Malaria is marching north, and global warming will make it worse, with mosquito armies colonising Europe, the US and highland regions of the South. That鈥檚 the picture being painted by a panel of UN scientists and several national governments. But for one of the world鈥檚 senior entomologists, this is not honest science. Paul Reiter is chief entomologist at the US government鈥檚 dengue research lab in Puerto Rico. And he鈥檚 afraid that attributing the spread of malaria to global warming could detract attention from much-needed efforts to combat the disease itself and save lives now. Ehsan Masood asked him why he鈥檚 decided to speak out.
What evidence do you have that global warming won鈥檛 lead to an increase in malaria and dengue fever?
First of all, most people think of malaria as a tropical disease. That鈥檚 completely wrong. Until very recently it was widespread in Europe and North America. In the 1880s, virtually all the US was malarious, and even parts of Canada. When the organisation I work for, the Centers for Disease Control and Prevention (CDC), was founded in 1946, its principal mission was to eradicate malaria from the US. In Europe, the disease was endemic as far north as Norway, Sweden and Finland. In the 1920s, epidemics killed hundreds of thousands in the Soviet Union, right up to the Arctic Circle. One of the last European countries to be freed of the disease was Holland. That was in 1970. As for dengue, the principal vector has been living happily in North America for about 300 years. At times the disease has been rampant. Indeed, the world鈥檚 first recorded epidemic was in Philadelphia in 1780. In 1922, all the southern states were affected. There were an estimated 500 000 cases in Texas alone.
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What were average temperatures when malaria and dengue were prominent in Europe and North America?
It was a little cooler than it is now, but that鈥檚 pretty irrelevant. The crucial difference between the tropics and the temperate regions is not that the tropics are hotter but that they don鈥檛 have cold winters. If you look at a weather map, you鈥檒l see that for many months of the year temperatures in the US and much of Europe are at least as high as in many tropical countries. If you plant in springtime you can grow 鈥渢ropical鈥� crops like peanuts, rice, sweet potatoes and sugar cane. It鈥檚 the summer鈥檚 warmth that counts, not the winter, and it鈥檚 the same for malaria. All you need is a mean of about 15 掳C for at least a month each year.
Why did malaria decline in the northern hemisphere?
In some places, it was drainage schemes, insecticides, anti-malarial medicines, and so on. But the most important factors were complex changes in the way people lived, which reduced their contact with mosquitoes. You can see this even today. For example, Texans love air conditioning. They live for much of the year with all their doors and windows closed. Over the border in Mexico, most people use natural ventilation assisted by electric fans. Between 1980 and 1999, there were 68 cases of dengue in Texas. In three adjoining Mexican states, there were 62 514.
Climate change researchers claim that an increase in extreme weather events will lead to more pools of stagnant water where mosquitoes could breed and that higher temperatures kill mosquito predators . . .
I find this very frustrating. Specialists in my field have had little voice in this debate. Take the Intergovernmental Panel on Climate Change (IPCC) which produced a global assessment of climate change in 1996. The bibliographies of the nine lead authors of the health section show that between them they had only published six research papers on vector-borne diseases. Nevertheless, they devoted a third of their chapter to speculation on the future of those diseases. On the other hand, if you take those of us who don鈥檛 toe their line, you will find we have well over 600 publications on the subject. It beats me why the IPCC is given such credence while we are branded as sceptics.
OK. But what of their specific claims? Does more rainfall lead to more malaria?
Such claims may sound credible, but they are highly simplistic. Malaria is definitely associated with rainfall in many parts of the world, but there are also lots of places where epidemics only occur during drought. For example, in south-eastern Sri Lanka, drought causes rivers to dry up. The resulting pools produce hordes of anopheline mosquitoes. Dreadful epidemics have occurred in drought years.
But what about predators? Surely hot weather kills mosquito predators like lacewings and ladybugs?
That鈥檚 the first I鈥檝e heard of it. Lacewings prey on aphids, and dragonflies are not considered major predators of mosquitoes. Besides, why should predators prefer cold weather? If anything, I would expect the reverse: longer warm seasons could lead to higher, more stable predator populations.
Are you saying that more mosquitoes don鈥檛 necessarily lead to more malaria?
Again, it depends where you are. Anyway, numbers of mosquitoes or numbers of bites are not the main part of the story. The dominant factor is the survival rate of the insect. Mosquitoes transmit diseases when they inject their saliva while they are feeding. For a mosquito to transmit a virus or parasite that it has swallowed with its blood meal, it must itself be infected by these agents and they must eventually multiply in its salivary glands. All this has to happen before the insect dies. At summer temperatures, the process takes something like 10 days. If the mosquito dies before those 10 days are up obviously it won鈥檛 transmit anything.
If temperature is not the dominant player in malaria, then why is the disease making a comeback in certain highland regions of Kenya, Rwanda and New Guinea, which are getting warmer?
This is where I really lose patience, because all these examples are carefully described in the scientific literature. Before the Second World War there were perhaps a million so-called Stone Age people living in the highlands of New Guinea, totally isolated from the outside world. The land was heavily forested, and there was no malaria. On the other hand, the coastal lowlands were dangerously malarial. Shortly after the war, highlanders started to work on the coast. There was increasing concern that they would carry the disease back to their villages. In the 1950s, the law compelled people travelling to the highlands to go through a two-week quarantine with mandatory treatment for malaria, even if no infection was evident. In the same period, the highland forests were being cleared, creating an environment of open sunlit pools that were ideal for malaria mosquitoes. Increasingly large numbers of workers moved in from the coast, and the attempts at quarantine eventually broke down. The consequences have been tragic, but have nothing to do with the climate.
But what about Kenya and Rwanda. Surely climate change must have played some part in the return of malaria there?
Absolutely not. Let鈥檚 take the Kenya highlands. Just as in New Guinea, forest clearance created an environment of open sunlit pools, ideal for Anopheles gambiae and A. funestus, the principal malaria vectors in Africa. Devastating epidemics occurred from the 1920s onwards. After the Second World War, the disease was pretty well eliminated by well-organised DDT campaigns. Sadly, it has crept back. However, it has not reached anything like the altitudes of 60 years ago. What鈥檚 more, the tea estates kept good meteorological records. Average temperatures today are only marginally higher than they were then.
What about Rwanda?
This is the most annoying of all the examples. Until relatively recently, the people of Rwanda lived in isolated homesteads on the steep mountain slopes. There was little malaria. But one of the factors in the terrible turmoil in Africa has been a population explosion. Arable land is now scarce. People have started cultivating the valley bottoms, replacing papyrus swamps, where malaria mosquitoes are uncommon, with rice. The rice fields are surrounded by villages. That鈥檚 where most of the new transmission is occurring. Also, people are travelling much more, carrying the parasites with them. There鈥檚 also resistance to anti-malarial drugs. But perhaps the most important factor in Rwanda is that from 1984 to 1988 UNICEF and other organisations spent several million dollars to improve malaria surveillance and control. When you improve surveillance, you get more recorded cases.
So what causes malaria and dengue to spread if it isn鈥檛 higher temperatures?
Population increase, deforestation, new agricultural practices, irrigation, increased mobility of people, breakdown of health and sanitary services, resistance to anti-malarials, insecticide resistance, war and civil strife, a whole truckload of reasons. Dengue has its own story. It鈥檚 mainly an urban disease. The virus is transmitted by Aedes aegypti, a mosquito that lives close to human habitation and breeds in all kinds of man-made containers. Rampant urbanisation and the throw-away society have been wonderful for the mosquito. Also, highways and jet airliners provide efficient transport for the virus.
OK. But can you see why some scientists go on about climate change and infectious disease? It鈥檚 taken a long, hard fight to get the US to take global warming seriously, and scientists don鈥檛 want to throw that away. Even the slightest contrarian messages can be used by the oil and auto lobby to obstruct efforts to address global warming . . .
You seem to be implying that the ends can justify the means. I disagree. The people who are most vociferous in this debate are simply not familiar with the epidemiology of diseases like malaria and dengue. My interest is in trying to keep the science straight. I love my subject and so do my colleagues. We are greatly concerned that a distorted picture has been presented to the public and is being used to drive policy.
What鈥檚 wrong with emphasising the risks of global warming if it鈥檒l lead to greater public awareness and investment in things like climate change research?
We are in the midst of a wonderful revolution, the new biology. There are lots of exciting possibilities just waiting to be explored: new ways to diagnose and treat infections, make vaccines, control vectors and lots more. With funding, commitment and organisation, we can deal with the mosquito-borne diseases. After all, it was precisely the advancement brought about by our modern economies that put these diseases at bay. If we jeopardise the opportunity for others to share in that advancement, on the mere basis of emotive arguments founded on an uncertain climate science, I believe we will be committing a serious mistake.