7. The Pathogenicity of Viruses - Ebola virus

Question 1

Ebola Virus

Answer 1

Ebola Virus

Question 2

What is the family name of viruses to which ebola virus comprises part of?

Answer 2

Filovirus

Question 3

What disease is caused by ebola virus infection?

Answer 3

Ebola Haemorrhagic Fever

Question 4

Describe the genome of the ebola virus.

Answer 4

Single stranded negative sense RNA approximately 19, 000 nucleotides in length.

Question 5

How many proteins are encoded by the ebola virus RNA and what are their names.

Answer 5

Seven proteins including:

1. Nucleoprotein (NP)
2. Virion protein VP34
3. Virion protein VP40
4. Glycoprotein (GP)
5. Virion protien VP30
6. Virion protein VP24
7. L protein RNA dependent RNA polymerase

Question 6

How does ebola virus enter the body of the host?

Answer 6

Via mucosal surfaces, breaks in the skin or parenteral introduction

Question 7

Which host cells are targeted by the Ebola virus for entry?

Answer 7

A wide variety of cell types including:
1. Endothelial cells
2. Epithelial cells
3. Monocytes
4, Macrophages,
5. Dendritic cells
6. Neutrophils
7. Fibroblasts
8. Hepatocytes
9. Adrenal cortical cells

Question 8

How does the Ebola virus enter each of these host cell types described?

Answer 8

The exact mechanism is not fully understood however it has been established that the Ebola virus cell surface protein GP binds to host cell surface protein NPC1 (Niemann-Pick C1 protein) and studies on vero cells have also shown an ability for GP to bind to the TIM-1 cell surface protein.

Studies have also shown a soluble form of GP termed, sGP is used to mediate binding to the CD16b cell surface protein of neutrophils directly or indirectly.

Once bound ebola virus is thought to enter the host cell by endocytosis.

Question 9

How does the ebola virus exit the endosome in the host cell?

Answer 9

The increasingly acidic environment of the endosome activates the proteases CatL and CatB which function to remove the GP1 protein resulting in a conformational change in the GP2 trimer which triggers the deployment of fusion machinery that function to insert the GP2 fusion peptide into the endosomal membranes.

Question 10

Where does transcription of the ebola virus vRNA take place?

Answer 10

The cytoplasm of the host cell.

Question 11

Describe the process of Ebola virus transcription.

Answer 11

Transcription begins at the transcriptional start site of the vRNA which is a length of 12-14 nucleotides in length ending in the consensus sequence 3’-CUUCUAAUU. mRNAs are synthesised from the vRNA in a 3’ to 5’ direction by the RNA dependent RNA polymerase complex until a stop site is reached characterised by 11-12 nucleotides with the consensus sequence 3’-UAAUUCUUUUU/U. Transcription occurs in a starting and stopping manner as the polymerase complex encounters start and stop sites along the genome. The ebola virus mRNA is then capped at the 5’ end by the L protein. Polyadenylation also occurs in a stuttering manner as the polymerase encounters the 5-6 uridine residues of the stop site.

Question 12

What causes the switch from transcription to replication of the ebola vRNA?

Answer 12

An increase in ebola virus protein levels, NP in particular, in the host cell causes a switch from transcription to replication.

Question 13

Detail the process of replication of the ebola virus RNA.

Answer 13

The negative sense vRNA is used as a template to form complementary positive sense vRNA which is then used as a template to synthesise new genomic negative sense ebola vRNA.

Question 14

Describe the process of ebola virus release from the host cell.

Answer 14

Once sufficient levels of negative sense nucleocaspids and envelope-associated proteins are reached, these components gather at the plasma membrane.
Budding occurs to release the virion fascilitated by VP40, GP and NP.

Question 15

How does the ebola virus cause disease?

Answer 15

Infection by and budding of virions ultimately results in cell necrosis

1. Extensive necrosis in liver, spleen, kidney and gonads:
2. Destruction of vascular endothelium and epithelium result in the haemorrhagic properties of the disease which is exacerbated by the reduced ability to synthesis clotting factors due to hepatocyte and adrenal cortical cell infection.
3. Neutrophil, dendritic cell, monocyte and macrophage invasion inhibits the immune response.
4. Cytokine storm causes a fatal inflammatory response

Question 16

What are the symptoms of the ebola virus?

Answer 16

Fever, haematemesis, diarrhea with blood, retrosternal abdominal pain, rapid evolution of death.

Question 17

Describe the results from studies detailing the importance of VP40, GP and NP in Ebola virus export.

Answer 17

Studies carried out on VP40 showed that by itself ZEBOV VP40 is capable of mediating its own release resulting in virus-like particles budding off from host cell. This process was unregulated when GP and NP were also present.

Question 18

How is ebola transmitted?

Answer 18

bodily fluids

Question 19

How does the ebola virus alleviate the immune response?

Answer 19

By directly infecting a number of leucocyte types causing their destruction.

The release of chemokines from the viral infected cell recruits more leucocytes to the site of infection allowing for further infection of cells and dysregulation of the immune response.

Inhibition of type 1 IFN response due to the VP35 protein acting as a type I IFN atagonist blocking the interferon regulatory factor IRF-3 activation and possibly preventing IFN-beta transcription

Also results in the loss of lymphocytes by apoptosis, even though it does not infect lymphocytes.

Question 20

How is the Ebola virus speculated to cause apoptosis of lymphocytes if it does not infect them directly?

Answer 20

1. Infection of dendritic cells impairing their function
2. Release of pro-apohttps://www.brainscape.com/decks/3697412/cards/quick#ptotic soluble factors such as nitric oxide (NO) from virus infected monocytes and macrophages.
3. Direct interaction of viral antigens and lymphocytes

all of which are speculated to collectively lead to apoptosis via TNF-related apoptosis-inducing ligand (TRAIL) pathway or Fas death receptor pathways.

Question 21

What is the reservoir of the ebola virus?

Answer 21

Fruit bats

Question 22

What is the incubation period of the disease?

Answer 22

2-20 days

Question 23

What is the prodrome phase of ebola infection?

Answer 23

Days 1-10 of infection characterised by severe flu-like symptoms.

Question 24

When do haemorrhagic symptoms of Ebola virus infection begin?

Answer 24

7-15 days after infection

Question 25

What happens once haemorrhagic symptoms kick in?

Answer 25

Multi-organ failure and death

Question 26

What are the two known strains of ebola virus and what is their fatality rate?

Answer 26

1. Zaire Ebola virus: 90% fatality

2. Reston Ebola virus: 0% fatality