Day 4: Tackling drought could increase disease

People living in arid climates have taken great pains to adapt, by re-using their wash water and collecting rainfall for secondary purposes, like gardening. The practice, tuned to a fine art in places like Arizona, is widely accepted as a conservation strategy — alleviating the need to irrigate with drinking water — and a way to deal with drought.

But what if another drought-afflicted creature also benefits, and then contributes to outbreaks of disease?

Michael Kearney, a zoologist at the University of Melbourne, is lead author on a new paper that predicts how a disease-carrying mosquito, Aedes aegypti, will respond to climate change. Aedes aegypti, also called the yellow fever mosquito, carries both yellow fever and chikungunya, a painful inflammation of the joints. It resides throughout the tropics and is found in limited pockets within the United States, particularly around the Gulf of Mexico. The new study was conducted in Australia, using computer models that rely on predictions for that region’s climate patterns. But the UN’s Intergovernmental Panel on Climate Change warned in a landmark report in 2007 that climate change could enhance the spread of all mosquito-borne diseases, especially malaria and dengue.

“One of the serious concerns about climate warming is that it will allow disease bearing mosquitoes to enter the US from the south and allow the ones already here to move north, bringing with them a variety of different kinds of diseases,” added Warren Porter, a co-author on the new study from the University of Wisconsin in Madison. He said the model used for Aedes aegypti is “a very generic one that could be applied to any species of mosquito for which we have temperature dependent and photoperiodic response data.”

The new study appears in this week’s issue of the journal Functional Ecology.

Kearney and his team suggest that climate change and evolutionary change could act together to accelerate and expand the mosquito’s range. But they say human behavior –- in the form of storing water to cope with the climate change -– is likely to have an even greater impact.

“The potential direct impact of climate on the distribution and abundance of Ae. aegypti is minor when compared to the potential effect of changed water-storage behaviour,” Kearney said in a press release. “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.” 

The authors found that public education campaigns, urging people to dump standing water and keep their lawns mowed, worked well to control Aedes aegypti in northern Australia. But the pests have shown an ability to adapt within a few generations to the conservation-related rain collection barrels under the ends of household gutters.

Roger Nasci, an entomologist and chief of the Arboviral Diseases Branch of the Centers for Disease Control and Prevention, said several US species could react the same way. Besides the limited numbers of Aedes aegypti, the tiger mosquito, ranging from New Jersey to Florida and southeastern Texas, is “an aggressive biter with vector potential,” he said, meaning it has the potential to carry disease. “A variety of other species that are important West Nile vectors … will grow in those containers if they’re available,” Nasci said.

The CDC, as always, urges people to eliminate standing water from their lawns, be it in old tires, fish ponds or unused flower pots. Containers not meant to hold water, such as children’s toys, should be dumped within seven days or turned over so water doesn’t collect.

One solution for the newest mosquito nurseries, the rainwater barrels, is nearly as cheap and simple: screen them, said Terry Sprouse, a researcher at the University of Arizona in Tucson.

“We use screens, like the material found on screen doors, to prevent mosquitoes from entering the water.”

Nasci offered other alternatives, ranging from chemical larvicides to predatory fish. One organophosphate has been marketed for such purposes, Tenephos, and when it’s used according to the package directions, the compound has been labelled safe even for drinking water by the World Health Organization, Nasci said. The bacterium Bacillus thuringiensisis a specific stomach poison for the mosquito and its relatives, and is only toxic during the spore stage — so, after it’s added to a rainbarrel, Bt shouldn’t pose a risk to insects in the garden, he said.

“Another way would be to introduce some method of larval control,” Nasci suggests. “Put fish in there, like guppies, that will eat the mosquitoes.”

And if the fish accidentally get dumped when you water?

“It’ll just be a little extra fertilizer for your plants,” he said.