Moritz Kraemer, Univeristy of Oxford
There are now more than 7 billion of us on Earth, and we’re more mobile and living in higher densities than we ever have previously. Together with climate change, this is creating novel niches for diseases and the vectors that transport them. Two viruses, dengue and chikungunya, which both cause symptoms of fever, prostration, and arthritis, and can be fatal, appear to be becoming significantly more common and widespread. They’re transmitted by two strains of Aedes mosquito – Aedes aegypti and Aedes albopictus. But our knowledge of where these mosquitoes are and how they’re spreading is limited, which makes monitoring, anticipating, and controlling outbreaks of these diseases much more difficult. Coming to the rescue is Oxford University’s Moritz Kraemer who’s pulled together data from across the world to produce the first global distribution map for these mosquitoes.
Moritz - So, the context of this study was largely to identify areas where we have mosquitoes that are transmitting two viruses that are of global concern. These two viruses are dengue fever and chikungunya fever, and the mosquitoes that primarily transmit them are Aedes aegypti and Aedes albopictus. Both of these viruses are now sort of increasing their geographical spread but also their numbers and overall health burden on the population.
Chris - What about the sources of both of these agents? If we wind the clock back, when did the first really rise prominence and begin to be a problem? And how rapid is the trajectory of spread?
Moritz - Initially, of these two viruses, one chikungunya emerged in Tanzania, it was sort of the first report, and dengue in Asia. But only over the past sort of like 20 – 30 years they actually became a big public health problem, and there was an estimate of 2013 where we have estimated that 100 million apparent infections happen every year. For dengue fever and for chikungunya, we actually don’t even have these numbers on a global scale, but here in 2014, 1 million infections were recorded in the Americas alone.
Chris - And why do we think this is happening? Obviously, we need the mosquitoes there, but mosquitoes have been around for millions of years, as have our human ancestors. So, why is this happening now?
Moritz - So, a lot of people have hypothesized, and we sort of assume that as well, that human mobility and also the increased mobility of trade behaviour between countries has sort of amplified that. Mosquitoes are spread in used tires where they are transported from one place to another, and this happens a lot now between Asian countries, for example, and Europe, and also Asian countries and the United States where there’s a lot of economic relations. On the other hand, we do fly a lot more, we do travel a lot more as humans and people go on vacation to certain places. We introduce the virus once we get infected. And then, plus the mosquito might be there. In Italy for example, there was this case of this Indian traveller who came back and then infected 200 people with chikungunya.
Chris - So, it all comes down to population dynamics, overall population overland movement, doesn’t it? How have you sought to try and map where the mosquitoes are going?
Moritz - For getting the data together, we started just sort of understanding, okay, what is information given out in the published literature? So, we did a whole lot of research just trying to find out, okay, how many papers are there discussing these mosquitoes and discussing them specifically with the information of the geography of these mosquitoes. So, we did a whole lot of research and downloaded all these papers and had four people working on it full-time trying to understand where do we find these mosquitoes. And we wanted that on a high spatial resolution because we wanted to map it on 5 kilometre by 5 kilometre spatial scale.
Chris - You’re nonetheless relying on other people’s data for this, aren’t you? So, there must be gaps. Because if they haven’t looked in an area, you won’t have anything to go on for that particular geography.
Moritz - Right. So, what we also did was contact a lot of health officials from different countries. This included the ECDC, the European Centre of Disease Control, but also the Taiwan CDC, Brazil for example, and Indonesia, and tried to find out what kind of information they actually collect on a daily basis. Because dengue and chikungunya have been a problem there for quite a long time now. So, we sort of gathered that data together and put them into our database and made them all work together in this database, and then put them into our model.
Chris - Were there any pointers towards, perhaps, novel ways that we might consider mitigating or controlling the spread, either of the mosquitoes or the disease, and any other surprises in here. Things that you really were quite shocked to see in the data.
Moritz - We haven’t specifically actually looked at any sort of control measurements that were put in place. Our understanding here, like our approach here was trying to find out, okay, what is actually the global distribution of these two mosquitoes and this really hasn’t been done before in that comprehensive manner that we have done. And so, there was this sort of first approach that we go in from here, trying to understand, okay, if we have information about control measures in a certain region, how effective are these measures, actually? And let me – this is a very good question but important also for future surveillance and guidance and control to understand – okay, if you put some measure in place at some point in geographic space, what does it actually mean and how can we contain the spread? But what we have sort of seen over the past years is that it’s very hard to contain the spread of these mosquitoes, even in areas where we have quite good health systems in place.