What is the causal agent of Chikungunya?

Correct — C, (c) Virus. Chikungunya is caused by the chikungunya virus, a single-stranded RNA virus of the genus Alphavirus in the family Togaviridae, and it is spread to human beings by the bite of infected mosquitoes of the genus Aedes — principally Aedes aegypti and Aedes albopictus, the Asian tiger mosquito.

The disease is best learnt beside dengue, because the two share their vector, their season and much of their presentation, and examiners exploit that. Fever comes on typically four to eight days after the bite, together with severe joint pain, and it is the joint pain that gives the disease its name and its clinical signature: patients are often bent double by it, and in some the arthritis persists for months after the fever has gone. Headache, muscle pain, joint swelling and a rash complete the picture. There is no specific antiviral treatment, so management is supportive, and control of the disease is control of the mosquito.

Knowing the vector settles the practical part. Aedes aegypti breeds in clean, stored water in and around houses — coolers, overhead tanks, flower pots, discarded tyres — and bites during the day, particularly around dawn and in the late afternoon, which is why bed nets alone are a weak defence and why source reduction, emptying and covering water containers, is the mainstay of control.

The historical outline is short and useful. The virus was identified after an outbreak in Tanzania in 1952, reached India and Cambodia in the early 1960s, and after an outbreak on the Kenyan coast in 2004 spread through the islands of the Indian Ocean to India and South-East Asia, causing explosive outbreaks that affected millions. India has had large epidemics in that wave, which is why the disease turns up in Indian question papers as reliably as dengue does.

Why the others are wrong

• (a)Non-chlorophyllous bacterium — No bacterium causes chikungunya. The phrase ’non-chlorophyllous bacterium’ is doing a particular job here: it sounds technical, and it describes a real category, since bacteria other than the cyanobacteria and a few photosynthetic groups do lack chlorophyll. But it is a description of most bacteria rather than of any pathogen in particular, and a candidate who is attracted to it is being attracted by vocabulary. Bacterial diseases worth keeping ready as contrasts include tuberculosis, cholera, typhoid, tetanus, plague, leprosy and anthrax; none of them is spread by Aedes mosquitoes, and all of them can be treated with antibiotics, which chikungunya cannot.
• (b)Nematode — A nematode is a roundworm, and nematodes do cause serious human disease — lymphatic filariasis, which produces elephantiasis, is caused by the nematode Wuchereria bancrofti and is indeed transmitted by a mosquito, in that case Culex. That real example is what makes this option more than filler: a candidate who remembers that a mosquito-borne disease is caused by a worm may attach the memory to the wrong disease. Ascariasis, hookworm infection and guinea worm disease are the other nematode diseases worth having. Chikungunya is not among them; its agent is far smaller than any worm and cannot be seen under a light microscope.
• (d)Fungus — Fungal diseases in human beings are mostly diseases of the skin, nails and mucous membranes — ringworm, athlete’s foot, candidiasis — with deeper infections largely confined to people whose immunity is compromised. None of them is spread by a mosquito. Fungi are eukaryotes with chitinous cell walls, orders of magnitude larger than a virus, and require a quite different treatment. Nothing about a mosquito-borne epidemic fever with severe joint pain points to a fungal cause, and the option is eliminated as soon as the mode of transmission is brought to mind.

Human diseases are classified by the kind of organism that causes them, and the classification determines almost everything else — how the disease spreads, whether antibiotics work on it, and what public health measure will stop it. VIRUSES are non-cellular, replicate only inside a host cell, and cause chikungunya, dengue, influenza, measles, mumps, chickenpox, polio, rabies, hepatitis and AIDS; antibiotics have no effect on them, and prevention rests on vaccination and on vector control. BACTERIA are single-celled prokaryotes and cause tuberculosis, cholera, typhoid, tetanus, plague, leprosy, diphtheria and whooping cough; these respond to antibiotics. PROTOZOA are single-celled eukaryotes and cause malaria, amoebic dysentery, kala-azar and sleeping sickness. HELMINTHS, the parasitic worms, include the nematodes responsible for filariasis, ascariasis and hookworm infection, and the flatworms responsible for tapeworm infection. FUNGI cause mainly superficial infections of skin and mucous membranes. The mosquito-borne group deserves particular attention because the vector, not the pathogen class, is what the diseases have in common: Aedes mosquitoes carry the viruses of dengue, chikungunya, yellow fever and Zika; Anopheles mosquitoes carry the protozoan parasites of malaria; Culex mosquitoes carry the virus of Japanese encephalitis and the nematode of lymphatic filariasis. Three genera, four kinds of pathogen, and a question can be built at any intersection.

Key facts

• Chikungunya is caused by the chikungunya virus, a single-stranded RNA virus of the genus Alphavirus, family Togaviridae.
• It is transmitted by Aedes aegypti and Aedes albopictus mosquitoes, which bite mainly during the day, around dawn and in the late afternoon.
• Fever and joint pain usually appear four to eight days after an infective bite; headache, muscle pain, joint swelling and rash are also common.
• Severe joint pain is its signature, and in some patients it persists for months after the fever resolves.
• There is no specific antiviral treatment; management is supportive and control depends on suppressing the mosquito vector.
• The virus was identified after an outbreak in Tanzania in 1952 and reached India and Cambodia in the early 1960s.
• An outbreak on the Kenyan coast in 2004 spread through the Indian Ocean islands to India and South-East Asia, affecting millions.
• Aedes aegypti breeds in clean stored water in and around houses, so source reduction is the mainstay of control.
• The same Aedes mosquitoes transmit dengue, yellow fever and Zika; Anopheles transmits malaria and Culex transmits Japanese encephalitis and lymphatic filariasis.

Common traps

• Being drawn to an option because its wording sounds technical. ’Non-chlorophyllous bacterium’ describes most bacteria and identifies no pathogen.
• Assuming that every mosquito-borne disease is viral. Malaria is protozoan and lymphatic filariasis is caused by a nematode.
• Confusing chikungunya with dengue. They share the Aedes vector but differ in their signature features.
• Expecting antibiotics to be relevant to a viral illness.

• practice — not a real PYQ

  Chikungunya and dengue are both transmitted to human beings by mosquitoes of the genus

  • (a)Anopheles
  • (b)Culex
  • (c)Aedes
  • (d)Mansonia

  Answer(c) Aedes

• practice — not a real PYQ

  Lymphatic filariasis, which causes elephantiasis, is caused by

  • (a)a virus
  • (b)a bacterium
  • (c)a protozoan
  • (d)a nematode

  Answer(d) a nematode