The ATTACK of the KILLER MOSQUITOES : Global Warming Could Bring a Deadly Increase in Insect-Borne Epidemics

THE SEED WAS SOWN A LITTLE MORE THAN FIVE years ago in Houston, in a pile of scrap tires fresh off a boat from Japan. Destined to be recapped and resold, the tires seemed innocuous enough. But inside that apparently lifeless mountain of rubber, an alien population was breeding: visitors from another country, shaking off the cobwebs of the journey, beginning to take nourishment, to grow and accustom themselves to their new home. The surroundings suited them just fine, and it wasn’t long before entomologists took note and identified the stowaways: Aedes albopictus , better known as the Asian Tiger mosquito, carrier of yellow fever’s close cousin, dengue.

Dengue (pronounced DENG-ee ) is endemic in Asia and Africa and recently has been carving an epidemic path through Central America and the Caribbean. Characterized in its milder forms by flulike symptoms and in more severe cases by bleeding, prostration, shock and death, dengue is just the sort of disease from which Americans tend to consider themselves exempt--a horrible but distant scourge that they will never have to confront unless they happen to visit some corner of the Third World. Even when told that the Asian Tiger mosquito is here, most people assume that the United States has the medical technology and know-how to keep the foe at bay.

Medical entomologist and Asian Tiger mosquito specialist George Craig knows better. “We’ve already thrown up our hands on that one,” he says from his office at the University of Notre Dame. “When the mosquito first showed up in Houston in 1985, we had no idea that it had come from Japan, but we thought it would stay down on the Gulf Coast. Now it’s doing very nicely in places like Chicago and Baltimore and St. Louis.”

Indeed, the Asian Tiger mosquito has shown impressive fortitude, prospering in anything from old tires to Coke cans. “Anything that will hold a quarter-inch of water,” Craig says, “will (act as a breeding ground for) this mosquito. We’ve established new laws on quarantining and on bringing it into the country, but that’s locking the gate after the horse has left. It’s here now, and there’s no way we’ll ever get it out.”

Some scientists worry that the United States could be in danger of a real dengue epidemic, comparable to the 344,000 cases in Cuba in 1981 or the 400,000 cases in Ecuador in just three months of 1988. In recent years, the United States has had about a hundred cases of dengue each year, all of them contracted abroad. “But now we have a new vector (spreading agent),” says Craig, “much more widespread, breeding in all kinds of water containers. So the opportunity for spreading is there. And there is no vaccine or cure for dengue.”

Lately, several medical researchers, notably Joshua Lederberg, the 1958 Nobel prizewinner for medicine and physiology, have been warning of the foolishness of considering epidemics a thing of the past. “There is no reason that a great plague could not happen again,” Lederberg, a geneticist, recently told Science magazine. “Certainly, another influenza pandemic is in the cards. Look at the ravaging epidemic of AIDS. People still do not understand that (the recurrence of epidemics) is a natural, almost predictable phenomenon.”

In the 20th Century, humanity has succeeded in virtually eradicating two of its most terrible epidemic diseases, smallpox and polio, and these victories have given many people the mistaken impression that all such diseases have been vanquished. But two battles do not make a war. Myriad deadly tropical diseases still threaten a great part of the world’s population, and the possibility exists that their spheres of influence could spread dramatically in the coming decades, even into North America and particularly into the Gulf States and California. The reason: a warming world.

The key lies in the agents of contagion of these diseases--insects. It is worth noting that smallpox and polio have no insect vector. The major threats for epidemics today are insect-borne--with AIDS the obvious exception--and many noted entomologists tell us that our intelligence and technology may be no match for these tiny enemies. For all the millions of dollars we’ve spent trying to get rid of various insects, they’re as prevalent and as dangerous as ever.

“It’s getting harder all the time to control mosquitoes,” Craig says. “We’ve lost a lot of the weapons: Drainage of land can’t be done anymore because of wetlands (protection laws), we can’t larvicide because we’re contaminating the water, and we can’t adulticide because we’re contaminating the air. The kinds of controls that were feasible 20 years ago aren’t feasible anymore, and there isn’t anything to replace them.”

Now, with global warming added to the recipe, prospects grow even bleaker. While debate still rages over how serious global warming will prove to be, there is nearly unanimous scientific consensus that the carbon dioxide and other greenhouse gases that humanity has been throwing into the atmosphere most of this century will serve to raise global temperatures at least a few degrees Fahrenheit in the coming decades. Last January, NASA’s Goddard Institute for Space Studies in New York City announced that the seven warmest years on their century-old record occurred in the last decade, and 1990 was the warmest of them all.

Most theoretical atmospheric models indicate that the average global temperature will increase by 3 to 9 degrees within the next few decades. This is a greater warming than any encountered in all human history. Although changing climatic patterns may make some regions drier and others wetter, the overall result will be a warmer and wetter world: If temperatures rise, more water will evaporate, and this necessarily leads to more rain.

The changing climate will affect the distribution and abundance of many species of insects. Mosquito eggs laid in warm stagnant water, for example, cannot survive a northern winter. But as the world warms and the winters become milder, such mosquito populations will be able to move farther north. This is more than merely a nuisance: The plasmodium parasite in anopheles mosquitoes causes malaria, and the only thing that keeps malaria from decimating our population as it does those in tropical countries is our colder weather. (Or, in Southern California’s case, its dry climate.)

In most parts of the United States, we take for granted our insulation from tropical diseases, but in fact their distribution is governed by an ecological interplay among the microbes that cause them, the insects that spread them, and ourselves. If the climate changes, so will this interplay.

“Tropical diseases are going to move northward,” Craig says. “There’s no doubt about it.” He is not alone in this opinion. Recently, a World Health Organization (WHO) task group focusing on the potential health effects of climatic change reported: “It should be clear that climatic change will affect the distribution and prevalence of vector-borne diseases dramatically, and that these changes cannot be ignored.”

What is less clear is exactly which diseases will flourish and where. Because climatologists are largely in the dark regarding the exact manifestations of global warming--where it will be more humid and where it will be drier, or how various locales will react--it’s almost impossible to make detailed predictions. But as the ominous probability of changing disease vectors hovers before us, epidemiologists are already trying to foretell the future.

MALARIA IS A GOOD PLACE TO START. BY THE 1960S, public-health programs and mosquito larvicide had coincided with colder temperatures, widespread urbanization and land reclamation--mosquitoes prefer rural environments of ponds, puddles and ditches--to eradicate the disease in this country. Still, malaria remains one of the great scourges of the planet. The WHO estimated in the summer of 1990 that about 270 million people were infected with the disease and that each year about 1.5 million children under the age of 5 die from it. More than 90% of the people living in sub-Saharan Africa are infected. The story of humanity’s fight against malaria is a potent illustration of the power and resiliency of the enemy.

In the 1950s, in an attempt to destroy the anopheles mosquito population that spreads malaria, India was doused with DDT. At first it appeared to work. The number of cases of malaria there, which had held steady at about 50 million overall since accurate counting had begun, dropped to 50,000 by 1961. But instead of continuing to decline, the incidence of the disease leveled off and then began to climb again despite the continued use of DDT. By 1965, there were 150,000 cases; by 1969, 350,000, and a decade later the number of malaria cases had grown right back to the 50-million level. Today there is more malaria in India than ever before, and with nearly 300 million persons infected worldwide, the campaign is remembered as an infamous failure.

The reasons are twofold. First, the DDT also killed predator insects and fish and disrupted the reproductive cycles of birds, all of which fed on the mosquitoes. Second, anopheles developed a resistance to the chemical more efficiently than did the predators, a general phenomenon that scientists do not yet understand and cannot predict. So after a few years of spraying, the mosquitoes were no longer being killed by DDT, and as they began to reproduce again, they found that their larvae were swarming into a world free of natural predators.

Though some experts find it improbable that malaria could ever reach epidemic proportions in the United States, the possibility remains, particularly in a warmer, wetter setting. There are species of anopheles here that are capable of transmitting the disease, and a study conducted for the Environmental Protection Agency in 1988 concluded that “as the climate becomes warmer, it may be expected that these (insect) vectors will increase in their geographic range.”

A more probable enemy in North America, though, is dengue. The vector mosquito is already in place in many cities and threatens to spread even more. “The Asian Tiger mosquito has moved well out of the hot zone, yes, but still it’s limited,” Craig says. “It’s not in Minneapolis, for example, and the reason is temperature. Will it go to Minneapolis if global warming occurs? The answer is yes.”

With the vector well established, all that might be necessary is for enough people to import the disease itself from places like Puerto Rico, the Virgin Islands and Jamaica to set the epidemic wheels in motion. And with the immigrant population swelling, the wheels are already greased.

A warmer, wetter climate in North America might accommodate another mosquito-transmitted tropical disease: encephalitis, or sleeping sickness, an inflammation of the brain that causes nerve-cell damage, muscular weakness and lethargy. Craig, who has studied encephalitis, points out that there is no vaccine for any of the 50 strains of the disease, six of which are already found in the United States. “Once you get it, there is no protection. It’s another example of the falseness of the easy statement that there’s a vaccine for everything,” he says.

This past winter there was an outbreak of St. Louis encephalitis in Florida, with 280 confirmed cases statewide. According to Jonathan Day, a University of Florida entomologist, one major reason for the outbreak was an unusually large amount of rainfall during the winter dry season, giving the female mosquito an abundance of puddles in which to lay its eggs. Of course, last year’s anomalous weather pattern in Florida probably had nothing to do with the greenhouse effect, but the case is still a frightening example of the possible effects of climatic change.

Many experts think that another repercussion of global warming will be the upheaval of various Third World societies that are unable to adapt their agriculture to new climatic conditions. As migrant populations fleeing from agricultural failure flow into the cities, new ghettos will arise and new epidemic situations will follow. Diseases like leishmaniasis, for example, may prosper as never before. Leishmaniasis is spread by the bites of sand flies; in the Mediterranean regions, the Middle East and the tropical Americas, there are already 12 million victims. Though mild cases present symptoms no worse than those of a bad flu, the disease can develop to cause horrifying facial disfigurations, sometimes completely destroying the victim’s nose. There is no recognized vaccine, and treatment is prolonged and painful: up to 30 days of injections of drugs with terrible side effects and with no guarantee of a cure. The disease’s insect vector has shown a great affinity for urban environments in South America and Central America, and Enrique Bucher, of Argentina’s University of Cordoba Centre for Applied Zoology, warns that “urban forms of leishmaniasis may spread northward into newly warmed-up areas.”

Bucher has been particularly concerned with the link between global warming and vector-borne diseases. He warns of another disease, this one already endemic in South America and likely to spread as the greenhouse effect worsens. Bucher has been studying Chagas disease, or American trypanosomiasis, in Argentina, Bolivia and Paraguay. Trypanosomes are microscopic single-celled animals that swim through the bloodstreams of larger animals, such as humans, and live there parasitically, causing a variety of diseases. The trypanosome that causes Chagas disease often lies undetected for years in the blood or muscles of the victims, who don’t even know they’re infected until at age 40 or 50 they suddenly die of massive heart failure: The heart muscles have been invaded and debilitated.

Chagas disease claims 18 million victims worldwide every year; there is no cure and no vaccine. Its carrier, the triatome (or “kissing bug”), is not directly affected by rainfall as many vectors are but could thrive on the changing patterns of human beings. The kissing bugs are not found in the wild but almost exclusively in the cracks and thatched roofs of badly maintained houses. “If arid areas expand into previously wet ones,” Bucher says, “then you can expect that the process of deterioration of productivity will continue, and poverty will expand. And poverty will always be associated with poor housing, and poor housing in turn invariably leads to Chagas disease.”

Perhaps the world’s grossest disease is already the world’s fastest spreading, with the number of people currently affected estimated at 400 million worldwide. As the world warms and becomes even more hospitable to the mosquitoes carrying the microscopic nematode worms that cause it, elephantiasis may constitute the most serious and immediate threat. Elephantiasis is characterized by a horrible and permanent thickening of the skin, together with enormous swelling of the limbs, lymphatic system and genitals. There’s nothing anyone can do to cure the disease, and Bucher says that in a warming world, the mosquito vector could not only spread into new regions but could reproduce more quickly.

This consequence of warmer temperatures applies to most of the insect vectors. “Generally speaking,” Bucher says, “increased temperatures would raise the reproductive rates of disease vectors, adding to vector-control difficulties. As a consequence, the shorter generation times for these vectors could favor the emergence of strains resistant to pesticides.” Not a comforting thought, considering how easily the anopheles mosquitoes adapted to DDT at normal temperatures.

A less deadly but perhaps more immediate increase in medical problems may come with skin diseases. David Taplin, a professor with the department of dermatology at the University of Miami School of Medicine, has been conducting field research into insect-related skin diseases normally associated with the tropics. “As the world gets warmer, and the tropics spread into what are now subtropical regions,” he says, “we expect to see a serious increase in skin problems in areas where they are now ignored.” Such maladies, he says, are more than minor irritations; they include serious diseases characterized by pullulating, painful sores and can be as debilitating as they are disgusting.

Finally, all of the factors involving the insect carriers of these diseases could be intensified by expansive human migrations spurred by environmental changes. The problem of environmental refugees is already a monumental one in Africa, where great droughts have driven whole societies--tribes and villages in Ethiopia and the Sudan, for example--away from the land that had supported them for centuries. Further climatic change could easily worsen this situation, possibly even catalyzing a massive migration to more favorable areas.

Jesse Ausubel, a Fellow in science and public policy at Rockefeller University and an expert on climate change, says that “one of the possibilities is that climate change may be a further spur to human migrations, particularly away from desiccation. And when people move, they bring all sorts of illnesses with them. So it may be that the vector turns out to be us.” The WHO task group asserted that unprecedented human migration “could create new vector breeding habitats and increase the prevalence of vector-borne diseases, which would, at best, strain the existing health-care system and, at worst, overwhelm it.”

ALTHOUGH NEARLY ALL SCIENTISTS NOW BELIEVE that the world is heating up, some experts find no real cause for alarm. Robert Gwadz, a medical entomologist at the National Institute for Health in Bethesda, Md., is a specialist on the anopheles mosquito and the malaria that it transmits. “If we are able to maintain our current standard of living despite a modification of climate,” he says, “then we will not tolerate the encouragement of a disease inside the United States. The United States can and will do what is necessary to prevent the introduction of most of these diseases.”

But in an era when poverty and homelessness are increasing rapidly, the United States may not be able to maintain its already eroded standard of living. Many experts believe that it is possible for an outbreak of a tropical disease--particularly among poor people who cannot afford medical care--to reach epidemic proportions in the United States before it is even identified.

But Ausubel agrees with Gwadz, for the most part. “The situation, to me,” he says, “is neither conclusive nor frightening. That’s not to say that there’ll be no effect, but I think that issues like the 55-mile-per-hour speed limit or attitudes toward smoking will continue to loom much larger in the epidemiology of a country like America than a change in temperature of a few degrees.”

W.H. Weihe, a researcher at the Central Biological Laboratory in Zurich and one of the leaders of the WHO task group, contradicts his own group’s findings as he echoes the confident tones of Gwadz and Ausubel: “I personally don’t think this is a very big problem, because we do know, especially in our very well-organized societies, that we can keep a control over insect populations.”

George Craig stifles a laugh when he hears claims of humanity’s ability to subdue insect vectors at will. “In the 1960s, the United States spent $100 million to get rid of Aedes aegypti , the yellow fever mosquito, and didn’t eradicate it in a single county. It was an absolute farce. And at that time we had things like DDT and other insecticides. Now we aren’t allowed to use those tools anymore. If we couldn’t do it in the ‘60s, how are we supposed to do it in the ‘90s?” A former president of the American Mosquito Control Assn., Craig insists that we are far less prepared to deal with mosquitoes than we were 25 years ago. “What we have done as far as mosquitoes are concerned is give up weapon after weapon, and we’ve gotten nothing back in exchange.”

Cornell University’s Thomas Eisner, past president of the American Naturalist Society and current holder of the Tyler Award for Environmental Achievement, is equally incredulous at claims of human superiority. “It’s folly for anyone to tell you that we can cope with spreading insect populations. I’m anxious about that kind of technological optimism. We tried to wipe out malaria, and what have we got? DDT-resistant insects, drug-resistant plasmodium and a vaccine that’s not working. I’m a very jovial, optimistic person by nature, but in this case I don’t find much to be optimistic about. The very fact that you can’t be certain--the very fact that we’re changing the world in a way we don’t really understand--means it’s insane not to take preventive action.”

Craig chimes in despairingly: “We can’t do anything about it because we’re the United States. And the United States does not practice preventive medicine; the United States practices only reactive medicine. We don’t do anything until we’re in the middle of the bonfire.”

At any rate, both sides in the debate readily agree on one thing: We need much more basic research on insects and the diseases they carry. “We have a very faint amount of material to draw from now,” Weihe says.

But even as growing insect populations are warming up to a more hospitable planet, the entomologist population is thinning. As Gwadz puts it, “DDT didn’t eliminate malaria, but it did eliminate the malariologists.” Thirty years ago, the prevailing sentiment was that insect problems were on the way out, so federal funds were redirected to other fields, drawing most of the young scientists with them. Now, on the brink of global warming and tropical-insect expansion, there is a shortage of people with the potential to devise a defense system.

To get them back will take two things: awareness of the potential problem and money. Since the 1970s, research support has declined 28%--to current levels of about $70 million annually from the National Science Foundation, which funds animal-behavior studies (only a few of which include insects), and $30 million from the USDA. Individual states also finance some research projects. Says Dan Haile, an agricultural engineer working on computer models of future tropical-disease expansion for the USDA in Florida: “When you get to talking about research dollars and the present budget, it gets very difficult. We have a hard time staying afloat in any case.”

With luck--and money--perhaps our scientific minds in the coming years will foray back into the fight against vector-borne diseases. Perhaps they will heed bacteriologist Hans Zinsser’s call from half a century ago, in his 1935 book “Rats, Lice and History”: “Infectious disease is one of the few genuine adventures left in the world. The dragons are all dead, and the lance grows rusty in the chimney corner. But however secure and well-regulated civilized life may become, bacteria, protozoa, viruses, infected fleas, lice, ticks, mosquitoes and bedbugs will always lurk in the shadows ready to pounce when neglect, poverty, famine or war let down the defenses.”

The specter of infectious disease is no less threatening today. As Joshua Lederberg has said, “Pandemics are not acts of God but are built into the ecological relations among viruses, animal species and the human species, and we had better understand that or we will rue it.”

Infectious diseases have killed far more people than all the wars in history, and of those, insect-borne diseases are the worst that humans have ever endured. Among the four horsemen of the apocalypse, pestilence is the standard-bearer. And, as it turns out, this particular killer is not riding on a white steed; it is flying out of the steaming tropics, propelled by the drumbeat of a billion tiny wings.