Last week, Radio 938Live asked me for an interview on mosquitoes and dengue. The interview that went on air was of course much shorter than the chat I’d had with the radio man, so here’s a more extensive Q&A:
Does the weather have to do with the increase in mosquito numbers in Singapore?
The weather probably does have some impact on mosquito populations, although I think we still don’t really know to what extent that is the case. Rainfall will increase the number of containers with water, which are potential breeding sites for the mosquitoes, but lots of rain can also wash away larvae in puddles and drains. The temperature will have some impact on how fast the mosquitoes develop, and therefore how fast the population grows. But there are many other factors that also affect mosquito populations – including NEA and the Mozzie Wipe-out – so conclusively pinning down any fluctuations on the weather is very difficult. Some mathematical modelling studies have been able to link temperature to dengue incidence, but what role mosquito densities play in this is not clear.
What causes a mosquito epidemic?
We have to separate a growth in mosquito population from a dengue epidemic here, and the two are not necessarily related. It is possible to have a dengue cluster or epidemic with relatively few mosquitoes. This is further complicated by the fact that measuring mosquito densities can be very tricky. If we read in the papers that the number of households found to have mosquitoes breeding has increased, that could be either because there are more mosquitoes or because the NEA is checking more households. The NEA is monitoring mosquito populations with ovitraps and gravitraps, but these have their limitations too.
You studied aspects if the Aedes aegypti and Ae. albopictus, vectors of the dengue virus. What can you tell us about their unique behaviour as compared to “normal” mosquitoes?
They are normal mosquitoes! If you mean whether dengue-infected Aedes mosquitoes behave differently from healthy Aedes mosquitoes, I couldn’t tell, because we didn’t have any infective insects in our lab, and I’m not aware of anyone else studying that. If you mean whether these two Aedes species are different from other species, then there are many differences. They prefer to lay their eggs in different kinds of water bodies, they bite at different times of the day, you tend to find them in different areas etc.
In your time with the National Environment Agency, what have you learnt about this type of mosquitoes and how to control them?
Most of what my colleagues and I were studying was related to what the mosquitoes are attracted to, so we could trap them more effectively, and how they behave towards the traps we were designing. We designed a gravitrap, which to a female mosquito wanting to lay eggs looks and smells like a perfect breeding site, but is actually a lethal trap for both her and any of the eggs she managed to lay before being trapped. While some people were hoping to use this as a new weapon to control mosquitoes, it was actually meant as a monitoring tool, an improvement on the ovitraps that are being used routinely but can turn into breeding sources themselves if not maintained properly.
What can Singaporeans do to support the fight against the spread of dengue and the Aedes mosquito?
Follow the advice of NEA and prevent mosquito breeding wherever you can. This has been the same message for decades, and it is actually working – we have a lot fewer mosquitoes in Singapore than other places with similar conditions. The problem is that the few that are remaining are still enough to allow dengue to spread. And reducing a relatively low mosquito population even further is so much more difficult. So public education is important, to try and make everyone understand the need to look out for potential breeding sources, and that every household’s effort does make a difference. NEA is always finding new ways to put out that message, and we here at the Science Centre are doing our bit, too. We have a section on dengue in our Living with Viruses exhibition, as well as SARS, HFMD and more. And the Megabugs Return! exhibition that opens this weekend has a huge, moving Aedes albopictus with some info on these mosquitoes and dengue, too.