Tuesday, May 5, 2009

Climate change won't cause the largest spread of dengue fever - we will.

ResearchBlogging.orgOf course, the climate change won't help, either. At least that is the claim of a new study published in PLoS Neglected Tropical Diseases. The researchers used computer models using ecological modelling ("GARP") to project the distribution of dengue fever's carrier, the mosquito Aedes aegypti, under today's climate and under climate change scenarios for 2030 and 2050 given published temperature rises. They wanted to know if climate change was likely to expand the mosquito's range in Australia, leading to increased risk of dengue outbreaks.

Dengue fever is nothing to be trifled with. It's caused by four closely related virus serotypes of the genus Flavivirus. It's found just about anywhere tropical, and wherever mosquitos can live that connects to it. The WHO states that 2.5 billion people - that's roughly 2/5 world's population - are now at risk from dengue and estimates that there may be 50 million cases of dengue infection worldwide every year, with dengue fever being epidemic in more than 100 countries. And unlike malaria and most other mosquito-borne tropical diseases, dengue is just as prevalent in the urban areas as it is in the rural ones, meaning its potential to infect is ubiquitous.

Dengue is more than just a fever. It causes flu-like symptoms at best and severe hemorrhagic shock at worst, a condition that is usually fatal. Most cases present with a characteristic rash, but positive ID can only be made with special tests. Treatment usually consists of closely monitoring the patient's vitals and responding quickly to any shock-like symptoms. There are no specific antiviral medications which dengue currently responds to, and there are no dengue vaccines. The only medication used to treat dengue are those to manage symptoms.

In Australia today, dengue epidemics are limited to Queensland, where the mosquitos are most prevalent. Increased temperatures, however, could allow the mosquitos to spread to other areas because winter temperatures won't drop low enough to kill the bugs, putting more people at risk of outbreaks. To understand how temperature might affect the mosquitos distribution, scientists from the Asia-Pacific Institute for Tropical Medicine and Infectious Diseases used models to determine the mosquitos habitat preferences.

The temperature is definitely important - higher mean temperatures mean the mosquitos can survive longer in a larger area. But there's more to the story than that. To spread and infect people with dengue, the mosquitos must do one thing: reproduce. To do that, they need water. Like other mosquitos, Aedes aegypti lives as larval stages in water. So for them to spread, there must be sources of accessible standing water for them to lay their eggs in. And as climate change increases temperatures in Australia, its expected to cause a severe drying up of a large part of the climate. That's good, right?

Well, not for us. The drying area caused by climate change will force people to seek water, too, but we're far more capable of acquiring it than a mosquito. The expected solution is that people will increase the instillation of water storage devices. Which, unfortunately for us but luckily for the mosquitos, are perfect breeding grounds.

So, you see, it won't be the climate's fault the mosquitos spread - it'll be ours. Our own adaptations to changing conditions will have the biggest impact on mosquito densities, according to the models in this paper. The authors write "in Australia, climate - and in particular temperature - plays a less important role in determining the range of this species due to a combination of its intimate relationship with humans and our propensity to store water... the synthesis of our GARP [ecological] modelling, the theoretical climate limits and the historical distribution of this mosquito strongly suggest that a distributional expansion is possible and could expose the majority of Australia's population to this dengue vector."

The key to preventing this possibly terrible outbreak, the authors stress, is to control how we store water. Previous research has shown that, in Australia, poorly screened and maintained domestic rainwater tanks have been identified as huge mosquito breeding grounds. And in Brisbane, Queensland's capital city, over 75,000 tanks were installed in 2007 alone. "If the installation and maintenance of domestic water storage tanks is not tightly controlled today," the authors add, "Ae. aegypti could be spread by humans to cohabit with the majority of Australia's population."

Their results echo an all to familiar warning about climate change and mosquito-borne diseases. The rising temperatures are only going to increase the ranges of these winged carriers of malaria, west nile virus, and a whole host of other deadly and costly diseases. Many of the real distribution changes, though, will come from our own adaptations to changing climate. If we don't prepare and look for solutions now, these infectious diseases are only going to become more and more pathogenic in the future.

Beebe, N., Cooper, R., Mottram, P., & Sweeney, A. (2009). Australia's Dengue Risk Driven by Human Adaptation to Climate Change PLoS Neglected Tropical Diseases, 3 (5) DOI: 10.1371/journal.pntd.0000429


Gabba said...

In Australia today, dengue epidemics are limited to Queensland, where the mosquitos are most prevalent.Yes and no. Dengue is found all across northern Australia. The reason Queensland gets the epidemics is because that is the only area in northern Australia that has (relatively) high population numbers. The rest of northern Australia is very sparsely populated indeed, with the possible exception of Darwin region, with around 120 00 people, and they see individual Dengue cases there fairly regularly.

Sheldon said...

Dengue fever is really a worldwide disease. We can really not easily get rid of it so as much as possible being a careful person is necessary.
Having a test using elisakits after experiencing some signs and symptoms is a good idea. You can easily know if your infected or not.