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Historical dengue mosquito elimination holds clues for Brisbane’s future

Researchers fear the growing popularity of rainwater tanks on Brisbane properties could provide the right conditions for the dengue mosquito Aedes aegypti to return to the city, almost 60 years after its elimination.

Although endemic to North Queensland and found as far south as Gin Gin and Goomeri, neither the mosquito nor its larvae has been found in Brisbane – outside first ports of entry or quarantined facilities – since 1957.

Scientists from QIMR Berghofer Medical Research Institute’s Mosquito Control Laboratory have trawled through historical data to catalogue the role of state and council-imposed mosquito regulations, and a decline in the use of rainwater tanks until very recently, on the public health fight against dengue in the city.

The study has been published in the journal PLoS Neglected Tropical Diseases.

Researcher Dr Jonathan Darbro said the last recorded case of locally acquired dengue fever in Brisbane was in 1948.

He said the Aedes aegypti mosquito, which transmits dengue, likely became extinct in Brisbane around 1960.

“Dengue and the Aedes aegypti mosquito were once prolific in Brisbane and even further south in New South Wales,” Dr Darbro said.

“We know this because our early historical records document outbreaks as early as 1887, although it wasn’t until 1906 that a Brisbane scientist, Thomas Bancroft, first suggested that particular mosquito was the vector,” he said.

“Deaths due to dengue peaked in Brisbane during 1905 but the last outbreak wasn’t until 1943, which resulted in 646 cases.

“Without a reliable source of drinking water until the commissioning of the Somerset Dam in 1954, households in Brisbane relied on rainwater tanks for potable water.

“Unfortunately, rainwater tanks were also the ideal environment for the dengue mosquito to breed in.

“Authorities introduced anti-mosquito regulations in 1911, which allowed them to fine and prosecute residents who owned rainwater tanks that harboured mosquito larvae.”

PhD student Brendan Trewin, who carried out the review, said permanent water storage containers like rainwater tanks provide the perfect refuge for dengue mosquitos during the dry season, as well as protection from the cold during winter.

“Following the introduction of Somerset Dam to supply Brisbane’s water in 1954, the number of rainwater tanks used by households fell dramatically,” he said.

“By 1971, there were fewer rainwater tanks relied on by households and those that did exist were more likely to be compliant with anti-mosquito regulations and less likely to harbour the vector.”

Mr Trewin said the study found that improved living conditions and infrastructure in Brisbane following World War II, the isolated use of chemicals like DDT, and making households directly responsible for compliance, contributed to eliminating dengue mosquitos from Brisbane.

However, rainwater tanks have grown in popularity in Queensland in recent years.

Dr Darbro said drought conditions in the early 2000s led to more than 300,000 rainwater tanks being installed in greater Brisbane.

“About 40 per cent of homes in Brisbane now have a rainwater tank,” Dr Darbro said.

“This has been driven by a culture of people wanting to save water and necessitated by legislated water restrictions during times of drought.

“With the current distribution of Aedes aegypti just 170km north of Brisbane, it’s vital we use our understanding of past threats to inform ongoing mosquito surveillance and rainwater tank monitoring.”

Mr Trewin said the global emergence and re-emergence of mosquito-borne viruses including dengue, chikungunya and Zika virus, highlighted the importance of early detection and response to disease-carrying mosquitos in urban areas.

He said dengue fever continued to exert a huge economic and health burden in many parts of the globe, affecting an estimated 390 million people each year.