Viruses 4 - Classification

Question 1

What are acute self-limiting infections?
and
What are examples?

Answer 1

• virus causes rapid, self limiting infection
• infection cleared by the immune system, leading to recovery
• influenza virus
• rotavirus

Question 2

Whare are chronic infections with a carrier?

Answer 2

• viruses can persist fir many months in animal after symptoms have cleared such as FMDV in cattle = 24 months
• will eventually be eliminated
• doesn’t tend to kill the host so we have to

Question 3

What is a latent infection?

Answer 3

• Virus persists at very low levels
• limited recognition by immune system of infected cells
• no/few viral proteins expressed

Question 4

Where do latent infections replicate?

Answer 4

• in immune privileged sites
• special relationship between tissue and immune system

Question 5

Latent infections have the potential to reactivate - what does this mean and what is this usually due to?

Answer 5

• switch from latent to productive infection
• usually due to immunosuppression/external stimuli (sunlight/stress)

Question 6

Name some examples of latent viruses?

Answer 6

• IBR virus invades via nasopharynx or genital tract - proceeds to ganglionic neuron - latency

Question 7

What is a chronic infection?

Answer 7

• virus persists at very low levels in immune privileged sites, with slow growth

Question 8

Name toe examples of chronic infection?

Answer 8

• FeLV - retrovirus in oncornavirus group - causes cancer - slow, progressive disease, can persist after initial infection
• Jaagsiekte “chase disease” = slow continuous growth

Question 9

Classification is the same for all organisms how are they classed?

Answer 9

• species
• genus
• family
• order
• class
• phylum
• kingdom
• realm

Question 10

What is the Baltimore system of classification based on?

Answer 10

• based on system of messenger RNA (mRNA) synthesis

Question 11

The Baltimore system of classification has 7 groups what are the specifications for each of these groups?

Answer 11

• group I = double stranded DNA viruses - DsDNA
• Group II = single stranded DNA viruses ssDNA
• Group III = double stranded RNA viruses dsRNA
• Group IV = positive sense single stranded RNA viruses = ssRNA (positive sense)
• group V = negative sense single stranded RNA viruse = ssRNA (negative sense)
• Group VI = single stransed RNA viruses with DNA intermediate in lifecycles = ssRNA (positive sense, reverse transcribing)
  Group VII = double stranded DNA viruses with an RNA intermediate in their lifecycle = dsDNA (reverse transcribing)

Question 12

What is Avian Influenza?

Answer 12

• 7 genera but only 1 (A) known to infect birds
• highly contagious disease if birds - domestic and wild
• significant economic impact
• potential zoonosis

Question 13

What is the mortality rate in affected flock? (AI)

Answer 13

• up to 50% mortality

Question 14

How is AI spread?

Answer 14

• spread by migratory wild birds

Question 15

What are the two forms of AI?

Answer 15

• highly pathogenic
• low pathogenic

Question 16

How do you determine if AI is highly or low pathogenic?

Answer 16

• determined by IVPI (intravenous pathogenicity index)

Question 17

How do you asses Intravenous pathogenicity index?

Answer 17

• inoculate virus into wing vein of 10 x6 (4-8) week old chicks
• examine every 24hrs for 10 days
• score number if birds on symptoms = normal, sick, paralysed or dead
• add up score after 10days = have an index
• IVPI > 1.2 = HPAI (max 3)
• or H5 or H7 with specified amino acid at Haemagglutinin cleavage site)

Question 18

Group VI -Group VI viruses have a positive sense, single-stranded RNA genome (so normally would replicate in cytoplasm as can be read directly into the ribosome and translation into proteins) but what do they do instead?

Answer 18

• but replicate through a DNA intermediate.
• The RNA is converted to DNA by reverse transcriptase
• then the DNA is spliced into the host genome for subsequent transcription and translation using the enzyme integrase.

Question 19

Group VII - viruses have a double-stranded DNA genome, but unlike Class I viruses, how would they replicate?

Answer 19

• they replicate via a ssRNA intermediate.
• The dsDNA genome is gapped, and subsequently filled in to form a closed circle serving as a template for production of viral mRNA.
• To reproduce the genome, RNA is reverse transcribed back to DNA.

Question 20

What is Haemagglutinin?

Answer 20

• homotrimeric (3 identical polypeptide chains) glycoprotein on the surface of influenza viruses
• amino acid sequence of the connecting peptide that cleaves H - critical factor

Question 21

What is Haemagglutinin important for?

Answer 21

• HA important for entering cells

Question 22

What is Neuraminidase important for?

Answer 22

• NA important for exiting cells

Question 23

How is an genome released into cytoplasm?

Answer 23

• As pH dops , structure of HA changes, virus and ,lysosome membranes fuse and release genome into cytoplasm

Question 24

What is HA essential for? and how dies this work in AI?

Answer 24

• binding viruses to cell surface
• binds to sialic acid-containing receptors in target cells in respiratory tract
• results in endocytosis - internalisation of the virus
• creates endosome =- cell digests contents - dropping pH transformation to lysosome

Question 25

Where is the HA cleavage site?

Answer 25

• protein cleaved unit HA1 and HA2

Question 26

What happens if there is no cleavage?

Answer 26

• no fusion
• no cell penetration

Question 27

What does cleavage require?

Answer 27

• require specific enzyme

Question 28

What is LPAI HA?

Answer 28

• Single type of amino acid = arginine at the hemagglutinin cleavage site
• trypsin-like proteases catalyse the cleavage process

Question 29

What happens in LPAI HA acquires multiple amino acids?

Answer 29

• open to multiple proteases (e.g., furin, PC6)
• virus can then multiply systemically (proteases elsewhere)
• significant increase in replication , virulence, pathogenicity

Question 30

What does virulence mean?

Answer 30

• Virulence, a term often used interchangeably with pathogenicity, refers to the degree of pathology caused by the organism.

Question 31

What is the extent of virulence usually correlated with?

Answer 31

• The extent of the virulence is usually correlated with the ability of the pathogen to multiply within the host and may be affected by other factors

Question 32

What is the difference between ducks and chickens AI?

Answer 32

• Chickens – excessive, delayed cytokine response; poor cellular immune response
• Ducks - fast, lower cytokine response; strong cellular response

Question 33

What are the general carrier of AI?

Answer 33

• In general: waterfowl and waders – carriers – disseminate virus; get localised/widespread outbreaks in wild birds

Question 34

Where do outbreaks of AI in poultry mostly occur?

Answer 34

• Outbreaks in poultry mostly occurred in areas where the location of farms or trade areas overlapped with habitats for wild birds

Question 35

Where do outbreaks in wild birds mainly occur?

Answer 35

• Outbreaks in wild birds…mainly found in areas where food and shelters..” Spread to poultry via both poultry and wild birds
• Contact with wild birds alone seems to drive the outbreaks in wild birds.