Exotic diseases

Among the species that may find new territories to colonize, some are insects that transmit diseases. Fears have been expressed that tropical diseases may spread into countries of the temperate regions. Malaria is the most dreaded of these.

Such fears are greatly exaggerated. Indeed, the risk may be nonexistent. Malaria was once common in parts of Britain. Known as "ague," it occurred in marshy areas in the east of the country, along parts of the Thames Estuary, and in the low-lying marshes of Somerset in the west of England. The last recorded case of English malaria was in 1911. True, the Anopheles mosquito that transmitted ague was of a different species from those that transmit tropical malaria, but the disease was the same and it was caused by the same protozoan parasite, Plasmodium. Malaria was prevalent in England during the centuries when the climate was distinctly cooler than it is today (see "The Little Ice Age" on pages 87-93).

It disappeared for a number of reasons. People moved away from the marshy areas, and these were largely drained. Public health provision improved and living standards rose. Draining the marshes destroyed the habitat in which mosquitoes breed. Better living standards included better housing, with fewer opportunities for mosquitoes to enter homes and feed on people while they slept. Better health provision included better education in preventing disease.

Sea levels and storms

Malaria has also vanished from the other industrialized countries of the temperate regions and for the same reasons. It is difficult to see how the disease could return. Warmer weather will not restore the marshes, and without these wetlands migrating mosquitoes will lack breeding sites. If the Plasmodium parasite should be discovered, the public health authorities would have no difficulty in dealing with it.

In July 2000 there was an outbreak of dengue fever in El Salvador. For several years this disease slowly advanced northward through Central America. It is thought of as a tropical disease—the same "dengue" is from a Swahili word—and another of those that warmer weather might bring to temperate regions. Interestingly, though, the disease advanced as far as the United States border, but no farther. Its advance was checked by the efficiently organized U.S. health authorities, which were ready for it and took the appropriate steps to prevent its spread.

Dengue fever is also spread by mosquitoes, and it used to be well known in Europe. In England it was known as "breakbone fever" or "dandy fever." This referred to its most distressing symptom, which is pain and stiffness in the joints, leading to a peculiar, "dandified" gait with the neck and shoulders held stiffly. Although it is temporarily incapacitating, dengue fever is very rarely fatal. It disappeared from Europe, just as malaria did, and it is just as unlikely to return on a large enough scale to present a serious threat to public health.

Nowadays the most serious health risks arise from the extent and speed of travel. Even in the days before widespread commercial air transport, a global pandemic could have devastating consequences. The outbreak of influenza in 1918 swept across the world. Fewer than 3 percent of its victims died, but so many people caught the illness that it killed more than 20 million people. Health experts fear that a new infection, such as severe acute respiratory syndrome (SARS), also with a low mortality rate, might cause similar devastation. This risk is far more serious than that of a tropical illness moving because of warmer weather and overwhelming the health services.

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