Mosquito outbreaks rising with the tide

My scientific colleague and regular collaborator, Corey Bradshaw of ConservationBytes, has been hitting the media this week to talk about mosquitoes, models and environmental change. Together with our postdoctoral researcher Guojing Yang, we’ve published a couple of papers on the population dynamics of mossies in northern Australia. The latest one has just appeared in the (freely accessible) online peer-reviewed journals from the Public Library of Science: Yang, G-J, BW Brook, CJA Bradshaw. (2009). Predicting the timing and magnitude of tropical mosquito population peaks for maximizing control efficiency. PLoS Neglected Tropical Diseases 3: e385.

Here is our general summary of the work:

Mosquitoes carry several diseases that are potentially fatal to people. The risk of disease transmission is high when mosquitoes are abundant in an area, and it is therefore the job of health professionals to control or prevent mosquito outbreaks in certain areas, especially those close to human habitation. Biologists that study mosquito populations have the ability to predict peaks in mosquito population abundance by relating measures of these with environmental variables, such as tidal events and rainfall. Here we analysed data of mosquito (Aedes vigilax) populations from northern Australia over 15 years. We compared the highs and lows in mosquito numbers to possible drivers of these, such as tides. We found that low tide events prior to the mosquito peaks were followed by a boom in mosquito numbers. We also found the highest population growth rate is in September, which is two months earlier than the peak of mosquito abundance. Thus, following low tide events in the dry season, targeted control (such as spraying in earlier September) of mosquito breeding areas may allow for more effective control of mosquitoes close to human settlement, and therefore reduce the likelihood of disease outbreaks.

Clare Peddie from the Adelaide Advertiser sums up the story and implications well in this brief piece, which I reproduce below. Below this, I’ll then have a go at explaining more fully the climate link with reference to some previous work.

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RESEARCHERS have found a way to predict plagues of disease-carrying mosquitoes up to two months ahead.

The warning system could be the latest weapon in the fight against dengue and Ross River fever, University of Adelaide ecologist Associate Professor Corey Bradshaw says.

“This model is a tool that helps predict when there is going to be a higher-than-average outbreak so that population control efforts can be implemented when they are going to be most effective and are most needed,” he said.

The researchers analysed 15 years of data on the northern Australian mosquito that transmits the Ross River and Barmah Forest viruses. They compared population size to environmental factors, such as tides and rainfall.

“Basic environmental monitoring data can be coupled with relatively simple population models to assist in predicting the timing and magnitude of mosquito peaks which lead to disease outbreaks in human populations,” Associate Professor Bradshaw said.

Salt-loving species tend to peak after very high tides. But the frequency of high tides and the amount of rainfall in the preceding months when mosquito numbers are low are critical – dictating the magnitude of eventual peaks.

“Previously, we didn’t know how big that peak would be,” he said.

“With this model, mosquito control efforts can be scaled according to the expected size of a future peak.”

Associate Professor Bradshaw said the same model could be applied to other mosquito species, for example dengue- or malaria-transmitting species, and others in tropical regions worldwide.

The research is detailed in a paper published online in the Public Library of Science journal PLoS Neglected Tropical Diseases at www.plosntds.org

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So what’s the climate change link? Well, we’ve shown that we can have a decent go at predicting the timing and magnitude (peak density) of mossie outbreaks based on tides levels and amount of recent rain. We have shown previously, in two earlier papers, that to make predictions of abundance and growth patterns with reliability, we need to consider simultaneously both external drivers (like rainfall and tides) and internal effects (density dependence — where the abundance of mossies is partially controlled by how many there were in previous time periods, due to competition, attracting predators such as fish, etc.).

Overall, our work indicates that climate change, via observed (ongoing) and expected future increases in tropical rainfall (more precipitation and heavier downpours), combined with higher expected frequencies and intensities of high tide events associated with sea level rise, is going to have a major impact on mosquito abundance trends in northern Australia.

For instance, more rain will increase the availability of ephemeral freshwater breeding pools, likely helping freshwater breeding species, such as Culex anulirostris. In addition, sea level rise will lead to increases in the frequency and intensity of high tide events. Our models clearly predict that increases in the frequency of high tide events will provide more suitable breeding habitat for Aedes vigilax (the Dengue mosquito) and other saline breeding species, which may, by proxy, increase the frequency and transmission of mosquito-borne diseases in northern Australia. Be prepared for some serious adaptation work in the health system as a result…