Warming Climate Spawns Disease Epidemics

By Cat Lazaroff

WASHINGTON, DC, June 25, 2002 (ENS) - The changing, warming climate around the globe is triggering unprecedented numbers of disease outbreaks in both land and ocean based wildlife populations in habitats ranging from coral reefs to rainforests. Ecologists and epidemiologists express concern over this rising trend in a new report in the June 21 issue of the journal "Science."


As mosquito populations move into previously cooler climes, the diseases they carry - such as malaria and West Nile Virus - spread with them. (Photo courtesy Centers for Disease Control and Prevention)
Their comprehensive two year study, developed by the National Center for Ecological Analysis and Synthesis (NCEAS) is the first to analyze disease epidemics across entire plant and animal systems, both on land and in the oceans. The study investigates these recent disease outbreaks and examines the mechanisms related to temperature or seasonality changes that could influence them.

"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," said lead author 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," added coauthor Richard Ostfeld from the Institute of Ecosystem Studies in Millbrook New York. "We donšt want to be alarmist, but we are alarmed."

The team of experts has concluded that there are common themes likely linked to global warming.

"Climate change is disrupting natural ecosystems in a way that is making life better for infectious diseases," said 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 team documented examples of viruses, bacteria and fungi associated with diseases that develop more rapidly with slight rises in temperature. Many vectors of disease such as mosquitoes, ticks and rodents, as well as the viral, fungal, and bacterial pathogens are sensitive to changes in temperature and moisture.

As temperatures increase, these carriers are likely to spread into new areas and may have potentially devastating effects on wildlife populations that have not been previously exposed. Reproduction, growth, and biting rates of insects all go up with increases of temperature.

spotted fever

This child's right hand and wrist display the characteristic spotted rash of Rocky Mountain spotted fever - a disease spread by ticks. (Photo courtesy Centers for Disease Control and Prevention)
Winter is the limiting time for many pathogens, killing back populations each year. With milder winters, this population bottleneck may be removed for many species.

Warmer, longer summers also mean that the period of time of disease transmission is longer. Warmer summers may increase the susceptibility of host species to diseases due to the stress of excessive heat, particularly in the oceans.

"We have to get serious about global change," Dobson warned. "It's not only going to be a warmer world, itšs going to be a sicker world."

Marine bacteria and fungal growth rates are both linked to increasing temperatures, for example. During the unusually warm 1998 El Niņo year, corals suffered massive die offs worldwide. A new large bleaching event has just occurred in Australia.

Harvell and other scientists have shown that once bleached or stressed by heat, some corals become susceptible to disease.

"The disease may be what actually kills them," said Harvell, but she and her colleagues attribute a dieback of soft, Caribbean corals called sea fans to a fungal epidemic. Aspergillis - a fungus that occurs everywhere - tends to be a problem for immune compromised hosts.


Coral bleaching is a product of warmer temperatures and disease, the researchers argue. (Photo courtesy Florida National Marine Sanctuary)
"Once stressed, corals become susceptible to diseases," said Harvell, whose group has isolated the fungus and found that it grows fastest at 30-32 degrees Celsius - exactly the temperature at which many of the corals in the Florida Keys start to bleach.

Warming winter temperatures can be as much of a problem as summer temperatures in affecting the geographic distribution of pathogens and parasites. The edible eastern oyster is plagued by a protozoan parasite called Perkinsus. A winter warming trend in the mid-1990šs lifted a barrier to the parasite's northward movement and allowed it to spread into previously unexposed Maine oysters.

In the Hawaiian Islands, mosquitoes are now spreading malaria into the last populations of honeycreepers ­ boldly colored songbirds that evolved only in Hawaii. In the 1960s, mosquitoes were restricted by temperature to elevations below 2,500 feet, but warmer temperatures have allowed them to move higher up the mountainsides.

The honeycreepers are now restricted to the highest forested slopes of the Big Island of Hawaii and Maui at elevations cool enough to stop the mosquitoes.

"Today there are no native birds below 4,500 feet," said Dobson.


Many Hawaiian birds, like this `Akiapölä`au, are declining due to the spread of malaria and other threats. (Photo courtesy Earthjustice Legal Defense Fund)
Humans are affected by spreading diseases as well. Rift Valley fever, a devastating viral illness spread by mosquitoes, occurs mostly in parts of East Africa and is linked to heavy rains. The last Rift Valley fever outbreak in 1998 killed thousands in East Africa.

"There is clear evidence that Rift Valley Fever outbreaks are linked to El Niņo years and we expect an increase in the frequency of El Niņos with climate change," warned coauthor Richard Ostfeld.

In the wetter conditions, mosquito populations explode, more mosquitoes acquire the disease, and more transmit the disease to humans and livestock. With warmer conditions, the researchers expect both faster replication of the virus within mosquitoes and an increase in the rate that they bite and infect other animals.

"The diseases we should be most worried about are the vector transmitted diseases," said Dobson.

As 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 individuals of each species. This dilutes the spread of pathogens between species.

In temperate areas, disease can hit harder. There are fewer numbers of species but many more individuals, making them more vulnerable to epidemics.


Dozens of lions died in Tanzania in 2001 after flies carrying distemper spread to eastern Africa. (Photo courtesy U.S. Fish and Wildlife Service)
With higher levels of biodiversity, there is a greater chance that a mosquito will bite a species in which the disease does not develop. In temperate areas, vectors have fewer choices about what to bite. As biodiversity decreases there is less and less buffering.

"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!" Dobson said.

Pathogens have also contributed to declines of threatened species such as lions, cranes, vultures and black-footed ferrets, among others.

"Human destruction of biodiversity makes this a double whammy ­ it means we are exacerbating the problem," said Dobson.

The authors acknowledge that response to their findings will be controversial and politicized.

"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," said Dobson. "This is true. Very little monitoring and few long term studies exist. What is apparent is the end result - when the epidemic strikes."

When diseases do break out, little is known about how to respond ­ and that needs to change, say the authors.


A parasite that attacks monarch butterflies is spreading into the insect's previously safe northern habitats due to warming temperatures. (Photo courtesy World Wildlife Fund)
Baseline disease data are critical. And while forecasting epidemics in crop diseases has received much attention, and the National Ocean and Atmospheric Administration (NOAA) has a sea surface temperature monitoring system for coral bleaching, there are no disease forecasting models for threatened wildlife populations or management protocols for infectious diseases.

"We need to develop lines of defense," said 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."

Not including funding for HIV, there is very little research money going towards infectious disease work and training, anywhere in the world.

"The scary thing about the recent anthrax threat [in the United States] 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," said Dobson. "We need to pay better attention to this issue in an increasingly unnatural world."