Is Climate Change Aiding Spread of Disease?

Rick Smith
International Herald Tribune
September xx, 2002
As floods rage in Central Europe and China and drought plagues Africa
and North America, it is getting harder to dismiss the scientists who
claim that weather is becoming not only warmer but more extreme.

Both of those trends, each in its own way, have ominous implications for the spread of infectious disease.

With this year on track to possibly outdo 1998 as the warmest year on record, according to the British Meteorological Office, it is clear that rising temperatures are enabling carriers of disease such as insects and rodents to expand their range and thus their ability to infect people.

The mosquito, still by far humankind's greatest single enemy in the battle against infectious disease, is rapidly taking advantage of warmer temperatures. It proliferates faster and bites more as temperatures rise, and it has extended its range both in latitude and elevation.

"Not only have we underestimated the rate at which climate would change, we have also underestimated the rate at which biological systems would react," said Paul Epstein, associate director of the Center for Health and Global Environment of the Harvard Medical School.

New Orleans had an explosion of mosquitoes as well as termites and cockroaches after a period of five years, 1990 to 1995, in which the area had no killing frosts

Malaria, which is carried by the Anopheles mosquito, can survive in areas where temperatures do not routinely fall below 16 degrees centigrade (60 degrees Fahrenheit), and has recently made inroads in the Korean Peninsula, Southern Europe, and the former Soviet Union.

Every year the disease kills at least one million people, mostly children. This figure is part of the 300 million victims annually that suffer crippling fevers and other health problems from the disease.

Greater Impact Expected

The danger of greater advances by all mosquito-borne infectious diseases is strong, furthermore, because global warming trends are uneven and are raising night and winter temperatures, which, more than day and summer temperatures, set the thresholds for killing insects.

There is much fear that temperatures, which have been rising fairly gradually over the last century, might at some point undergo upward jolts that spur quantum leaps in the spread of disease.

"Our sense is that change will be gradual, but what we learn from ice records is that we can have sudden flips of 2 to 3 degrees centigrade," said Epstein.

Then there is the second issue: the possibility that there will be more extreme weather events—floods, hurricanes, and droughts—and that this will have extreme effects on populations of disease carriers.

Some are predictable. The 1998 hurricane called Mitch, one of the strongest on record, left a trail of infectious disease in Central America as vectors of disease spread amid wasted infrastructures and weakened populations.

But other chains of events are harder to anticipate.

Nature abounds in examples of sudden epidemics spurred by jagged weather patterns that whipsaw from one extreme to the other. In 1993, there was an explosion in a population of rodents in southwestern United States that spread hantavirus syndrome, a lung infection. The situation occurred after a drought that killed off the rodents' predators was quickly followed by heavy rains that expanded the rats' food supply.

Warming and the associated spread of certain diseases is more than a cyclical phenomenon. After all, populations of disease carriers also waxed and waned in the trail of the Ice Ages.

Multiple Factors

But even if human actions are causing long-term global warming, they are also affecting many other things that indisputably spread disease, making it hard to isolate the actual role of climate change.

Mass travel and transport of goods top the list.

West Nile virus, which is rapidly spreading across the United States, arrived in 1999 in the New York area, a major port of entry for both people and goods.

Some migrating diseases have been followed to their source with impressive ingenuity. The Centers for Disease Control and Prevention in Atlanta traced the outbreak of a parasitic ailment at a Philadelphia wedding two years ago to raspberries in the wedding cake that in all likelihood came from a single farm in Guatemala.

The growing list of crisis areas can overwhelm health specialists and make the debate on climate change a theoretical luxury for many of them.

"If you go back to the 1960s, dengue fever was mostly confined to Southeast Asia and the Caribbean, but it is now widely distributed in Africa, Central and South America, and the Indian subcontinent, and is driven by factors such as population density, movement, and the transport of materials that would contain mosquito eggs," said Ray Arthur, project leader for the surveillance of dengue fever at the World Health Organization.

"Whatever the merits of the debate on global warming, our big concern now is to find a vaccine rapidly," he said.

Nevertheless the role of climate is getting more attention as more data accumulate.

"Back in the mid-1990s, data were indicative but not strong," said Richard Ostfeld, an animal ecologist with the Institute of Ecosystem Studies of Millbrook, New York. "But evidence has mounted steadily since."

Stepped-Up Disease Tracking

Some researchers try to eliminate the human factor as much as possible by focusing on the animal and plant worlds where species are expanding or changing their ranges in latitude or elevation to areas that were once too cold for them.

One good example is the northward advance of certain kinds of insect-borne encephalitis in Scandinavia. "Humans are the victims rather than the carriers in this case, so it is the insects that are moving for some reason," said Ostfeld.

If climate is, in fact, causing long-term shifts in disease patterns, it is clear that developing countries will suffer most. Wealthy societies can rapidly marshal a wide range of weapons, from fish that eat mosquito larvae to insecticide pellets in sewers.

"As dengue fever marched northward and entered Florida and Texas, it fizzled out in the United States, where homes had screens and air conditioning," said Ostfeld. "But the solutions aren't as readily available in the Third World."

In the meantime, disease specialists are turning to both mundane and sophisticated techniques to get a better overview of what is actually happening.

The World Health Organization is perfecting its Global Outbreak Alert and Response Network, an Internet search program that was launched two years.

"It continually scans for key words from press, public health e-mail discussions, and all other publicly available information," said Iain Simpson, spokesman on infectious diseases for the World Health Organization. In this way, he explained, the organization is alerted to more than half of the outbreaks of disease that it tracks.

More ambitiously, experts are trying to establish correlations between weather patterns and subsequent disease outbreaks so that they can make predictions and take preventive action.

Scientists have established, for example, that outbreaks of Rift Valley fever, a mosquito-borne viral disease, tend to occur in the Horn of Africa about five months after certain patterns occur in ocean surface temperatures in parts of the Indian and Pacific oceans.

If such a correlation is well established, livestock herds can be vaccinated and mosquito breeding sites targeted in time to prevent, or at least modify, major outbreaks.

Copyright 2002 International Herald Tribune

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