Dengue fever carrying mosquitoes could
thrive in water tanks, which are likely to
become more common as climate change
continues.
Image: iStockphoto

A new study to predict the impact of climate change on the dengue fever-carrying mosquito Aedes aegypti in Australia could help limit its spread.

A key finding of the study is that climate change and evolution could act together to accelerate and expand the mosquito’s range, but human behaviour – in the form of storing water to cope with climate change – is likely to have an even greater impact.

“Dengue fever is a viral infection that causes a severe flu-like illness, and sometimes a potentially lethal complication called dengue haemorrhagic fever. According to the World Health Organization, there may be 50 million infections worldwide every year”, said the study’s lead author, Dr Michael Kearney from the Department of Zoology. “In many Australian cities and towns, a major impact of climate change is reduced rainfall, resulting in a dramatic increase in domestic rainwater storage and other forms of water hoarding and possible breeding sites for the mosquito.”

Ae. aegypti probably arrived in Australia in the 19th century on ships carrying water barrels infested with mosquito larvae. It has unusual breeding practices, laying eggs in artificial containers only where a plentiful supply of its human food-source is available. The mosquito was once widespread in urban Australia, stretching as far south as Sydney and Perth. By the late 1960s, Ae. aegypti was restricted to the northern half of Queensland (where it currently resides) thanks in part to removal of old galvanised tin rainwater tanks, installation of piped water, insecticides and new power lawnmowers that helped people keep their back yards tidy.

This new research conducted by Australian and international collaborators, and published in the journal Functional Ecology, has major implications for public health interventions in Australia and other areas of the world affected by dengue and other mosquito-spread diseases.

“While we think it’s a great idea environmentally to install water tanks, our results highlight that dengue-vectoring mosquitoes could spread to areas where they are currently not found once suitable breeding sites, such as containers, become available.

“We are concerned that they could return to the areas they historically inhabited,” said co-author Professor Ary Hoffmann from CESAR/Bio21 Institute, University of Melbourne.

The predictions come from a new “bottom-up” model that takes into account the mosquito’s biology and its physiological characteristics, such as the ability of its eggs to tolerate drying out.

To construct the model, Kearney and his colleagues needed to predict the microclimates mosquitoes experience.

“Like all mosquitoes, the dengue mosquito has an aquatic larval phase and it is very particular about breeding in artificial containers like water tanks, buckets and old tyres. We modelled two extreme types of container – a large (3600 litre) water tank receiving rainwater from a roof catchment, and a small (20 litre) bucket receiving rainwater only from directly above. We considered each container type in high and low shade,” Dr Kearney says. The authors also took evolution into account for the first time in such models.

“Evolution happens all the time in nature and can be very rapid, taking only a few generations to influence the fitness of populations.

“Without due caution with water storage hygiene, this indirect effect of climate change via human adaptation could dramatically re-expand the mosquito’s current range.”