Arboviruses

Question 1

What are arboviruses?

Answer 1

Arthropod-bourne viruses.
A virus transmitted to vertebrates by blood feeding arthropods

Question 2

What are some examples of arthropods?

Answer 2

Mosquitos
Ticks
Sandflies

Question 3

What are the key features of arboviruses?

Answer 3

• Must replicate in both arthropod and vertebrate hosts
• (mostly) RNA genomes - high mutation rate
• Zoonotic (maintained in animal reservoirs)

Question 4

What are 3 common examples of arboviruses?

Answer 4

Dengue
Zika
West Nile

Question 5

What are the vectors of the 3 previously mentioned viruses?

Answer 5

All are mosquito species vectors
- Dengue = Aedes aegypti
- Zika = Aedes spp.
- West Nile = Culex spp.

Question 6

How do arboviruses differ from other viruses?

Answer 6

• Dual-host requirement
• Environmental sensitivity
• No sustained human to human transmission

Question 7

What does it mean for arboviruses to have “dual host” requirement?

Answer 7

Must be able to infect and replicate in (cold-blooded) arthropods and (warm-blooded) vertebrates.
E.g. Dengue virus grows in Aedes aegypti mosquitoes (25–30°C) and humans (37°C).

Question 8

What does it mean for arboviruses to have “environmental sensitivity”?

Answer 8

Climate affects vector competence (e.g. warmer and humid climates accelerate spread and replication in mosquitos)

Question 9

What are reservoir/amplifying hosts?

Answer 9

A vertebrate host in which the virus replicates efficiently, producing high viraemia, allowing vectors to become infected when feeding

Question 10

What is viraemia?

Answer 10

The viral presence in the bloodstream (of its host); it’s essential so that a feeding vector can become infected.

Question 11

What are dead-end (incidental) hosts?

Answer 11

A host that can become infected by the arbovirus, but does not produce enough viraemia to infect new vectors

Question 12

What is the reservoir, and the incidental hosts in West Nile virus?

Answer 12

The reservoir host = birds
The incidental host = humans and horses

Question 13

What is the reservoir hosts for Dengue virus?

Answer 13

Humans! Which is unusual. No animal reservoir is needed for sustained human transmission

Question 14

What is the general lifecycle for arboviruses?

Answer 14

1. Infected vector bites vertebrate host
2. Virus is transmitted via saliva during bite
3. Virus replicates in host’s blood
4. A new vector feeds on viraemic host (if not dead-end host), becoming infected
5. Virus replicates in vector’s midgut, then travels to salivary glands

Question 15

What distinguishes the West Nile virus cycle from the Dengue virus cycle?

Answer 15

WNV relies on a bird–mosquito–bird cycle with humans as incidental

Dengue relies on a human–mosquito–human cycle.

Question 16

Give 3 factors that can influence spread of arboviruses

Answer 16

Climate
Urbanisation
Animal migration

Question 17

Give 2 examples as to how climate influences spread of an arbovirus

Answer 17

Warmer temperatures = faster viral replication in mosquitos = shorter extrinsic incubation period = faster spread

Warmer humid climate = vector range expansions = more and new areas affected

Question 18

Give 2 examples as to how urbanisation influences the spread of arboviruses

Answer 18

Urbanisation means more water tanks, pots, buckets = Aedes aegypto like to breed and lay eggs in standing water

Dengue epidemics in megacities (lots of humans), no animal reservoir necessary (urban cycle)

Question 19

Give an example as to how animal migration can affect spread of arboviruses

Answer 19

Birds can carry West Nile virus from Africa to Europe/North America; cause of 1999 WNV outbreak in New York

Question 20

What are the reservoir hosts for Zika virus?

Answer 20

Reservoir = primates and humans

Question 21

Unlike the other two, how can Zika virus be transmitted?

Answer 21

• Sexually
• Via blood
• Vertically (mother to fetus)

Question 22

What is vector competence?

Answer 22

The ability of a vector to:
- Acquire the virus during blood feeding
- Support viral replication in midgut/salivary glands
- Transmit virus via subsequent bites

Question 23

What are the 4 main barriers to vector competence?

Answer 23

• Midgut infection barrier
• Midgut escape barrier
• Salivary gland infection barrier
• Salivary gland escape barrier

Question 24

What is the midgut infection barrier?

Answer 24

After a blood meal (by the vector), the virus must infect midgut epithelial cells.

Otherwise the virus will be digested and transmission fails

Question 25

What is the midgut escape barrier?

Answer 25

The virus must be able to exit the midgut (hemocoel) and enter the body cavity.

Question 26

What is the salivary gland infection barrier?

Answer 26

The virus must be able to infect the salivary gland. If blocked, the virus cannot be present in saliva, therefore no transmission

Question 27

What is the salivary gland escape barrier?

Answer 27

The virus must be secreted in saliva during feeding. If blocked, the virus is stuck in the gland, so transmission fails

Question 28

What is the extrinsic incubation period?

Answer 28

The time it takes from when a virus infects a mosquito from a blood meal to the point at which the mosquito becomes infectious

Question 29

What is the intrinsic incubation period?

Answer 29

The time it takes from when mammalian host is infected by a mosquito to the point that host is viraemic and can transmit to another uninfected mosquito

Question 30

What is meant by the “evolutionary balance” in arbovirus transmission?

Answer 30

Arboviruses must balance effective replication with not harming hosts too much, ensuring both vector and vertebrate remain alive and mobile enough for transmission.

Question 31

Why must arboviruses limit their virulence in vectors?

Answer 31

To avoid killing or debilitating the vector, which would interrupt transmission. The virus needs to replicate enough to reach the salivary glands without compromising vector survival.

Question 32

What is a real-world example of a viral mutation affecting vector interaction?

Answer 32

The CHIKV E1-A226V mutation increased Chikungunya virus infectivity in Aedes albopictus, leading to a large outbreak in the Indian Ocean (2005–06).

Question 33

What was the downside of the CHIKV E1-A226V mutation?

Answer 33

It reduced viral fitness in Aedes aegypti (a different Aedes species) — showing that adaptations for one vector can come at a cost in another.

Question 34

What is the trade-off between arbovirus virulence and transmission in vertebrate hosts?

Answer 34

If a virus causes severe disease (e.g. hemorrhagic fever), the host may become immobile or die, reducing contact with vectors and limiting transmission.

Question 35

How do arboviruses optimise transmission in vertebrate hosts?

Answer 35

Through two main strategies: 1. Acute infections with short, intense viraemia (e.g. Dengue virus). 2. Persistent infections in reservoir hosts (e.g. birds for West Nile virus), allowing long-term transmission potential.

Question 36

What is the example of a chemical vector control mechanism?

Answer 36

Use of insecticides like pyrethroids (wall spraying and insecticide nets). However, kdr mutations have caused resistance to pyrethroids

Question 37

What is an example of an environment vector control method?

Answer 37

Removal of standing water (breeding sites for mosquitos)

Question 38

What are two examples of vaccines for arboviruses?

Answer 38

Dengvaxia - Dengue YFV-17D - Yellow fever vaccine

Question 39

What is a challenge to Dengvaxia vaccine?

Answer 39

Risk of antibody dependent enhancement of Dengue (enhancement of entry of Dengue into host) for serological negative individuals (those who have not been exposed to Dengue infection)

Question 40

How successful had the yellow fever vaccine 17D been?

Answer 40

Very effective, granting lifelong immunity for most

Question 41

What are two examples of how vector competence can be manipulated for control?

Answer 41

- Genetic modification of vectors - Wolbachia infection - Population suppression/habitat targetting

Question 42

Give an example of how genetic modification of vectors can be used for control

Answer 42

Engineered mosquitoes can be made resistant to viral infection (e.g., inserting antiviral genes using CRISPR). These mosquitos are released into the wild to breed and pass on resistance. Aedes aegypti resistant to Dengue and Zika have been created in the lab.

Question 43

What is wolbachia infection?

Answer 43

Wolbachia is a bacterial symbiont that reduces viral replication inside mosquitos. When mosquitoes carry Wolbachia, their vector competence is lowered dramatically Has been used in field trials in Australia, Brazil, Indonesia with great success in reducing Dengue cases.

Question 44

What treatments are their for arbovirus infections?

Answer 44

There are no licensed antivirals for human arboviral infections. Treatment is mainly supportive, including fluids and basic painkillers

Question 45

What are the conserved symptoms for most arbovirus infections (like Dengue and Zika)?

Answer 45

Fever Rash Joint/muscle pain

Question 46

What are 3 main reasons why there are no antivirals on the market for arboviruses?

Answer 46

- Most infections are self-limiting (resolve on own) - Complex viral lifecycles in multiple hosts complicate targeting - Lack of large scale investment (affects lower income countries)

Question 47

3 diagnosis methods for arboviruses

Answer 47

- Molecular techniques (PCR) - Serological tests (ELISA) - Cell culturing