As the world becomes smaller and warmer, diseases which were once confined to the tropics are now appearing in temperate zones. Robert Desowitz says it's time to get concerned

Not far from London, in the Wash and in the marshes of Sussex and Kent, the natives were sick with malaria. This was not during some remote age when hippopotami cooled their blood in Thames mud. Between 1850 and 1860 more than 60,000 patients were admitted to London's St Thomas's hospital, diagnosed as having the ague - the acute, cyclical fever of malaria.

By October 1865, 29 people had contracted yellow fever; 17 had died. These victims were not Nigerians, Brazilians or Cubans but Welshmen from Swansea. The Hecla, a modest wooden cargo ship carrying iron ore from Santiago, Cuba, had put into Swansea. On board, James Saunders, a sailor, was dying of yellow fever. During the next weeks dock workers came down with this mysterious, frightening disease. Then Swansea citizens with no connection to the port were stricken. Finally, autumn cold set in and mosquito transmission of the virus (whose mosquitoes - Cuban stowaway or local Welsh - was never determined) ceased and with it the mini-outbreak was over.

Two infectious, mosquito-borne diseases emerged from the tropics to infect people living in the temperate world. If this sounds familiar it is because the media - in books, articles, films and television programmes - have assailed us with the same epidemiological facts of life and death. They would have you believe that infections coming to us from a mysterious tropical cauldron are unique occurrences of our age. But this is not true - malaria, yellow fever, hookworm, the plague and, probably, syphilis came from that cauldron at an earlier time, as HIV/Aids, Ebola and Marburg viruses have come in our time. There is no "cold barrier". We are not, nor have we ever been, isolated or insulated from the scourging microbes and parasites of the tropics.

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The essential elements, immigration and ecological change, that brought new pathogens to populations in 997 remain operative 1,000 years later. People move, and with them move their worms and germs. Immigrants can vitalise their host nation by their labour and culture. They can also be a hive of germs. Those reluctant immigrants, the African slaves, brought malaria, leprosy, hookworm, filariasis (elephantiasis), and yaws into the New World. The willing immigrants, colonists and missionaries, brought smallpox and common childhood diseases such as measles and whooping cough, devastating to immunologically naive adults, to the Amerinds as well as to other isolated populations throughout the world.

We are, so far, less sure of what the new immigrants may bring to us, or have already brought as slumbering epidemic potential. It has been estimated that there are some 100,000 immigrants from Latin America living in the United States who harbour the protozoan parasite Trypanosoma cruzi in their blood and tissues. The reduviid blood-sucking "assassin" bug is waiting in many parts of America ready to take up transmission. It causes the almost untreatable Chagas's disease, the major cause of heart failure in the tropical Americas, and has already appeared in dogs and people in such unlikely places as the environs of San Francisco. Some super-orthodox Lubavitcher sect Jews of Brooklyn have contracted neurocysticercosis. This caused by the larval form of a tapeworm - the pig tapeworm. The source of infection (the worm egg) is traced to their Mexican domestic help. Mexico is the world's pig tapeworm capital. Baggage handlers at a Paris airport get malaria. The Aids virus is thought to have come out of Africa via immigrants.

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In 997, or even 1897 for that matter, the immigrant came by slow boat. Plenty of time to develop symptoms and even die on the way over. The laws of quarantine were based on the slow boat principle. Now you can get from anywhere to anywhere in less than 24 hours and pathogens have rapid transit. Nor has there ever been such a multitude of mass movement of peoples from the Third World to the industrialised world. Ecological degradation is the second element. There is an infectious disease consequence to each tropical forest felled, grassland invaded by suburban sprawl, or greenhouse-induced rise in temperature. Many of those environmentally-influenced infections are zoonoses transmitted by blood-sucking arthropods.

Zoonoses are diseases in which the causative pathogens are transmitted from animals to humans. But what is virulent for man is often harmless in its natural animal host. In the stable, undisturbed ecosystem those microbes can remain hidden, unknown to science. We are unaware of their existence until they cross the biological chasm and announce their presence in the sick human. Then comes the frantic effort to find the natural animal host source, to identify the circumstances that caused transmission and finally, to seek some means of cure and prevention.

These new zoonoses have been called emerging diseases but often as not, like Ebola, even the most vigorous search fails to reveal from where and how they have emerged. And that indeterminacy is frightening. And like Ebola, too often the cure and prevention cannot be furnished by scientific research. Twenty-four-hour discoveries to save the world from microbial Armageddon exist only in the imagination of the screen writers.

Mosquitoes transmit malaria, filaria (elephantiasis), and a myriad of viruses such as dengue, yellow fever and Japanese encephalitis; fleas transmit plague and murine typhus; lice transmit typhus and trench fever, while ticks transmit Lyme disease, Russian spring-summer encephalitis and Kyasanur forest disease - to name a few. Undoubtedly other vector-borne potential zoonotic pathogens are circulating but unknown in intact tropical ecosystems. When global warming makes London subtropical and Rome a city of equatorial climate those tropical and subtropical mosquitoes, blood-sucking flies, ticks and mites will follow. A former forest-breeding mosquito will become a city dweller and there will be urban malaria, urban dengue and yellow fever, urban encephalitis and urban elephantiasis.

The prospect that makes us fearful is the emergence of a pathogen so virulent that it threatens the existence of the human species. Great killing epidemics have occurred in the past. These include the plague of Athens, a mysterious disease that killed one-third of the Athenian population in a 20-year period beginning in 430 bc; the black death of Europe beginning in 1348 so reduced Italy's population that it took almost 400 years to regain pre-plague size; the killing syphilis in Europe, beginning in 1493 after Columbus was thought to have brought it from the New World; and the great influenza pandemic of 1918, thought to have killed between ten and 20 million people.

The human species has, so far, managed to survive nature's worst microbial assaults. However, we are living in an age of an unprecedented concatenation of ecological degradation, massive global migrations (especially from the tropics), global warming, and failing chemotherapeutic standbys. From this has come an unprecedented number and variety of new, untreatable infections that kill almost all they strike - Marburg, Ebola, Lassa, and Aids. These may only be curtain-raisers for the main act yet to come. We are also living in an age when military microbiologists have genetically engineered microbes into exquisite pathogenicity.

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A few days ago I was listening to a radio interview of an American general who was about to be nominated as chief of staff. He was saying that we must be prepared for all unknown threats, to plan for every contingency, to constantly develop new weaponry and, of course, to be prepared to pay for it all. The unspoken implication was that his political masters had decided that it was in the nation's best interest to deal with overseas conflagrations, be they in Somalia, or Haiti, or Bosnia. The homology of his remarks to the emerging diseases-microbiological imperative was striking. The global military and medical needs seemed essentially the same. The disparity is in their relative fulfilment. How then, should we address the known and unknown threats of microbial and parasitic pathogens?

First of all we must accept the unpleasant realities. The priority of our concern is over the pathogens that may emerge from the tropical Third World. Surveillance in the way of preventative strikes, ie local epidemiological research and clinical case detection with competent, technologically modern laboratory support is practically non-existent in most of those countries. They have enough trouble in coming up with that Pounds 2 or so annual per capita health expenditure - and that includes dealing with Aids.

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If not the tropical nations then who should we look to as the "emerging disease police"? A supranational organisation such as the World Health Organisation? Others will disagree but it is my observation that the WHO has been rendered ineffectual by staffing and budgetary deficiencies and distracting political subversions. This is particularly true at the regional level, the semi-autonomous WHO offices that actually would be responsible for planning and operations.

If not the WHO, then the governments of the industrialised nations, former colonial powers, themselves? Before the demise of colonialism those countries had in place a remarkably effective network of interacting research and operational health units. Could they be expected to re-establish, for their own self-interest, their medical research services - a sleeping sickness service, a malaria service, a virus research service? Would the now sovereign nations permit such an intrusion?

Then there are the learned professional societies such as Britain's Royal Society of Tropical Medicine and Hygiene. They are potentially powerful agents for planning. They could be a strong voice to influence government policy. Unfortunately the effectiveness of these societies is now mitigated by the partisanship of their membership - molecular research types, clinicians, epidemiologists, health policy planners; each faction compartmentalised by their need to compete for the limited funds necessary for their professional pursuits.

What about the military? When Ebola began killing the monkeys held in a commercial facility in Reston, Virginia, only the United States army had the authority, resources and nerve to "nuke'' the monkey house. Civilian organisations seemed paralysed and powerless at the threat, which fortunately turned out to be a non-starter.

I would favour the professional societies to do the best job and to prompt their governments to begin planning for foreign and domestic health contingencies. I know that this means an endless series of committees, meetings, and reports. But it would be a start, and we must start somewhere. If we are very lucky there will be no great emerging tropical killer to strike us in our temperate zone sanctuaries. If we are very lucky there will be no new wars and our military establishments can be disbanded. But don't bet your life on it.

Robert Desowitz is professor emeritus of tropical medicine and medical microbiology, University of Hawaii. His book, Tropical Diseases: From 50000 bc to ad 2500, is published by HarperCollins, Pounds 18.99

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