Outdoor spraying? It's a bit hit and mist

By Dr Eddy Bajrovic, Medical Director, Travelvax Australia.

Mosquitoes and the diseases they carry are topics that come up often on our travel health advisory phone service (1300 360 164).
Callers often assume that staying in luxury accommodation will make them immune from risk.
“Oh, we’re in a luxury apartment right on the beach,” they say. “We’ve stayed there before - they use those fog machines to kill the mozzies…”
So, do the machines that spray clouds of synthetic pyrethroid insecticides around the gardens and grounds of your hotel or resort really work?
The answer is ‘yes… well, mostly’.
Outdoor spraying is important in tropical countries where diseases like dengue and Chikungunya are present, according to Sydney-based entomologist, Dr Cameron Webb*.
It keeps the mosquito population down and also interrupts the disease transmission cycle by killing female mosquitoes carrying a virus before they can infect humans.

‘Don’t be complacent’

However, to be effective spraying has to be carried out regularly and by a skilled operator because the insecticide must reach the majority of the disease-carrying mosquitoes in the gardens and grounds.
More often than not, our confidence in fogging is misplaced: the insecticide kills only some – not all – of the mosquitoes in the vicinity.
“There is definitely an art to fogging,” Dr Webb told the Travelvax Report.
“For it to be effective you need to spray regularly and do it when mosquitoes are actually flying around. And, there are limits to how well it works – especially if there is any wind or it’s raining.
“It is important to remember that even if the resort is doing a great job at controlling mosquitoes, you’ve got a risk of infection during that day trip to the local village or markets.
“Regardless of whether their accommodation is five-star or budget, my message to travellers is: Don’t be complacent.”

Then there’s Generation Next

Regardless of how well it’s done, the respite from mosquitoes is relatively brief. The spray kills adult mosquitoes only, not their larvae, which hatch regularly in water left after rainfall or watering.
The next generations of mosquitoes only need to bite a person infected with dengue or Chikungunya to become infective themselves, and pass on the virus throughout their entire lifespan of around 3 weeks. 
And, it’s not unusual for a female mosquito to bite more than one person in any single feeding session.
Indeed, infected female mosquitoes can pass on dengue (and other pathogens) to their offspring, who then infect any people they bite, Dr Webb added.
Known as ‘vertical transmission’, it is (thankfully) thought to be relatively rare. Although laboratory studies have shown a rate of around 10%, in reality it is probably a little less.
“There are probably a range of factors at play, including level of infection in the adult female mosquito and the size of her batch of eggs,” he said. 

Do you feel lucky?

So, relying on spraying alone to avoid insect bites leaves much to chance – or luck.
But, luck deserted the 1026 Australians who’ve returned home from holidays with dengue fever to date this year, based on current national disease surveillance figures.
Australia is on track to at least reach - if not surpass - last year’s 1840 dengue cases, which was the highest number in more than two decades. So far, 33 Australians have returned from holiday with Chikungunya this year. 
Among Australian travellers, Asia is undoubtedly the main source of these two pervasive viruses. Not surprising given the region contains some of the world’s most popular holiday destinations.

The facts on dengue and Chikungunya

– Their symptoms are similar, although Chikungunya is generally a milder illness. 
– Dengue infection may result in fever, headache, pain behind the eyes, joint and muscle pain, rash, nausea/vomiting, easy bruising, and mild bleeding from the nose or gums. Read more.
– Fever and joint pain are also typical with Chikungunya, but some people also experience headache, muscle pain, joint swelling, and/or rash. Read more.
- Most infections clear up on their own within 2-4 weeks, although dengue infection can progress to dengue haemorrhagic fever, a potentially fatal condition. Chikungunya infections rarely result in death, but the symptoms can be severe and disabling, with joint pain persisting for months.

Reduce bites, reduce your risk

Unfortunately, there is no vaccine for either of these mosquito-borne viruses. But, the fewer the bites you receive, the lower your risk of infection.
– Use insect repellent on all uncovered skin. Look for a repellent that contains one of the following active ingredients: DEET, Picaridin (KBR 3023), oil of lemon eucalyptus/PMD, or IR3535 and follow the instructions on the label.
– Wear loose, long-sleeved shirts and long pants when outdoors – especially in the early morning and around sunset.
– For added protection, soak your clothing and bed net (if your room is not screened) with permethrin. This contact insecticide repels mosquitoes and other insects, and also kills them when they land on the treated material. (Permethrin shouldn’t be applied directly on your skin. Read more about permethrin.) 
– If using sunscreen, apply it before you apply insect repellent.
Read more on protecting yourself against insects.
Finally, if you experience fever after travelling overseas, seek medical care right away. And, don’t forget to tell your doctor about your recent travel.

Travelling to the tropics? Get more advice on avoiding dengue and Chikungunya, as well as the recommended vaccinations and other health-related issues for your destination from Travelvax Australia’s travel health information service on 1300 360 164 (toll free from landlines within Australia).

* Dr Webb is based at the Department of Medical Entomology, Pathology West – ICPMR Westmead & the University of Sydney. You can follow him on Twitter @Mozziebites