It bites, it kills, it's coming to Essex

Malaria, or the ague, was rife in 17th-century Britain. Now many researchers believe global warming could bring the disease back to these shores

Winters were bitterly cold in the days of the Tudors and Stuarts. Between 1550 and 1700, Britain suffered a little ice age. The Thames froze over; sheets of ice extended off Brighton beach; and the Arctic ice pack extended so far south that Eskimos took to landing their kayaks in Scotland. Oh, and England was gripped by malaria.

Winters were bitterly cold in the days of the Tudors and Stuarts. Between 1550 and 1700, Britain suffered a little ice age. The Thames froze over; sheets of ice extended off Brighton beach; and the Arctic ice pack extended so far south that Eskimos took to landing their kayaks in Scotland. Oh, and England was gripped by malaria.

Malaria? The tropical disease? The very same. They were dying by the dozen in the salty confines of the Kent and Essex marshes from a disease known as the ague. The English mosquito, Anopheles atroparvus, loved to breed in the brackish waters. Child death rates in coastal Essex were more than one in ten.

According to Mary Dobson of the Wellcome Institute for the History of Medicine in Oxford, Essex vicars refused to live in their marsh parishes. And Daniel Defoe wrote of "a strange decay of the [female] sex here". Men brought up in the marshes had some immunity to the disease. "But they always went into the hilly country for a wife [who] presently got an ague or two, and seldom held it above half a year. And then the men would go to the uplands again and fetch another."

Until recently, researchers doubted that this ancient ague was true malaria. But there is now little doubt, according to Dobson. The disease took classic malarial forms such as tertian or quartan fever, which recurred on alternate days or every third day. William Harvey, discoverer of the blood circulation system, described a thickening of the blood in its victims - the calling card of the most virulent malarial parasite, Plasmodium falciparum. And it responded to quinine in a potion developed by a drop-out apprentice apothecary, Robert Talbor. (His breakthrough came just too late to save Oliver Cromwell, who died of the ague in 1658, but just in time for him to be knighted by Charles II.)

Remarkably, says Paul Reiter, the author of a new study called From Shakespeare to Defoe: Malaria in England in the Little Ice Age, "the entire story took place in a period when temperatures were probably colder than in any other period in the past 10,000 years". But why the history lesson? Simple, says Reiter, a Briton now working for the US Centers for Disease Control in Puerto Rico: it makes a nonsense of the widespread claim that Europe, the US and many other parts of the non-tropical world are threatened by malaria as global warming gathers pace.

The issue is serious. More than 300 million people suffer from malaria and over a million die a year. Most are children under five living in tropical Africa. But the infection is also endemic from Afghanistan to Indonesia and from Mexico to Brazil. And while other killers retreat, malaria extends its grip, penetrating central Asia, highland regions of central America, east Africa and New Guinea. Many medical researchers claim the rise is directly attributable to global warming. "The resurgence of malaria and other mosquito-borne diseases is occurring at high latitudes and high elevations contemporaneously with climate change," says Paul Epstein of the Harvard Medical School in Boston. In 1998, the warmest year of the last millennium, there were major outbreaks in the highlands of New Guinea, Kenya, Uganda, Tanzania and Rwanda. It returned to the Kenyan capital, Nairobi, for the first time since the 1930s.

If malaria does follow temperature, then we are in big trouble. Epstein predicts that "conditions suitable for transmission will increase from the present 45 per cent of the globe's surface to 60 per cent" by the middle of the next century. For Reiter, such claims "disregard history".

So what is going on? The disease is a complicated one, caused by a single-cell parasite called plasmodium. Of the four species that cause malaria in humans, Plasmodium falciparum is the most feared, being responsible for some 90 per cent of deaths today. The parasites are passed between humans in the saliva of anopheles mosquitoes as they bite. The disease can be picked up almost anywhere. Hang around an international airport long enough and you will probably meet a malaria-carrying mosquito just off the plane. But for it to take root requires conditions that sustain both parasite and mosquito through their complete life cycles. High temperatures speed these life cycles. But below 16C the parasite's life cycle is unlikely to be completed. This has led Epstein to claim that "malaria generally extends only to places where winter temperature reaches no lower than 16C".

But hold on. That is hard to equate with the stories of malaria in ice-bound 17th-century Britain. The truth turns out to be more complicated. In many parts of the world malaria, or the potential for malaria to spread, is seasonal. Those areas, according to World Health Organisation maps, include Britain, which has always had summer conditions suitable for the falciparum parasite.

Philippe Martin, who researches the disease at the European Commission's Joint Research Centre in Ispra, Italy, argues that in such regions the potential for "epidemics, causing widespread debilitation [and] increased mortality among unprepared or non-immune populations... is high". He concluded a recent paper: "Malaria is today a developing country issue. With climate change, it could become a problem for developed countries, too."

Undoubtedly the climatic potential for malaria to invade Britain exists - but then it always has. And its departure, begun during the 19th century and completed only in the 1950s, had nothing to do with climate and everything to do with draining the marshes, cheaper quinine and better public health and housing.

An important test for the competing theories about malaria spread should come from the highlands of Kenya. During the past century, the disease was gradually eradicated from these regions, thanks largely to colonial insect spraying. Now it is returning. Steve Lindsay, a malaria specialist at the University of Durham, believes that climate is probably playing a role in that return, but says: "I don't believe climate change is driving the epidemic at present. It has more to do with failing health services, resistance to drugs, and ecology."

Human development, he argues, has provided "wonderful new habitat for malarial mosquitoes in highland Kenya". We clear forests to warm surface temperatures; then make pools of water from wheel ruts, borrow pits and irrigation canals so that the insect can lay its eggs; and finally we offer it lifts to new areas for further colonisation.

Tony McMichael at the London School of Hygiene and Tropical Medicine agrees with Lindsay. He rejects the "crudely dualistic models" of Reiter and Epstein. "To suggest an increasing influence of climate in a warming world is not to diminish the far greater influence of many of those other factors," he says.

So what of the English case? "Yes, malaria was present in Europe during the little ice age. But if we recreated 17th-century England today, but had warmer conditions, would there be more malaria now than there was then? That is the scientific question," says McMichael. "Other things being equal, those conditions would allow more intensive, more prolonged and more extensive transmission of malaria. But whether it actually happened would depend on public health defences."

Clearly the map of where malaria flourishes has as much to do with public health provisions and economic development as it does with temperature - arguably much more. Global warming and more intense rainfall could trigger the return of the disease to places where eradication programmes are precarious. But the prospects of its return to Europe? Former Yugoslavia, maybe, if it continues in its dislocated condition. An occasional case near an international airport. But elsewhere, only a major breakdown of law and economic order would make it likely.

The fear of killer diseases could help persuade the world to combat global warming. But, as Reiter says, if our concern is really about malaria, "concern should focus on ways to deal with the realities of transmission, rather than on the weather".