Global Warming And Vector-Borne Disease: Is Warmer Sicker?
One of the scarier claims made by supporters of an international climate treaty is that global warming will spawn epidemics of deadly "tropical diseases" malaria, dengue fever, Yellow fever not only in countries where such scourges are already entrenched but in North America and Western Europe as well. Many prominent individuals in government, academia, and media embrace the "warmer is sicker" hypothesis. Indeed, some claim the expansion of disease vectors due to global warming is already under way. But is it true?
Concerned that serious issues of public health are being distorted for political purposes, the Competitive Enterprise Institute, on behalf of the Cooler Heads Coalition, invited Dr. Paul Reiter, Chief Entomologist of the Centers for Disease Controls Dengue Fever, to lecture on the subject of climate change and insect-borne diseases. Dr. Reiter spoke to an audience of congressional staff, journalists, and citizens.
More than just an academic researcher with multiple publications in professional journals, Dr. Reiter has repeatedly risked his own health battling infectious diseases in countries as exotic and different as Kenya, Jamaica, Ecuador, Zaire, and Sudan. He speaks with the authority of experience.
In his lecture, Dr. Reiter developed the following points.
- Malaria, dengue fever, and Yellow fever are not actually "tropical" diseases. These illnesses were once common in North America (including Canada and Alaska), Western Europe (including Scandinavia), and Russia. Moreover, they were common in those places during the 19th century, when the world was by all accounts colder than it is today. Insect-borne diseases are not diseases of climate but of poverty.
- All attempts to link specific recent outbreaks to climate change cannot survive a confrontation with the facts. In all cases, local conditions (such as the banning of DDT, land use changes, or foreign contact) account for expansions of disease vectors or increases in infection rates.
- Developed countries like the United States need not fear the spread of insect-borne diseases provided they remain prosperous. Whatever the climate, developing countries will remain at risk until they acquire window screens, air conditioning, modern medicine, and other amenities most Americans take for granted. As a matter of social policy, the best precaution is to improve living standards in general and health infrastructures in particular.
Marlo Lewis, Jr. Vice President for Policy & Coalitions
GLOBAL WARMING AND VECTOR-BORNE DISEASE: IS WARMER SICKER?
DR. PAUL REITER CHIEF, ENTOMOLOGY SECTION DENGUE FEVER BRANCH CENTERS FOR DISEASE CONTROL
I am a scientist. My specialty is "diseases transmitted by insects," I am classified as a "Medical Entomologist." I have been a full-time researcher in this field for nearly 30 years. At present, I work for the Centers for Disease Control and Prevention (CDC), at the Dengue Branch, in San Juan, Puerto Rico. I've been with the CDC for 17 years.
Most of my work has been outdoors, in "the field." I began my career working for the British Medical Research Council, exploring new methods for the non-insecticidal control of malaria mosquitoes in Kenya, East Africa. I also tackled the mosquitoes that transmit filariasis (elephantiasis). After that I was employed by the World Health Organization (WHO), in West Africa, on a massive project for the control of onchocerciasis (river blindness), a worm disease transmitted by black flies. Then I crossed the Atlantic to work for the CDC. I spent 4 years in Memphis, Tennessee, investigating the epidemiology and control of St. Louis encephalitis, a mosquito-borne virus that is native to the United States. CDC then transferred me to the Dengue Branch. Dengue is an urban, mosquito-borne viral disease that we'll talk about a little later. I've been in Puerto Rico for 13 years. Much of my work on dengue has been conducted there, but in addition I have directed extensive field studies in Jamaica, Venezuela, Ecuador, and other countries. I've also been involved in the investigation of various outbreaks for CDC, WHO, and other organizations around the world: a massive epidemic of dengue in Guayaquil, Ecuador; the first recorded epidemic of yellow fever in Kenya; an outbreak of hemorrhagic disease in southern Sudan; the Ebola outbreak in Kikwit, Zaire; an outbreak of dengue in the Amazon region of Ecuador, and so on. I tell you all this because in virtually all these studies, climate and weather have been a priority issue. In other words, for a person who is interested in the natural history and dynamics of these diseases, climate and weather are crucial factors, and have always been a matter of great interest for me. Perhaps thats partly because I'm an Englishman and we're always talking about the weather!
So, I am a specialist in a well-defined branch of science, one of a number of similar specialists who are scattered around the world. What Id like you to know is that in recent years, my colleagues and I have been quite astonished by some of the statements that are being made about our subject by people who are outside our field. We are not rocket scientists, so we dont publish papers on rocket science. But it's quite incredible how many scientists from other fields -- including rocket scientists have been publishing articles and making public statements about mosquito-borne disease, all in the context of global climate change.
I'm not a climatologist either, so I wont pronounce on that subject. For the sake of this discussion, lets just assume that some of the predictions being made about the world's climate, in the framework of the global warming paradigm, will eventually come true. How would such changes really affect vector-borne diseases, both in this country and the rest of the world? I'm sure you know some of the predictions: mosquitoes that transmit diseases like malaria, yellow fever, and dengue will move into the United States ... they will become established ... they will bring the diseases with them ... tropical diseases will become common ... millions more people will die in Africa and other tropical regions....
Malaria, Dengue Fever, and Yellow Fever: Not Tropical Diseases
What do those of us who work in this field really think about this?
Well the very first thing I'd like to impress upon you is that malaria is not a tropical disease! Most people think of it as tropical because, today, it occurs mainly in the tropics. But in the past malaria was present nearly everywhere in the world. Hippocrates described "autumnal fevers" that plagued the ancient Greeks in the Mediterranean region. His description of these "tertian" and "quartan" fevers were written with such careful, perceptive observation, and such meticulous detail, that we can be absolutely certain that at least two species of the malaria parasite probably Plasmodium vivax, and P. faliciparum were present. The association of wetlands with malarial fevers was well known, and he even described the association between enlarged spleen a symptom of chronic malaria infection and marshy, wetland areas. In fact it is quite possible that at the end of the Ice Age, as the ice caps were retreating and humans moved northward, the mosquitoes and the malaria moved with them. The bone pathology of Neolithic peoples buried in Anatolia and Macedonia is strongly suggestive of chronic malaria, and the prevalence of Mediterranean Anemia (thalassemia - a hereditary disease which affords some protection against malaria) in southern Europe also suggests a long history of contact with the pathogen. And so I suppose one could say that malaria in Europe is truly a result of global warming, although it happened rather a long time ago!
The word malaria is actually Italian. In ancient Rome, the disease was a major problem. The patricians of Rome lived high up on the seven hills, because to live down below, close to the Pontine marshes, was very unhealthy. All the major Roman authors described malaria as a health curse. Three species of malaria parasite vivax, malariae, and falciparum were certainly present at that time. Some historians have even suggested that the fall of the Etruscan and Roman Empires was hastened by massive epidemics of the disease...who knows, perhaps the result of El Niño events?
Here is some translated verse from Dantes Inferno:
As one who has the shivering of the quartan so near, that he has his nails already pale and trembles all, still keeping the shade, such I became when those words were uttered
That was written in Tuscany in the 13th century. This slide is a page from the 15th century health manual, Hortis Santatis, published in 1495. It shows a wretched man tormented by insects. It looks autumnal to me. In fact, throughout the Middle Ages, malaria was a major problem, not only in Italy, but right up into such untropical regions as my country, England. Chaucer wrote about malaria, the ague as it was called in those days. Shakespeare quotes the disease several times. Remember King Lears dreadful curse on his daughter?
Infect her beauty, You fen-sucked bogs, drawn by the powrful sun To fall and blast her pride
I never thought I'd be quoting Shakespeare, in a scientific context, in Washington! Shakespeare, like many other people in England, was familiar with the association between bogs wetland areas and malaria. And it was a major problem much later than Shakespeare. The south of England was repeatedly wracked by epidemics of "harvest ague" that were responsible for many deaths. The 17th century physician Thomas Sydenham wrote:
when insects do swarm extraordinarily and when fevers and agues (especially quartans) appear early as about midsummer, then autumn proves very sickly.
A few years later, King Charles II the one who didnt get his head chopped off caught malaria, probably at Windsor or Winchester, but was cured by the "Jesuits bark," chinchona imported from Peru. The bark, a natural source of quinine, was already in use in catholic Europe, but had been branded as "popery" in Protestant England. After his spectacular recovery and evident good health, it became very popular in Britain! Nevertheless, the disease only went into significant decline after the start of the Industrial Revolution.
Clearly, the concept of malaria as a purely tropical disease just doesn't hold true. In fact, right through the Mini-Ice Age period from the early 17th century to the first decade of the last century when it was significantly colder than it is now, malaria continued to be a major problem throughout Europe.
If we look at this map, the 15o isotherm shows the approximate northernmost limits of the disease, at least until very recently. It's not an exact limit, for there was even some malaria in the Grampian Highlands of Scotland, which I assure you never feel tropical! And as you can see, the isotherm includes the south of Norway, much of Sweden, Finland, and way up in northern Russia. To the south of that line, every country was affected, many until quite recently. Holland, for example, was only declared malaria-free by the World Health Organization in the early 1970s. In the last century, outbreaks in the countryside around Copenhagen, Denmark, killed thousands. I can show you texts on the distribution of malaria in Sweden. Finland had a major problem until after the Second World War. Germany and France, too. Perhaps the worst problem was in Eastern Europe. I don't know if any of you've been to Poland. I've been there with my wife in the wintertime, when the temperature was 27o C (-33oF) ... very, very cold! It doesn't surprise me that so many Poles end up in Chicago Warsaw and Chicago have a similarly nontropical climate in the wintertime!
Figure 2 is a curve for malaria in Poland after the Second World War, until it was eliminated in the early 1950s. I wonder how many of you have really thought of malaria in Poland, even in the global warming context?
Let me impress upon you how far north this problem went. In the 1920s and 1930s, the Soviet Union suffered dreadful epidemics of malaria, much more than any other country in the Northern Hemisphere. In the period 1923 to 1925, there were around 16 million cases, with 600,000 deaths. Archangel, which is at the same latitude as Fairbanks, Alaska, had 30,000 cases, roughly 30% of which were fatal. Think of it, malaria in Archangel. You need an icebreaker on the sea for 6 months of the year!
So, I hope that I've convinced you -- we're talking about a disease that is not truly tropical! And yet this map [Figure 3] is the kind of thing that is being fed to the public.
I took it off the Global Warming web site of the World Wildlife Fund. It clearly implies that with global warming we are going to see malaria move northwards into northern Spain and southern France. Theres no malaria there today anyway ... but why have they excluded the north of Russia?
If we switch to this side of the Atlantic, this is the area where malaria was known to occur at the end of the last century. East of the Rockies, nearly all of the United States experienced malaria. There were also areas of transmission on the Pacific side. And it didn't stop at the border: there was malaria in Canada. If you care to look at the records, youll find that significant numbers of people died of summertime malaria during the great construction projects that opened up that country in the 19th century.
It is interesting to remember why the CDC is headquartered in Atlanta. Not everyone is aware that the principal reason for the creation of the Communicable Disease Center, the original name for CDC, was to eradicate malaria from the United States. The disease had begun to decline in the 19th century, but major outbreaks still occurred, apparently on a cycle of 10 or 15 years, especially in the South. The last major peak was in the 1930s. By the time of the Second World War, the residual foci of transmission were relatively obscure, but it was feared that a new peak was imminent. And so, when the new wonder insecticide, DDT, became available, CDC was set up to get rid of the disease. Actually, another reason that Atlanta was chosen was because it had become the major city of the South. Until 1878, Memphis, Tennessee had held that position the site of the cotton exchange, a major port on the Mississippi, the crossover point for the railroads, etc. But Memphis had suffered multiple epidemics of yellow fever, a viral disease that can cause massive mortality. The worst of these was in 1878, when 19,000 people were infected and at least 9,000 died. It was a major catastrophe. Many people, especially the richer folks, fled to cities like Atlanta and St Louis, and never came back. Memphis lost its city charter, and has never really returned to its former importance, despite its strategic location.
When I worked on St. Louis encephalitis in Memphis, I made a lot of collections of local mosquitoes, to isolate the virus. One of my favorite spots was in an old cemetery where there are hundreds of gravestones of local people who died of yellow fever in 1878. There's a mound in that cemetery where 6,000 victims were buried in a mass grave. In the cemetery record office there are several huge volumes for 1878. Nearly every line of every page records a person with the words "Yellow fever" in the Cause of Death column. Transmission had begun in New Orleans and spread relentlessly north, through Memphis, and even along the upper Mississippi and Ohio rivers. At least 100,000 people were infected and 20,000 died in that terrible year. The map [Figure 5] shows the northern limits of Ae. aegypti, the vector mosquito. Aedes aegypti, yellow fever and dengue (another viral disease, which we'll deal with in a moment), all originated in Africa. They were imported with the slave trade. But Ae. aegypti is now well entrenched it's almost a native species here. In the southern part of its range it is present all the year round. You can still find it around that cemetery in Memphis.
Dengue (often called "breakbone fever") is a viral disease closely related to yellow fever. It was not recorded so often in earlier times, probably because people feared yellow fever much more, but it was well known. From the beginning of the 19th century until the 1940s, there were at least eight major pandemics of dengue, some of which appear to have moved from the United States into the Caribbean region. (I doubt that many people have ever thought of the United States as an exporter of tropical diseases!) Anyway, one of the biggest epidemics was in this century, in 1922. There were half a million cases in Texas alone. Most of the south was affected. In Savannah, Ga, something like 30% of the 30,000 cases had hemorrhagic symptoms. We tend to associate such severe manifestations of dengue with Southeast Asia. This was occurring in the United States, in our century!
Today, dengue is a major public health problem in most tropical countries. This year, we are seeing huge epidemics in Southeast Asia. During the recent El Niño event the area was affected by devastating drought. We are also seeing large epidemics in some parts of Latin America and the Caribbean. Those areas had above-normal rainfall during the same period. That tells you how confusing the climate/dengue relationship is. In 1995, the disease swept through Mexico, right up to Rio Grande. In the state of Tamaulipas, on the Mexican side of the border, nearly 6,000 cases were recorded. On the Texas side, there were only six! So why is that? Why is it that all these diseases used to be present in this country and in Europe? (I should have mentioned that Europe also suffered from major epidemics of dengue and yellow fever. In fact, one of the biggest epidemics of dengue and dengue hemorrhagic fever ever recorded was in Greece in 1927-1928. And I should also have mentioned that the first major urban epidemic of yellow fever was recorded in Philadelphia in 1793, and that the disease continued to revisit that city, as well as Boston and New York, for another century). Why, then, is this not occurring now?
Wealthier Is Healthier
Well, many, many factors are involved. The epidemiology of these diseases is complex. Suffice to say that the natural history, ecology and behavior of their vectors play such a crucial role that they are much less predictable than, say, influenza. But quite clearly, in modern times, people in this country and in most of Europe live under conditions very different from those in countries where dengue, yellow fever, and malaria are still endemic. People no longer live in homes that are contiguous with, or even built above, their animal stables. House construction is far more sophisticated it's hard for a mosquito to get into a building where there is no gap between the roof and the top of the walls, and the windows are sealed to the walls. Most Americans don't tolerate a lot of insect bites, especially indoors. They spend millions on cans of bug spray and repellent. And there is air conditioning. Most houses in the United States have insect screens. Most people, even the poorest people, especially in the southern states, use air conditioning in mosquito season. So the mosquitoes simply can't get to the people! They're indoors watching television (the people!). There are other major differences I could go into. But the bottom line is that we have so changed the interaction of mosquitoes with people that these diseases hardly have a chance any more.
The spots on this slide represent the numbers of cases of malaria imported into France in 1996, and the cities where they were detected. I put this in because France keeps careful records of these imports. At least 5,000 cases occur every year. Most come in from West Africa and Southeast Asia. Also from Haiti. We have the same problem. Not only malaria. Two years ago, a man from Tennessee went on a fishing vacation on the Amazon, in Brazil. He came home, got sick, and died of yellow fever. He wasn't vaccinated. Check the CDC weekly report! We record thousands of cases of malaria every year. Nearly all are travelers who are infected abroad. They simply are very unlikely to pass the disease on once they get here.
Of course, it does happen, sometimes. There was a recent case in Toronto, Canada. Someone came in from Asia...they had the parasite in their blood...a local Anopheles mosquito bit them...the weather was warm...the mosquito became infected (thats what happens)...it passed the disease on to a local. The same has happened in California, in Michigan, even in Queens, New York! Frankly, it would be a little surprising if it didnt!
Incidentally, I came home from Zaire in 1995. I got sick 6 weeks later... falciparum malaria. Fortunately it wasnt fatal!
Another point to ponder: in the summer months, it's a lot hotter in many parts of the United States than it is in the tropical countries where these diseases occur. If you look at the temperatures in San Juan, and compare them, say, to Disney World, in Orlando, you can see that it is very much hotter in Orlando than in the Caribbean. Theres a heat wave in the United States at this very moment. Temperatures in three figures are quite widespread. It doesnt often get above 92oF in San Juan. The record is around 96, and that is rare. So let's not even talk about global climate change. Check todays USA Today! It's warmer here in Washington than over there!
So the message is that these diseases are not "tropical." They are capable of being transmitted here, but it simply doesnt happen very often.
Lets move to the tropics. What would happen if, let's say, sub-Saharan Africa got hotter? Frankly, its awfully hard to make even an educated guess. The malaria infection rates are so high that it's hard to think there would be very much change. I worked for some time on malaria in Kisumu, Western Kenya. Up to 95 percent of local children were malaria positive by the age of five. Malaria is somewhat seasonal there, so a change in climate could alter the epidemiology of the disease, but we can only speculate as to whether people would suffer more, or less. I can mention, however, that French colleagues have published clear evidence that in the last 50 years there has been a gradual decline of malaria in the northern Sahel belt of West Africa. They attribute this to increasing aridity.
What about the High Country? In the past few years there have been many suggestions that diseases in the tropics will ascend to new altitudes. That makes a lot of sense. If it gets warmer, then presumably the mosquitoes and the pathogens they transmit will be able to survive at higher altitudes. Cities like Nairobi, Kenya, and Quito, Ecuador, could experience new levels of infection. (Incidentally, Yellow fever is endemic in the Langata Forest, a few miles from the center of Nairobi, and malaria was endemic in the environs of Quito until the DDT era.) What concerns me is that, in recent years, a number of claims have been made that this ascent is already occurring in many parts of the world. Allow me to review these claims with you, using this map.
Lets go from east to west, starting with Papua-New Guinea, over here, just to the north of Australia. Papua-New Guinea has had major epidemics of "highland malaria" in recent years, and these have been attributed, by some, to global warming. It is certainly true that, as everywhere else in the world, the temperature in Papua New Guinea has risen steadily since Napoleonic times. It is warmer now than at any period since before the Mini Ice-Age. But if you look at the malaria literature (which I suspect that many of the promoters of the global warming paradigm have not done), youll find that, 50 or 60 years ago, when remote areas of the highlands were being explored by outsiders, a number of so-called Stone Age tribes were discovered, living in total isolation. These peoples had been so incredibly separate from the outside world that they appeared free of malaria and many other diseases.
The Australian colonial authorities were concerned that these diseases would be imported from the coastal regions into the highland valleys (considered as land above 1600m), with catastrophic consequences. So they instituted a Quarantine Law: people were not allowed to move from the coast, which is highly malarious, to the highlands, unless they spent 2 weeks in a special quarantine camp. In those camps they were obliged to undergo medical tests, and a course of malaria chemotherapy, even if they appeared malaria-negative. Unfortunately, the quarantine eventually broke down, and predictions that were first made in 1947 have become reality. It has nothing to do with climate change. There is no indication of any significant change, except the contact with the outside world. "Development" has arrived. Patches of forest have been cleared. There are airstrips. There are villages. There is agriculture. There is mining. New vectors, not native to the forest, but which thrive in open land, are now common. Malaria has arrived. There is a human tragedy, but it's not a climate change tragedy.
Lets move westward, to Madagascar. There was a major epidemic of malaria in the mid-1980s. Some reports put the death rate as high as 400,000. More conservative estimates indicate it was around 100,000. Still an awful lot of people. Those people live on a mountain plateau. They grow rice on irrigated land. The first major malaria epidemics were in the 1870s, apparantly as a consequence of the irrigation development. There was colossal loss of life. The disease remained endemic until 1949, when the French colonial authorities began a major control effort. They did a lot of DDT spraying to suppress the vector. They established malaria clinics to manage patients, and dispensaries to distribute anti-malarial drugs. And they virtually stamped out the disease. That all ended after the colonial era. One by one the dispensaries closed down. And the mosquito control ended. So it's not surprising that 120 years after the first major epidemics, epidemic malaria has returned to Madagascar. Another public health tragedy, but nothing to do with the weather.
The next triangle is in Tanzania. A similar story to Papua New Guinea. Ive seen people on TV talking of new epidemics of malaria in the eastern Usambara mountains of northeastern Tanzania. My first boss lived there, and so did several other friends. There was a forest field station near a place called Amani (altitude 1000m), a station where the Europeans put their wives and children during the hot season. Cooler than the coast, and much healthier. Anopheles gambiae and An. funestus, the classic vectors of malaria in Africa were absent. They are not forest species. Things have changed now. Large patches of forest were cut down. Agriculture was introduced. People from the coast came and settled. The two species of mosquitoes that I mentioned came with them. Together, they colonized the area. And malaria came too. The meteorological station, first established by the Germans, showed that the temperatures went up. It always goes up when you take away the shade! Actually when new trees started to grow, trees planted for various reasons, the temperature started going down again. I suggest you check the articles that claim that the malaria in the Tanzanian hills is due to global warming. You will find they refer to a paper by colleagues of mine, published in 1987. It is true, they mention a 3oC change in temperature. The temperature did change. They even show it with a graph. It went down by 3oC in 8 years! Three degrees in the shade, the shade of the new trees! But it is no longer dense forest. The malaria has remained.
I take particular exception to that top triangle on the eastern side of Africa! That was the first recorded epidemic of yellow fever in Kenya. In one of the ecological journals, it is claimed that the outbreak occurred because "Ae. aegypti moved to new altitudes." It so happens that the results of the investigations of the entomology of that epidemic (a joint effort between the Government of Kenya, the WHO, and CDC) will appear in next months Journal of the American Society of Tropical Medicine and Hygiene. I happen to be the senior author. I spent 19 days offering my arms to the mosquitoes there, together with a French colleague and 19 African assistants. Our principal goal was to determine which species were present, which were responsible for transmission. This was not an urban epidemic, so, as we had expected, there was no evidence that Ae. aegypti had been involved. We isolated virus from Ae. africanus and Ae. keniensis. Other classic vectors of woodland yellow fever Ae. vittatus, Ae. metallicus, Ae. luteocephalus and Ae. Simpsoni were also present. The urban form of Ae. aegypti was absent. For the global warming advocates to make this claim, before our investigation has even been published...it's just, it's just not cricket!
The last triangle in Africa is in Rwanda. There is a paper in The Lancet that highlights a quantum leap in the number of malaria cases at a certain highland site in 1987. The text claims that until then, transmission in the area had been limited, ostensibly because the disease was at the fringes of its maximum altitude. In that year, according to the author, unusually high temperatures had allowed a spectacular increase in transmission. What puzzled me was that the disease was not really at the fringes of its altitudinal limits. Moreover, in the years after that spectacular increase, transmission rates had continued at the higher level, even though the temperatures went back to normal." A few weeks ago, at a yellow fever conference in Senegal, I had the fortune to meet a woman from USAID who had worked on malaria in Rwanda. She proudly told me how she had contributed to an improvement in malaria surveillance and treatment in the highlands of Rwanda. That was in the mid-1980s! It was her achievement that had resulted in more malaria being detected and recorded. It had nothing to do with the weather. This month, French colleagues of mine, including Jean Mouchet, a 75-year old scientist with a 50-year career in Medical Entomology, and more than 300 publications to his name, will publish a detailed paper that debunks this and a number of other such claims that have been made for sub-Saharan Africa.
Leaping over to our side of the Atlantic, this triangle is in Colombia. If you read the literature on global climate change, you will learn that dengue has ascended to new altitudes in Colombia, as high as 2,200 meters. Unfortunately the writer who perpetrated this myth did not distinguish between the presence of the vector, good old Ae. aegypti, and the disease. Aedes aegypti is certainly present at 2,200 meters. Two colleagues of mine, Marco Suarez and Mike Nelson, were the ones who had surveyed the area, looking to see whether the mosquito was up there. Their work was published by the WHO in 1981, and is the source quoted by the global warming advocates. In that source, however, you will find it clearly stated that dengue was not present anywhere near that altitude. The mosquito was there. It had probably arrived there in shipments of used tires. Nothing to do with the climate.
Do you see the triangle in Costa Rica? Claims have been made that dengue has arrived at high altitude in that country as a result of global warming. However, Costa Rica was one of 22 countries in the Americas that were declared free of the vector by the World Health Organization (incidentally, the United States was not!). That was around about the beginning of the 1970's. After the campaign ended, the mosquito regained all of its former territory. Costa Rica was one of the last countries to remain free of it. But eventually it did return, and when it did, so did dengue. A simple story. I don't think you need any more detail.
The triangle in Texas is for the six cases of dengue in 1995 that I have already mentioned. Nothing to do with global climate change! Remember, there were half a million cases there in 1922! The triangle in California refers to malaria. There have been several small outbreaks of malaria associated with groups of agricultural laborers from south of the border. As you know, it can be quite warm in California, and malaria was once endemic. The mosquitoes are still there. There are lots of people coming in from Central America, where the disease is endemic. So its not surprising that we see small numbers cases from time to time. Ive also put a triangle at Queens, NY. A few years ago there were three case of malaria there. Again, someone came in with malaria and transmitted it. It's damn hot in Queens at the moment. I wouldn't be surprised if we see more malaria cases, but I wouldn't ascribe it to global climate change. And then there are those triangles, in Switzerland and in England. There have been quite a number of cases of malaria associated with airports. Mosquitoes board an airplane in Africa, and get off at Gatwick. Then they bite the natives, and: Presto! We have malaria! But clearly not the first manifestations of global warming! Frankly, a lot of natives in my country might welcome a bit of global warming!
The square symbols depict the maximum altitude of malaria transmission for various countries, as reported in the period 1880 to World War II. As you can see, there was malaria in Kenya to around 2,600 meters. In Ecuador, quite close to Quito, there was quite a lot of malaria in the 1920s and 1930s at a similar altitude. In Bolivia, transmission occurred to just over 2,600 meters, and there was even a site at 2,700, transmitted by mosquitoes breeding in the thermal pools at 32o Centigrade! As you would expect, the further from the equator, the lower the altitude limits. However, the triangles are sites where, it is claimed, global warming has brought dengue and malaria to record altitudes. As you can see they're all well below the altitudinal limits that were recorded even in the last century. So, I think we can dismiss those claims without losing any sleep.
Conclusion: Economics Trumps Climate
To sum up, I believe that if global warming comes to pass, it is unlikely to give rise to epidemics of mosquito-borne disease in the United States. Unless, of course, society collapses. If we go back to the living conditions of the 17th century, the picture would certainly change, but I suspect we would be able to prevent that happening. I imagine that if it gets warmer, Americans will spend more time in air conditioned rooms, not less. I also think it likely that if the mosquitoes get more dangerous, society will take extra care to suppress them. In other parts of the world, the picture gets more complicated, but again, we are not helpless. If we treat the problems seriously, we can tackle them effectively. Let me repeat: economic factors, not climate, are likely to be the most important parameter.
Of course, science is not built of certainties. Every theory is subject to revision, if new observations come to light or new ways are available of looking at our data. Today, we have a new tool. We test our theories with models, virtual models, in a computer. These models are not simulations of nature. They are caricatures. They help us explore the validity of our theories. It is legitimate to play with these models, even to explore the future, so long as we remember that such explorations are built on even shakier ground. To craft our models, we start by speculating. Which observations and which parameters are most relevant to the future? For example, which factors are likely to affect:
...the prevalence of disease carrying mosquitoes... ...and their behavior... ...and their interactions with people... ...and the pathogens they transmit... ...and the transmission dynamics of the diseases they cause... ...in all parts of a world... ...if the climate is altered by increased atmospheric retention of heat energy... ...if this happens as a result of higher concentrations of greenhouse gases derived from fossil fuels...
I wish I had time to discuss this more. Clearly, these multiple systems, and all their interactions, are incredibly complex. To a scientist they are fascinating, endearing, and challenging. There is still so much we need to know, not least in the field of climate/disease interaction. But until we learn more, well .... I hope that I may have helped some of you sleep more peacefully at night!