TCS Daily

Fever Pitch

By Paul Reiter - August 6, 2003 12:00 AM

On his recent trip to Africa, President George Bush paid homage to the tens of thousands of slaves who were held in pens on tiny Gorée Island, Senegal before shipment to the United States. As he stepped ashore, he must surely have seen a marble Madonna, a memorial to 29 medical personnel who died in a terrible yellow fever epidemic in 1878.


Typical tragedy of the tropics, you may say. But wait. If you go to Elmwood Cemetery in Memphis, Tennessee, you will see a huge mound, a mass grave into which thousands of bodies were piled during the devastating yellow fever epidemic of ...1878. There were 100,000 cases in the United States that year, 19,500 in Memphis alone. The mosquito-borne pestilence moved by riverboat and railroad, from New Orleans to Ohio. Memphis was destroyed, and has never regained its position as capital of the South.


The 1878 epidemic fascinates me because I am a specialist in the ecology and epidemiology of diseases transmitted by mosquitoes. Malaria, dengue, yellow fever, St. Louis encephalitis, West Nile encephalitis, that sort of thing.


So I am not a rocket scientist, and if I were to write articles on rocket science it wouldn't be surprising if people didn't take me seriously.


However, perhaps it is surprising that the reverse isn't true.


My field has a small number of specialists, so we tend to keep contact with each other. About 12 years ago, we were piqued to see a growing number of articles that were in our field but were written by persons we had never heard of. Some were even rocket scientists!


The articles had a common theme: "global warming" is a threat to human health; it will cause major increases in the transmission of mosquito-borne diseases; the diseases will spread to new latitudes and altitudes around the world; and the process has already begun.


Nearly all the articles exploited common misconceptions: mosquito-borne diseases are "tropical," hot weather and heavy rainfall mean more mosquitoes, mosquitoes die if the weather is cold, and more mosquitoes mean more infections.


Despite their abject misinformation, their impact was increasingly obvious, not only in the popular press, but also in the prognostications of influential panels of "experts." For example, the Second Assessment Report on the Impacts of Global Change published by the United Nations Intergovernmental Panel of Climate Change (IPCC) devoted more than a third of its chapter on human health to discussion of the mosquito-borne diseases. Neither the nine lead authors nor the sources they cited were specialists in the field. Nevertheless, their authoritative pronouncements gave authenticity to a new crop of erroneous articles, many with inventive explanations of new situations.


A good example was a Scientific American article "Global Warming: The Hidden Health Risk" that appeared in July 2000. The bulk of the article gave all the usual examples, but there was also an extensive discussion of the disease of the moment: West Nile encephalitis in the United States. The gist was that global warming had exacerbated the proliferation of this mosquito-borne virus after its accidental introduction in 1999. The initial factor had been the warm winter of 1998-99, which had increased the survival of the Common House Mosquito, Cx. pipiens, one of principal suspects, and had thus helped the virus to proliferate in the New York area.


To the layman this might sound totally plausible, but is there really evidence that Cx. pipiens survives better in warm winters?


Every week through the winters of 1981-82 and 1982-83 I crouched my 6-foot-plus frame through 465 yards of a 5-foot diameter Memphis storm sewer, counting the adult Cx. pipiens that were sheltering there. I had marked them with fluorescent powders in the late fall.


I endured this routine to determine their survival rate -- the species is also an important in the transmission of another virus, St. Louis encephalitis.


Fortuitously, the first winter was bitterly cold, with temperatures down to zero degrees Fahrenheit, but the second was affected by an El Niño event -- you could dance outdoors some nights in December.


My travails showed that the survival rate was high and was the same in both winters. There was no hint of increased mortality during colder spells. Indeed, on a morning after zero degree weather, I collected females from a culvert where they were totally exposed to the cold. They were surrounded by ice, and I suspected they might be frozen and dead, but before I had warmed up in the cab of my pick-up, they were buzzing happily in their tube!


Mine is the only study of its kind. The results were not surprising: as with many insects, adult Cx. pipiens have a natural anti-freeze that protects them through the winter... Incidentally, the 1998-99 winter was much colder in Volgograd (formerly Stalingrad) than in New York, but the human toll from West Nile virus was much higher in Volograd. Unlike Hitler's army, Cx. pipiens is comfortable in the Russian winter!


The same Scientific American article made extensive claims about malaria moving to new altitudes in the tropics. And the same author has stated in other publications that a 1993 outbreak of yellow fever in the Tugen Hills, western Kenya was the result of Ae. aegypti, commonly known as the Yellow Fever Mosquito, moving to higher elevations because of global warming.


That claim particularly bugs me because I led the World Health Organization team of entomologists that investigated that outbreak. To identify the mosquitoes involved, we sat for 19 consecutive days, morning and evening, catching the mosquitoes that came to bite us. It was a tiresome job, but it was the only way we could collect the relevant species because they are not attracted to lights and they only bite primates.


We isolated the yellow fever virus from two forest species, Ae. africanus and Ae. keniensis. Our epidemiologists confirmed that this was a classic "sylvatic" outbreak, transmitted by mosquitoes between monkeys in the forest. The only human victims were people who got bitten when they ventured into the forest -- honey gatherers, charcoal makers, and women taking water from the streams. Ae. aegypti, the usual suspect in towns and villages, was not present in the area!


Thus the yellow fever claim was pure fiction. Moreover, the Scientific American article included statements that Ae. aegypti had transmitted dengue fever at new altitudes in Mexico and Costa Rica, and had ascended to new altitudes in Colombia and India. These too were fantasy: the professional reports on these issues unequivocally stated that there was nothing surprising about the altitude at which they occurred. Of course, there was no mention of yellow fever in the U.S. in 1878!


The sad fact is that there is little we scientists can do to challenege this campaign of disinformation. None of us denies that temperature is a factor in the transmission of mosquito-borne diseases, and that transmission may be affected if the world's climate continues to warm. But it is immoral for the political activists to mislead the public by attributing the recent resurgence of these diseases to climate change, particularly in Africa. The true reasons are far more cmplex, and the principal determinants are politics, economics, and human activities. A creative and organized appplication of resources to change the situation is urgently needed, regardless of future climate.


Paul Reiter worked for 22 years as a medical entomologist for the Division of Vector-Borne Infectious Disease of the Centers for Disease Control and Prevention (CDC). He now heads a new unit of Insects and Infectious Disease at the Pasteur Institute in Paris.

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