By Eloise King I First published: October 16, 2011
body+soul, The Sunday Telegraph
Mosquito season is here, but we may be protected from dengue fever, thanks to a scientific breakthrough.
When Australian scientists announced in August that they had introduced a bacterium called Wolbachia into mosquitoes that made them resistant to the dengue virus, newspapers were splashed with headlines such as: “Aussies defeat dengue danger”.
But with this year’s wet season about to start in northern Queensland, the scientific breakthrough will finally be put to the test.
Since the beginning of the year, researchers have been releasing mosquitoes carrying Wolbachia bacterium into the wild to monitor its spread through two mosquito populations in Cairns. By protecting mosquitoes from dengue, the Monash University program hopes to stop transmission into humans.
Five weeks after the final release, results showed 100 per cent of the mosquitoes at Yorkeys Knob carried Wolbachia, with Gordonvale at 90 per cent. But shortly after, the transmission rate dropped by about 10 per cent.
“The drop was relatively small so we are still pretty chuffed by the results,” says Professor Scott Ritchie, Eliminate Dengue program researcher. “But we need more sampling to see how well the bacterium holds in mosquito populations.”
Professor Ritchie says reasons for the drop could be lack of rain over the dry season or just a simple sampling issue.
“Dengue-carrying mosquitoes breed in puddles around the home, in pot plant bases, vases, roof guttering and fallen fronds, and it’s been a dry season,” he says. “We have enough Wolbachia in there that when the wet season begins and it starts raining, transmission rates should rise to 100 per cent again.”
Scientists will continue to monitor Cairns mosquito traps and the program is set to be repeated in other parts of Australia as well as Thailand, Vietnam, Indonesia or Brazil within the next 12 months.
A global issue
Dengue is a virus transmitted to humans through the bites of female Aedes mosquitoes. Ironically, the most common way mosquitoes contract it is through feeding on humans who are infected with the virus.
Dengue affects between 50 and 100 million people in tropical and sub-tropical regions each year. It occurs naturally in more than 110 countries across South-East Asia, the Western Pacific regions, Africa, the Americas and the Eastern Mediterranean.
The World Health Organization (WHO) estimates two-fifths of the world’s population, 2.5 billion people, are at risk of contracting dengue.
There is no continuous transmission of dengue in Australia. Outbreaks are initiated by travellers and residents arriving into Australia with the dengue virus, who are then being bitten by local mosquitoes. An average of 100 new cases are reported each year, almost all in North Queensland.
Queensland Health declared the worst dengue outbreak in over 50 years during the 2008-9 wet season, with more than 1000 cases reported.
The dengue virus circulates in the human bloodstream for two to seven days.
Dr Ronald McCoy, spokesman for the Royal Australian College of General Practitioners, says between 50 and 90 per cent of people with dengue will experience little more than influenza-like symptoms.
“The rest get classic dengue symptoms,” he says. “These include sudden high fever lasting two to seven days, intense headaches behind the eyes, muscle and joint pain [in the ankles, knees and elbows], nausea, vomiting, diarrhoea, flushed face and neck, limb rashes and extreme fatigue.” If treated, dengue fever is rarely fatal.
Dengue haemorrhagic fever (DHF) is a potentially deadly complication of dengue infection that causes the same dengue fever symptoms but is followed by a further, rapid deterioration as the fever subsides. Dr McCoy says, “A small proportion of people who get dengue go on to develop DHF, but there is no way of telling who it will be.”
DHF symptoms include liver enlargement, gut bleeding and, in severe cases, circulatory failure. The circulatory system begins to fail as the fever drops, the patient goes into a critical state of shock and dies within 12 to 24 hours if left without medical treatment.
It’s estimated 500,000 people with DHF require hospitalisation each year, the majority of whom are children. About 2.5 per cent of those affected die. There is no vaccine or specific treatment for dengue fever. Dr McCoy says paracetamol is recommended to manage the aches and pains.
“If someone has dengue fever, we keep an eye on blood counts and fluid levels from day three of the illness until a few days after the fever subsides,” he says. “A very small percentage develop DHF and this monitoring tells us if they have it or not.”
Dr McCoy says symptoms don’t usually extend past the fever, but there is a risk of post-viral infection which can cause longer-term tiredness and low moods.
“I was so weak I couldn’t eat and it felt like a bomb was going off in my head. Little purple bruises started appearing on my legs and I lost seven kilos in five days. The worst of it was over in about 10 days, but for the next year, I was still fatigued, weak and had very little appetite. All I could do was what I absolutely had to for work and my university studies and then I would crash out in bed again.”