By Dr Eddy Bajrovic*
The world has learned something new – and worrying – about dengue fever.
An estimated 390 million people are infected with dengue fever annually and 50% of the world’s population is at risk of infection, according to the WHO.
Almost all of the 1638 Aussies who had their diagnosis of dengue confirmed through a blood test last year were infected while travelling overseas – mainly in Asia. (A small percentage was infected in North Queensland, where imported cases spark localised outbreaks each summer.)
But, only about 25% of people infected with dengue actually get its symptoms – a high temperature, headache, pain behind the eyes, rash, and severe aching of muscles, joints, and bones (the reason dengue is also called ‘breakbone fever’).
It was previously thought that it was only these ‘symptomatic’ people who were able to pass on the virus to someone else through a mozzie bite. The reasoning was that only those who were sick would have enough of the virus in their blood stream to actually infect a mosquito that bites them, before passing on the illness by biting another person who entered their territory.
What we thought …
Conversely, those who were ‘asymptomatic’ – that is, with no tell-tale signs of illness – did not have enough of the dengue virus in their blood to make a mozzie bite dangerous to others. Or so we thought.
But, this theory has been disproven by research conducted in Cambodia by a team of Cambodian and French scientists, who studied people from Kampong Cham, a city 100km northeast of the capital, Phnom Penh.
The researchers tested volunteers who had no symptoms but lived with or near people who had been hospitalised with dengue symptoms and whose infection had been lab-confirmed.
Those asymptomatic family members and neighbours who tested positive for the virus were then exposed to healthy laboratory-bred Aedes mosquitoes – the species that spread not only dengue, but also the chikungunya, Zika, and yellow fever viruses.
When the mosquitoes were analysed later they were found to be infected at levels that would have enabled them to transmit the virus if they bit a human.
What we now know…
The study not only disproved the existing thinking, it has turned our understanding of dengue transmission on its head by also confirming that people without symptoms:
- Are markedly more infectious than the 25% with symptoms.
- Pose a greater risk of infecting others because they are in the general population, making it more likely they will be exposed to roving mosquitoes and encounter more healthy people. (People sick with dengue are very likely to be less exposed to either mozzies or people because they are usually bed-ridden – either at home or in hospital.)
Therefore, the 75% who don’t get sick – local residents and travellers alike – create a much larger ‘pool’ of infected people capable of passing on the virus to those around them.
For every four new infections, one person is likely to experience dengue’s excruciating symptoms, which can last for weeks.
Not to mention being at risk of the dengue ‘legacy’: those who get one of the four strains once are more susceptible to the more severe haemorrhagic form should they subsequently get infected with a second strain.
Dodge mozzies in 2016
Dengue fever will continue to be a significant travel health risk for Australians visiting tropical and sub-tropical regions of the world in 2016. Only the food and water bugs that cause traveller’s diarrhoea pose a higher risk.
From all the indications, it seems inevitable that the dengue, chikungunya and Zika viruses, along with the Aedes mozzies that spread them, will expand their geographic range as climate change continues to take effect in coming years.
Besides dengue, the rise in the incidence of many insect-borne infections makes taking active measures to avoid bites vital for anyone planning travel to the tropics.
But, when it comes to dodging mozzies, some things work and others don’t.
- Using an effective repellent. That is, one containing DEET (40% formulation lasts 6 hours), Picaridin (20% last 4 hours), or oil of eucalyptus. Low-dose DEET- or Picaridin-based repellents are suitable for children as young as 3 months.
- Eliminating nearby mozzie breeding sites. Aedes mosquitoes breed in containers close to people, so turn over or remove anything that can hold water on the balcony or in the garden.
- Missing the ‘rush hour’. Head inside at the peak feeding times of dawn and dusk. If you can’t, refresh your repellent and put on long pants and sleeves, and shoes and socks.
- Sleeping under a bed net. If you’ll be camping outdoors or sleeping in budget accommodation without screens on windows and doors, pack a bed net to sleep under. A permethrin-treated net is doubly effective. You can buy a treated net or purchase a DIY kit and use it to treat the net and clothing to give yourself a safe, long-lasting barrier against biting insects.
- Wristbands and phone apps. They just don’t.
- Personal repellents based on ‘natural’ ingredients.
- Vitamin B. Nothing you can eat or drink will prevent insect bites.
- Burning mosquito coils and sticks. At best, they merely reduce the number of mozzies indoors, but they can be just as toxic to humans sleeping in an unventilated room.
As part of you pre-travel planning, check if dengue or other mozzie diseases are a risk at your destination by calling Travelvax Australia’s travel health advisory service on 1300 360 164 (toll-free from landlines). You can also get advice about vaccinations recommended for the trip and make an appointment with travel medicine professionals to get any you need at your nearest Travelvax clinic – usually in a single visit – along with personalised advice.
* Dr Bajrovic is the Medical Director of Travelvax Australia.