While global warming will encourage the spread of mosquito-borne disease, it is by no means the only factor, reports Jearelle Wolhuter
The arrival of summer is always a happy event. The accompanying arrival of mosquitoes less so. While itching mosquito bites are often regarded as an irritating but harmless part of summer, in the more tropical regions of the world the high-pitched sound of a mosquito closing in can spell death. But mosquito-borne diseases are no longer confined to faraway hot countries and a handful of unlucky backpackers. Changes in our lifestyles and global weather patterns could soon see these diseases coming to a holiday resort near you.
Many people regard mosquito-borne diseases such as malaria, dengue fever and West Nile fever as being a distinctly tropical and third world kind of affliction. There is certainly some logic to this belief. Dengue fever is endemic in large parts of South America, Africa and Asia. The latest large outbreak was centred on Rio de Janeiro and affected hundreds of thousands of people. Malaria kills three million people in Africa annually, most of them children under five years of age. Mosquitoes spread these diseases between humans or between humans and animals. The diseases are where the mosquitoes are, and mosquitoes certainly prefer it hot, with plenty of standing water to breed in. Sporadic outbreaks in North America and Europe, such as when West Nile fever reached New York in 1999 and affected the Camargue horses in France in 2000, seemed just that: shocking, but uncommon.
Malaria can be deadly, but even when people survive, it incapacitates them for a while, just like the usually non-lethal dengue and West Nile fevers. This means large sectors of the workforce and school-going children are non-productive in countries already badly hit by Aids. These diseases are spreading quickly. Not just to North America and Europe, but also to previously disease-free areas in the tropics. West Nile virus first appeared in the USA in 1999, but scientists expect it to pose a risk throughout America by 2006. This disease is spread mostly from birds to humans and infected wild birds now provide a reservoir of the disease.
Mosquito-borne diseases were once quite prevalent in North America and Europe. In fact, the Centres for Disease Control and Prevention (CDC) was founded in 1946 to eradicate malaria from America and the disease was only stamped out in Europe in the middle of the twentieth century. This is why some scientists question the theory that global warming is leading to the spread of these diseases. As Paul Reiter, a scientist with the CDC pointed out in a 2000 interview with New Scientist, it is not that the tropics are that much hotter, they just don't have colder winters. "For many months of the year temperatures in the US and much of Europe are at least as high as in many tropical countries. All you need is a mean of about 15 degrees Celsius for at least a month each year."
Europe and North America eliminated mosquito-borne disease through good eradication programmes combined with cold winters. People also changed their lifestyles and reduced the contact they have with mosquitoes. As Reiter explains: "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. Between 1980 and 1999, there were 68 cases of dengue in Texas. In three adjoining Mexican states, there were 62, 514.†So while it is true that mosquitoes breed faster and more often in higher temperatures, there has not been enough of a rise in global temperatures to explain the resurgence of diseases - at least not yet.
According to Reiter, there are bigger problems than global warming that need to be addressed right now. There is widespread resistance to drugs and pesticides used to combat mosquitoes. Public health programmes combating the spread of mosquitoes are expensive, and in many third world countries the health system is basic at best. Social instability, poverty and war all lead to migration on a huge scale. When people move to cities in large numbers, they often end up in unhygienic shanty towns. This, combined with deforestation, irrigation systems and dams have created the ideal breeding ground for mosquitoes: large pools of standing water. For example, dengue fever is a largely urban phenomenon, since the mosquito that spreads it breeds mostly in man-made containers.
So not only are there more mosquitoes, there are more resistant strains of disease and scarce resources to combat its spread. And yet, more and more tourists are travelling to previously unexplored locations. Cheap air travel is in part responsible for more people coming into contact with ever nastier strains of these diseases. Some become ill overseas and bring the disease back with them, and sometimes mosquitoes hitch an unintended lift in the luggage of a passenger. While they are not ideally suited to it, local mosquitoes can transmit tropical diseases, since many mosquito control programmes were discontinued or cut back when the diseases they spread seemed to have vanished for good. Once infected travellers have returned there is a risk that doctors might not recognise the symptoms of tropical diseases, as the early symptoms can look remarkably like flu.
Vaccines have proved remarkably hard to manufacture, but seem to be one of the best long-term solutions. The development of new drugs, even when there is a market for them, takes time. Meanwhile, conditions are ripe for the spread of these diseases.
