Scientists warn that infectious diseases will rise as the world gets warmer. Human malaria, butterflies with parasites, diseased corals, and trees overgrown with fungus are some of the things awaiting us as the Earth warms up. Entire species of animals could be wiped out.
Human tropical diseases will become more common as tropical weather spreads. Dr. Richard Ostfeld of the Institute of Ecosystem Studies says, "Disease now has to be considered another main player on the climate warming stage. We need to be taking climate warming much more seriously than we currently are. By 'we' I refer to international agencies but also the U.S. Government."
The rise in infectious diseases will be caused by changes in temperature, rainfall, and humidity, all of which encourage the growth of insects and bacteria. Climate differences will also stress plants and animals, making them more susceptible to infection. A two year study by the National Center for Ecological Analysis and Synthesis (NCEAS) analyzed disease epidemics in plant and animal systems, both on land and in the oceans. The study investigated recent disease outbreaks and how they were related to temperature changes. "What is most surprising is the fact that climate sensitive outbreaks are happening with so many different types of pathogens - viruses, bacteria, fungi, and parasites - as well as in such a wide range of hosts including corals, oysters, terrestrial plants, birds, and humans," says Drew Harvell of Cornell University.
"This isn't just a question of coral bleaching for a few marine ecologists, nor just a question of malaria for a few health officials - the number of similar increases in disease incidence is astonishing," says Ostfeld. "We don't want to be alarmist, but we are alarmed."
"Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases," states epidemiologist Andrew Dobson of Princeton University. "The accumulation of evidence has us extremely worried. We share diseases with some of these species. The risk for humans is going up."
The study team found examples of viruses, bacteria, and fungi associated with diseases that develop more rapidly with slight rises in temperature. Many disease carriers, such as mosquitoes, ticks, and rodents, as well as viruses, fungi and bacteria, are highly temperature and moisture sensitive. As temperatures increase, the diseases they carry will spread into new areas.
Winter kills off many of them, but winters are rapidly becoming warmer throughout the Earth. If insects and other pathogens don?t die off every year, they will only become bigger and stronger. Warmer, longer summers also mean that the period of time that diseases can be transmitted is longer.
In Hawaii, mosquitoes are becoming so strong that they are destroying the honeycreepers, brightly colored songbirds that live only in Hawaii. The tops of volcanoes were once too cold for mosquitoes to live, but now that it?s warmer, they can go up to where the birds live and have spread avian malaria there. "Today there are no native birds below 4,500 feet," says Dobson.
The diseases and stress caused by global warming have contributed to the decline of many threatened species, such as lions, cranes, vultures, and black-footed ferrets. Mosquito-borne diseases affect humans too. Rift Valley Fever regularly devastates parts of East Africa and is linked to heavy rains. "There is clear evidence that Rift Valley Fever outbreaks are linked to El Ni?o years and we expect an increase in the frequency of El Ni?os with climate change," says Ostfeld. The last outbreak in 1998 killed thousands in East Africa.
During the unusually warm 1998 El Ni?o year, corals suffered massive die-offs worldwide. Now, a new bleaching event has just occurred in Australia. Once they are bleached or stressed by heat, corals become susceptible to disease. "The disease may be what actually kills them," says Harvell. Aspergillis is a fungus that occurs everywhere, but corals succumb to it when they become too warm. It grows fastest at 30-32 degrees Celsius, which is exactly the temperature at which many of the corals in the Florida Keys start to bleach.
Oysters are susceptible to a protozoan called Perkinsus. A winter warming trend in the mid 1990's allowed this parasite to spread northward into previously unexposed Maine oysters.
As the temperature warms, insects carrying disease from the tropics are spreading towards the poles. In tropical areas, there is a greater diversity of species, but smaller numbers of each, which is why humans and animals can survive there. But when some of these insects spread to temperate areas, they have a chance to reproduce in greater numbers. When insects species are more diverse, there?s less chance an individual insect will bite a human or animal that?s susceptible to the disease it carries. "Thus any pathogen which manages to spread from the tropics to the temperate zone in a warmer world is likely to have a bigger impact as it can focus on a few common and abundant susceptible species?maybe even us!" says Dobson.
"There are still people resistant to the idea of climate change at all, others will say it is hard to predict what type of outbreaks will occur or where they will happen," says Dobson. "This is true. Very little monitoring and few long-term studies exist. What is apparent is the end result - when the epidemic strikes."
"We need to develop lines of defense," says Harvell. "Now that we know these epidemics are arising, what can we do about it? It's highly likely to get worse with increasing temperature."
But there?s not much funding for research into tropical and other infectious diseases. "The scary thing about the recent anthrax threat was not just that it happened but how few people know anything about infectious diseases, and how little even these people know about their dynamics," says Dobson. "We need to pay better attention to this issue in an increasingly unnatural world."
Dr. Ute Collier, head of climate change at the World Wide Fund for Nature, says, "Climate change is a monster of many faces and this study underlines the multiple stresses it puts on people, wildlife and the environment. The increasing risk of infectious diseases will put an additional strain on species struggling to adapt to a climate that is changing faster than ever before. Our future health, as well as that of our planet, depends on us tackling this global threat now." "We have to get serious about global change," says Dobson. "It's not only going to be a warmer world, it's going to be a sicker world."
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