA genome of out any viruses we know
2. Cause significant respiratory and gastrointestinal disease in humans and domestic animals
   a) They are known to cause common colds

Question 38

What is the life cycle of coronavirus?

Answer 38

1. Spike proteins bind to surface of our cells
2. Virus is then taken in by cell via endocytosis
3. Hemagglutinin then allows for change in pH of vesicle so viral particle can open up and viral RNA can be released
4. Since RNA is positive sense, it is passed through ribosomes to generate 2 proteins (1A, 1B) which allow for double membrane vesicles to form
5. Viral genomic replication then occurs
6. Viral mRNA synthesis which leads certain protein synthesis (e.g. spike proteins, membrane proteins)
7. Along with the ER there is formation of virus particles (membrane formation)
8. Viral particles are exocytosed out of cell

Question 39

What is COVID19?

Answer 39

1. SARS-CoV-2 is of the Coronaviridae family, genus betacoronavirus
   a) It is the 7th coronavirus known to infect humans
2. Structure:
   a) Has a phospholipid layer
   b) Has spike glycoprotein responsible for receptor binding and membrane fusion
   c) Has envelope protein important for virus infectivity
   d) Has a matrix glycoprotein which interacts with E protein to form viral envelope

Question 40

What is the life cycle of SARS-CoV-2?

Answer 40

1. Attachment between ACE2 on epithelial cells and spike protein on virus
2. Endocytosis - virus enters cell
3. Leads to fusion of vesicle and virion within cell
4. Leads to translation - non-structural polyproteins formed
5. Proteolysis occurs
6. Replication occurs by RNA-dependent RNA polymerase
7. Structural viral proteins are then translated from subgenomic viral mRNA and assembled into new virion
8. Virion is released and exits cell via exocytosis