CDS (I) 2024 General Knowledge · Question 62 of 120

Correct — A, Flavi-ribo virus. Dengue virus sits in the family Flaviviridae, and its genome is a single strand of positive-sense RNA of roughly 9.2 to 11.0 kilobases. The printed option welds those two facts together — 'flavi-' for the flavivirus group and 'ribo-' for a ribonucleic acid genome — and on that reading it is the only choice that describes dengue at all. Four antigenically distinct serotypes, DENV-1 to DENV-4, circulate, which is why a second infection with a different serotype carries the higher risk of severe dengue. The virus reaches people through the bite of an infected female mosquito, principally Aedes aegypti, with other Aedes species acting as secondary vectors. None of the other three options is a flavivirus, and only one of them is even RNA-based.

• (b)Adenovirus — Adenoviruses carry double-stranded DNA, not RNA, and spread by droplets, contact and the faecal-oral route. They cause sore throats, conjunctivitis and gastroenteritis — no mosquito is involved.
• (c)Vaccinia virus — Vaccinia is a large double-stranded DNA poxvirus, best known as the virus used in the smallpox vaccine. It is neither a flavivirus nor mosquito-borne.
• (d)Nipah virus — Nipah is an RNA virus, which makes it the most tempting wrong answer, but it belongs to the Paramyxoviridae and its reservoir is fruit bats of the genus Pteropus. It spreads through bats, pigs, contaminated date-palm sap and person-to-person contact, not through mosquitoes.

Viruses are grouped first by the kind of nucleic acid they carry and then by family. Dengue belongs to the family Flaviviridae, a set of enveloped positive-sense RNA viruses that also contains yellow fever, Japanese encephalitis, West Nile and Zika — almost all of them arthropod-borne. The mosquito is the vector, not the causal organism: the causal organism is the virus itself.

The stem gives away half the answer by saying 'mosquito-borne virus', so the work is to spot which option names a virus group that is both RNA-based and carried by mosquitoes. The option as printed, 'Flavi-ribo virus', is not a name any virology text uses, and a student who has learnt the standard term 'flavivirus' can be thrown by it; the examiner appears to be spelling out the two features rather than quoting a taxon. It is a paper coinage, and the honest way to read it is as 'a flavivirus with an RNA genome'. One thing has moved since the exam: in the taxonomy adopted by the International Committee on Taxonomy of Viruses the genus Flavivirus has been renamed Orthoflavivirus, so dengue virus is now placed as the species Orthoflavivirus denguei — the family Flaviviridae is unchanged, and 'flavivirus' remains the ordinary clinical word.

• Dengue is caused by dengue virus (DENV), family Flaviviridae, with a positive-sense single-stranded RNA genome of about 9.2-11.0 kb.
• Four serotypes circulate, DENV-1 to DENV-4, and all four are placed within a single species.
• The principal vector is the female Aedes aegypti mosquito; other Aedes species, including Aedes albopictus, also transmit it.
• Adenoviruses and vaccinia are DNA viruses; Nipah is an RNA virus but a paramyxovirus spread by fruit bats.
• Other flaviviruses of exam interest include yellow fever, Japanese encephalitis, West Nile and Zika.

• Naming the mosquito as the causal organism when the stem asks for the virus.
• Picking Nipah because it is also an RNA virus that makes headlines in India, forgetting that its reservoir is fruit bats.
• Being thrown by the non-standard printed label 'Flavi-ribo virus' and rejecting the right option for that reason.

Asked as a one-line classification item where the stem already supplies 'virus' and 'mosquito-borne', leaving the family as the single thing to know.

• practice — not a real PYQ

  Which one of the following diseases is NOT caused by a flavivirus?

  • (a)Yellow fever
  • (b)Japanese encephalitis
  • (c)Chikungunya
  • (d)Zika

  Answer(c) Chikungunya — it is caused by an alphavirus of the family Togaviridae, though it shares the Aedes vector with the other three.

• practice — not a real PYQ

  The natural reservoir of the Nipah virus is

  • (a)the Aedes mosquito
  • (b)fruit bats of the genus Pteropus
  • (c)the common house rat
  • (d)domestic poultry

  Answer(b) fruit bats of the genus Pteropus — pigs and contaminated date-palm sap act as intermediate routes to humans.