What is the dengue virus?
There are currently thought to be at least five different 'strains' of the dengue virus, although only the first four seem to infect humans. The virus is from the same family of viruses as yellow fever, and as chikungunya virus (which can be similar to dengue).
Where did dengue come from?
Dengue probably only became a human disease about 800 years ago, when it 'jumped' into human populations from monkeys. It was restricted to a very few parts of the world until the middle of the twentieth century, when the disruption caused by the Second World War, followed by the increase in cargo transport and globalisation, carried the mosquitoes all around the world.
Why do some people get severe dengue?
Severe dengue normally occurs only with a second attack of dengue caused by a different strain of dengue virus to your previous attack. It mainly affects children and is extremely rare in returning travellers.
The symptoms of dengue are mainly caused by our own immune systems responding to the virus. In the case of severe dengue the immune system responds dramatically, producing destructive substances which damage organs and make fluid leak from blood vessels. Children and young adults tend to have the most reactive immune systems, and this is thought to be why they are more likely to get severe dengue compared with older people.
90% of severe dengue cases occur in children aged less than 15 years.
How do I catch dengue?
You can only catch dengue through being bitten by an infected mosquito.
Which mosquitoes carry dengue?
Dengue virus is transmitted by the bite of an infected Aedes mosquito - most commonly a mosquito called Aedes aegypti. This mosquito likes to be where there is human development - it prefers to lay its eggs in man-made water containers. The mosquito feeds in daylight in the early morning or late afternoon, so night-time mosquito nets are not sufficient protection. Only the female Aedes mosquito feeds on blood.
The mosquito has stripy legs. The same species of mosquito can also transmit chikungunya fever, yellow fever and Zika virus, if they are present in the local population.
In order for the mosquito to become infected it needs to feed on a person who has large amounts of virus in the blood; this happens early in a case of dengue, often before people know that they are infected. The virus needs to live for 8-12 days in the mosquito before it is capable of being passed on to another person. The mosquito remains infected with dengue for the remainder of its life, which might be days or a few weeks.
Can I catch dengue directly from another person?
No, you can't catch dengue from person-to-person spread. You can only catch dengue from an infected mosquito - but the mosquito becomes infected by biting another person.
How does a dengue epidemic occur?
Dengue epidemics occur when many infected people are close together in an area where there are many biting Aedes mosquitoes. If there are large numbers of people carrying the virus then the mosquitoes are more likely to become infected, and if there are a lot of infected mosquitoes biting then more people will be infected.
Is dengue dangerous in pregnancy?
If a pregnant woman contracts dengue in early pregnancy there is an increased chance of miscarriage. Dengue can also be dangerous to the developing baby later in pregnancy, particularly if it is the mother's second infection. Both she and her baby can develop severe dengue.
Further reading and references
Dengue and severe dengue; World Health Organization, July 2016
Dengue Clinical Guide - Treatment Algorithm; Centers for Disease Control and Prevention
Vasilakis N, Weaver SC; The history and evolution of human dengue emergence. Adv Virus Res. 200872:1-76. doi: 10.1016/S0065-3527(08)00401-6.
Dengue fever; National Travel Health Network and Centre (NaTHNaC)
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