Lecture 30 - Flaviviruses

Question 1

Where do arboviruses replicate?

Answer 1

In arthropod vector and animal host

Question 2

Are humans the preferred host for most arboviruses?

Answer 2

No. Incidental hosts. Except for yellow fever and dengue

Question 3

Example of a DNA arbovirus

Answer 3

None. All arboviruses are RNA

Question 4

Viral families that can be arboviruses 1) 2) 3) 4)

Answer 4

1) Reoviridae 2) Flaviviridae 3) Togaviridae 4) Bunyaviridae

Question 5

Types of togaviruses that can be arboviruses

Answer 5

Alphaviruses

Question 6

Flavivirus genome

Answer 6

Question 7

Type of gene product of flaviviruses

Answer 7

Polyprotein

Question 8

Immature vs mature flavivirus virion structure

Answer 8

Immature - PrM protein intact, in the ER. Mature, infectious- Pr protein cleaved from M protein. M and E proteins are joined. Flat capsid shape.

Question 9

Number of prM and E copies in a flavivirus capsid

Answer 9

180

Question 10

What happens once pr is cleaved from M on a flavivirus virion?

Answer 10

M and E form homodimers.

Question 11

Flavivirus replication 1) 2) 3) 4) 5) 6)

Answer 11

1) Receptor-mediated endocytosis. 2) Flavivirus envelope fuses with endosome, releasing genome into cytoplasm 3) Translation, proteolytic processing of polyprotein. This occurs on ER cytoplasmic membrane 4) Non-structural proteins formed, viral RNA replication. 5) Virions assembled on ER membrane. 6) Virions bud from ER, exocytosed through Golgi.

Question 12

Do flaviviruses have a virion polymerse?

Answer 12

No. They are +RNA viruses

Question 13

Number of denguevirus types

Answer 13

Four

Question 14

Is dengue infection localised or systemic?

Answer 14

Systemic, high viremia

Question 15

Similarity of denguevirus types

Answer 15

Not very similar. As dissimilar to one another as other flaviviruses are. Don’t confer neutralising antibodies

Question 16

Primary flaviviral antibody target

Answer 16

E surface protein (envelope protein)

Question 17

Dengue transmission cycle

Answer 17

Extrinsic incubation period - Time between mosquito acquiring virus and becoming infectious. Normally 7-10 days. Intrinsic incubation period - When the mosquito can be transmitted to humans from infected mosquito.

Question 18

Implications of a 7-10 day extrinsic incubation period

Answer 18

Infected mosquito can only transmit virus 7-10 days after blood meal that infected it.

Question 19

Most important denguevirus vector

Answer 19

Aedes aegypti

Question 20

Denguevirus genetic bottleneck

Answer 20

Needs to be able to replicate both in humans and mosquitoes

Question 21

Aedes aegypti features 1) 2) 3) 4)

Answer 21

1) Prefers laying eggs in artificial containers (EG: tyres, empty drink cans) 2) Adults tend to rest on surfaces inside homes 3) Daytime biting, preference for humans 4) Once infected, mosquito is infected for life

Question 22

Areas with the greatest dengue burden

Answer 22

India, SE Asia, South America

Question 23

Global dengue burden

Answer 23

100 million symptomatic cases 2.1 million severe cases 21,000 deaths

Question 24

Outcome of most dengue infections

Answer 24

Clinically silent

Question 25

Mild dengue symptoms 1) 2) 3) 4) 5) 6) 7)

Answer 25

1) Fever 2) Headache 3) Myalgia 4) Retro-orbital pain (very characteristic of dengue) 5) Vomiting 6) Anorexia 7) Joint pain

Question 26

Severe dengue symptoms 1) 2) 3)

Answer 26

1) Severe plasma leakage into interstitial tissue 2) Severe haemorrhage 3) Severe organ impairment

Question 27

Most common severe symptom of dengue

Answer 27

Severe plasma leakage

Question 28

What might lead to severe plasma leakage in severe dengue infections?

Answer 28

Thought to be from immune response. Critical stage is at day 4-6 of disease, when dengue viremia is in steep decline.

Question 29

What can vascular permeability in dengue lead to?

Answer 29

Dengue shock syndrome

Question 30

Risk factors for dengue shock syndrome 1) 2) 3) 4) 5)

Answer 30

1) Secondary infection with another dengue type 2) Young age 3) Female gender 4) Obesity 5) Maybe virus strain

Question 31

Why is being infected with a secondary denguevirus strain more severe?

Answer 31

Denguevirus antibody-dependent enhancement

Question 32

Denguevirus antibody-dependent enhancement

Answer 32

Preexisting antibodies to another dengue strain bind but fail to neutralise virus. Results in greater uptake of dengue virions by macrophages, and increased viral replication

Question 33

Challenges for dengue vaccine 1) 2) 3)

Answer 33

1) Infection with one dengue serotype only confers lifelong immunity to that one serotype 2) Longer vaccine followup for boosters required, because of possible severe heterotypic infection 3) Lack adequate animal disease models (can infect animals, but animals don’t develop the same symptoms

Question 34

Flavivirus E glycoprotein function

Answer 34

Binds to host cell receptor. Mediates virus-specific membrane fusion

Question 35

Manufacturer of dengue vaccine in stage 3 trials

Answer 35

Sanofi-Pasteur

Question 36

Outcome of Sanofi-Pasteur stage 3 denguevirus vaccine trial

Answer 36

56.5% efficacy in preventing symptomatic infection

Question 37

Design of Sanofi-Pasteur denguevirus vaccine

Answer 37

CYD-TDV is a tetravalent vaccine. Yellow fever 17D (attenuated) vaccine strain has PrM and E proteins replaced with those dengue

Question 38

Denguevirus treatment 1) 2) 3)

Answer 38

1) No licensed vaccine, no specific treatments 2) Analgesics (avoid ibuprofen, aspirin) 3) Avoid dehydration 4) Avoid mosquito bites

Question 39

Effect of Wolbachia infection of Aedes aegypti

Answer 39

Decreases mosquito susceptibility to denguevirus infection Infected male can’t mate with uninfected female. Infected males and infected females can produce offspring. Uninfected males and infected females can produce offspring. All offspring are infected with Wolbachia

Question 40

Yellow fever 1) 2) 3) 4)

Answer 40

1) Flavivirus 2) Causes haemorrhagic fever 3) Transmitted by bite of female mosquito of genera Aedes, Haemogogus, Sabethes 4) Maintained in nature in cycles of mosquitoes and sylbatic hosts (non-human primates)

Question 41

Is there a yellow fever vaccine?

Answer 41

Yes. 17D live attenuated strain. One dose gives immunity for 10 years.

Question 42

Yellow fever incidence

Answer 42

~200,000 cases.

Question 43

Yellow fever mortality

Answer 43

15-20%

Question 44

Where is yellow fever found?

Answer 44

Tropical, subtropical regions of Africa, Sough America

Question 45

Yellow fever symptoms 1) 2) 3) 4) 5)

Answer 45

1) Early symptoms are muscle pain, fever, headache 2) Toxic phase is where damage occurs to the liver, kidneys, heart. 3) Viremia is only in first 3 days of infection 4) In severe cases, jaundice, kidney failure, internal bleeding, circulatory collapse 5) Death in 6-7 days post-infection

Question 46

Yellow fever treatment

Answer 46

Only supportive therapy