Dengue virus

Question 1

What are arboviruses?

Answer 1

RNA viruses spread by moquito, tick or fly bites

Question 2

What is Dengue virus?

Answer 2

• positive ssRNA virus spread by mosquitoes
• 4 serologically distinct groups 1-4
• causes the most important arthropod-bourne viral disease of humansWh

Question 3

What family is Dengue virus from?

Answer 3

• Falviridae
• related to Zika virus and has some cross-reactivity

Question 4

What is the worldwide distribution of Dengue virus?

Answer 4

• countries at risk of infection directly correlate with where mosquito vectors live
• rang constantly expanding as temperatures rise

Question 5

How is Dengue virus transmitted?

Answer 5

• spread between mosquitoes and primates
• mosquito bites a human and the cycle of humans and mosquitoes infecting one another begins

Question 6

What are the two types of Dengue virus?

Answer 6

• Dengue / dengue fever
• severe dengue (progression of dengue fever)D

Question 7

describe the clinical features of dengue fever

Answer 7

• fever headache, aches and pains
• acute phase 3-7 days
• can be prolonged and lead to tiredness and symptoms seen in long covid
• acn sometimes progress to severe dengue

Question 8

Describe the clinical features of severe dengue

Answer 8

• usually in children
• acute phase the same as Dengue fever
• fever gives way to sudden circulatory collapse
• increase in vascular permeability leads to fluid leakage into the lungs and tissues
• can cause shock and death less than a day after first symptoms occur

Question 9

What is the structure of a flavivirus such as Dengue?

Answer 9

• viral envelope embedded with M + E proteins
• nucleocapsid comprised of viral RNA + a c protein at the 5’ end of the genome

Question 10

What is the genome organisation of a flavivirus such as Dengue?

Answer 10

• structural genes at 5’ (C, M and E)
• non structural include NS1, NS2 protease, NS4 polymerase

Question 11

How does Dengue virus enter the host cell?

Answer 11

• receptor mediated endocytosis
• endosome acidified and virus nucleocapsid is released into the cytoplasm

Question 12

Where does Dengue virus replicate and mature?

Answer 12

• replication complex is formed at the ER membrane
• packaged by the core protein and assemble at the ER
• matures as it travels through the golgi
• rearrangement of strucutral proteins prevents fusion with cell membranes before the virus exits

Question 13

How and where does the proteolytic processing of the polyprotein occur?

Answer 13

• polyprotein is threaded through the ER as it is translated
• capsid and replication proteins on the cytoplasmic side
• others in the ER lumen
• cleavage by NS2B

Question 14

Why is Dengue virus difficult to study?

Answer 14

• mice are ok for drug testing byt they dont respond to the virus like humans do
• primates show no symptoms
• studies must be done on epidemiological data and human volunteers

Question 15

What cells are the initial major target for Dengue virus?

Answer 15

• skin dendritic cells
• enter the blood and reach their major targer: monocytes and macrophages
• virus has also been isolated from lung, kidney, lymph nodes and more

Question 16

How does dengue virus appear to cause cell death?

Answer 16

• not killed by the virus itself
• capillary leakage may be host cytokine mediated

Question 17

What is a second infection with a different serotype of Dengue like?

Answer 17

• sequential infections are more likely to result in severe disease
• immune enhancement hypothesis
• pre-existing cross-reactive antibodies actually allow for worsened infection
• damage by activated CD8+ T cells

Question 18

How does Dengue viral load affect disease severity?

Answer 18

• high viral lead increases severity
• dengue 2 is most virulent
• strains exist within Dengue 2

Question 19

What 3 things do cells infected with Dengue release? What do host antibodies respond to for each?

Answer 19

• mature virions have their envelope protein targetted
• immature virions have their pre-M protein targeted
• NS1 protein has antibodies specific to NS1 targetting it

Question 20

What does the immune system usually do in response to mature Dengue virions?

Answer 20

• produces antibodies specific for envelope proteins
• can lead to neutralisation by blocking uncoating, entry etc
• can also lead to antibody-dependent enhancement

Question 21

What does the immune system do in response to immature dengue virions?

Answer 21

• antibodies specific for pre-M
• can lead to antibody dependent enhancement in secondary infeciotn

Question 22

What is antibody dependent enhancement?

Answer 22

• in secondary infection, cross-reactive, non-neutralising antibodies bind the virus
• virus/ab complex can bind fc receptors on monocytes and macrophages and have enhanced virus uptake in these cells

Question 23

Where are the epitopes on the Dengue polyprotein that are recognise by T cells?

Answer 23

• all across the protein
• mostly on NS3

Question 24

What is the role of CD4 T cellls in Dengue infection?

Answer 24

recognise antigens on mature virions, immature virions, NS1 and the polyprotein via MHC II and produce cytokines

Question 25

What is the role of CD8 T cells in dengue infection?

Answer 25

recognise antigens on mature virions, immature virions, NS1 and the polyprotein via MHC I and induce target cell lysis

Question 26

How can cross-reactive T cells worsen secondary Dengue infection?

Answer 26

• different serotypes have different but similar epitopes
• if T cells only have partial recognition of antigens it can lead to an aberrant T cell response
• insufficient cell lysis due to low levels of IFN-Y and TNF
• worsened infection

Question 27

What are some immune factors of severe dengue?

Answer 27

• multifactorial and not fully understood
• inflammatory mediates co-indice with vascular leakage - hallmark of severe disease
• neutrophil degranulation

Question 28

How has global distribution of Dengue serotypes changed?

Answer 28

• spread from parts of Africa and Asia to much larger surrounding areas
• large area around the equator growing
• Dengue 1-4 are seen in all affected regions now

Question 29

What leads to an increased probability of severe Dengue?

Answer 29

• increased transmission and worldwide movement increases the probability of virulent strain selection
• increased probability of secondary infection leads to increased probability of immune enhancement

Question 30

What are some challenges in developing a Dengue virus vaccine?

Answer 30

• immune enhancement means that inducing immunity for one serotype could worsen infection with others - need a tetravalent vaccine
• lack of suitable animal model
• many commonly affected countries can’t afford expensive vaccines and drugs

Question 31

What vaccine methods seem most promising at the moment and are in trials?

Answer 31

• recombinant live-attenuated vaccine in those who have had Dengue before
• a chimeric recombinant virus whereby yellow fever is cloned and the structural genes are replaced with those of all 4 types of Dengue virus
• could also use a dengue virus back bone?

Question 32

How can vectors be controlled to control the spread of dengue virus?

Answer 32

• release of Wolbachia sp.
• bacterial symbiote of mosquitoes that decreases female life span by 50%
• release in Indonesia lead to a 77% decrease in Dengue and fewer hospital admissions