Will global climate change worsen infectious diseases?
The growing body of research linking climate to the spread of human and animal infectious diseases includes some ominous predictions, but long-range forecasts remain uncertain.
With a solid scientific consensus on the reality of global warming, the next research hurdle is to describe the impacts and what can be done to mitigate them. A growing body of work links climate to the spread of human and animal infectious diseases, but the relationships between pathogens and their hosts are complex. Predictions of how these dynamics will play out over the long run in a changing climate remain controversial.
Human infectious diseases have been on the upswing since the 1970s and 1980s, says Duane Gubler, an epidemiologist at the University of Hawaii. Dengue fever, not considered a major public-health problem in the mid-20th century, now strikes 50–100 million people each year, he says. More than 3000 children die from malaria each day, according to the World Health Organization (WHO). And emerging diseases, including West Nile virus and Lyme disease, are spreading across North America.
Many scientists suspect that global climate change may be a major contributor to the emergence and resurgence of at least some of these infectious diseases. WHO estimates that infectious and noninfectious diseases—such as heat stroke and asthma from smog exacerbated by warmer weather due to climate change—already claim 150,000 lives each year. However, the scientists interviewed for this story say that links between human disease and climate change are difficult to tease out.
Fortunately, nonhuman diseases are not influenced by as many factors, and this makes them ideal systems to study, says Rick Ostfeld, a disease ecologist at the Institute of Ecosystem Studies. “There are now dozens of examples of diseases where changes in temperature and precipitation are clearly implicated,” he says.
Harlequin frog extinction
Climate change has altered patterns of a fungal infection, leading to the extinction of two-thirds of tropical harlequin frog species in Central and South America, according to new research published on January 12 (Nature 2006, 439, 161–167). The authors found that warmer sea and air temperatures generated clouds that created optimal conditions for a pathogenic chytrid fungus that kills frogs.
Likewise, warming Arctic soil temperatures are accelerating the growth of a nematode worm parasite of musk oxen, according to a new study by Susan Kutz, a wildlife pathologist at the University of Calgary (Canada). The recently discovered parasite matures inside ground-hugging snails that are eaten by musk oxen. The parasites then work their way to the lungs and lay eggs in large cysts that typically number 100 per animal. For most of the past century, cool temperatures slowed parasite development, forcing the worm to overwinter in its snail host and face a measly 1% chance of surviving until spring. But in 12 of the past 13 years, soaring soil temperatures have allowed the worm to develop in one season, boosting the infection rate. This may be one factor driving a 50% decline in musk oxen since 1980 in a heavily infected area of Canada’s Northwest Territories, Kutz says.
Climate trouble in paradise
Meanwhile, avian malaria is invading higher elevations on Hawaii’s mountains, where threatened native birds once found refuge from this deadly pest carried by introduced mosquitoes. At temperatures below 13 °C, the parasite is unable to mature inside mosquitoes to its infectious stage, says Lenny Freed, an evolutionary ecologist at the University of Hawaii. Although average annual temperatures have held steady at Freed’s study site, he has documented more consecutive days of summer temperatures above the 13 °C threshold for malaria infectivity. New research on archived blood samples reveals that malaria infection rates began to climb about a decade ago, he says. “It offers a glimpse of what could be expected with climate warming,” he adds. Because Hawaiian birds evolved in the absence of mosquitoes and malaria, they have few defenses and could become extinct if they lose their lower-temperature mountain refuges.
On the basis of these types of studies, fingering global climate change as a driver of disease is intuitively tempting, says Bob Holt, an ecologist at the University of Florida. Climatic conditions set the distribution and abundance of disease-carrying vectors, such as mosquitoes that transmit the malaria parasite, he explains. “The bulk of the vector-borne diseases are tropical, and it’s expected that a warmer world will boost their numbers and range,” he says.
But sensitivity to climate doesn’t always lead to more cases, Holt cautions. In theory, if climate change renders a host less able to mount an immune defense and it therefore dies more quickly from the disease, global warming could weed out infection from the population over the long run, he says.
Or, a warmer but more variable climate could work to the disadvantage of disease vectors, says Greg Glass, a disease ecologist at Johns Hopkins University. He and his colleagues analyzed 30 years of data on mosquitoes and climate in Maryland and found that one of the most important factors determining summertime abundance of some species—such as Culex pipiens, the species often considered the vector of West Nile virus in the eastern U.S.—is the maximum temperature in winter. For species that overwinter as fertilized adult females, a warm spell in January that causes them to emerge from safe hiding spots makes them vulnerable to subsequent cold snaps. If climate change leads to more variable winter weather, as some models predict, it could actually reduce populations of mosquitoes in temperate zones, he says.
Human activities complicate the situation
Human intervention adds another layer to understanding the complexity of vector-borne diseases, especially human diseases, Ostfeld says. Malaria transmission is influenced by public-health infrastructure, the use of bed netting and pesticides, agricultural practices, and nutrition, making it difficult to pull out a climate signal from the data.
“It’s a fairly sure bet that insects that are currently restricted by cold will spread north as the climate gets warmer,” says David Rogers, an ecological epidemiologist at the University of Oxford. But whether and how much the diseases they carry with them will spread are unknown, he says.
“To capture malaria in a model, you’ve got to quantify all the variables, such as mortality rates,” Rogers says. But values for some variables, such as the efficiency with which the malaria parasite travels from humans to mosquitoes and back again, simply don’t exist in the literature, he says.
Rogers and his colleagues took a different approach by mapping the global distribution of malaria and using the data to get the disease itself to tell the researchers what its climate limits are—how much temperature, rain, and vegetation it needs to survive. When the scientists projected this climate envelope 50–80 years into the future, they found that malaria’s geographic boundaries would change only 5–6% compared with the present. Although roughly 400 million people would be newly exposed to the disease at its northern boundaries, another 400 million people live in areas where the disease would disappear, so these effects would cancel each other out, he says. Although the net number of people affected by the disease won’t change, the effects won’t be trivial as the toll of malaria is lifted from one group and shifted onto another, Rogers adds.
Researchers have not yet been able to produce convincing evidence that malaria cases will grow with a warming climate, mainly because of the strong disagreement between different climate models at the regional level, especially for precipitation, says Francisco Doblas-Reyes, a climate forecaster at the European Centre for Medium-Range Weather Forecasting. However, a strong push exists for more cooperation between epidemiologists and climatologists, and lessons learned with short-term forecasts can build a base of understanding for eventually making long-term disease forecasts, he says.
Doblas-Reyes and his colleagues recently developed a climate model that can anticipate malaria outbreaks in Botswana up to 5 months in advance (Nature 2006, 439, 576–579). Researchers applied 3 coupled ocean–atmosphere climate models to 22 years of data on climate and malaria in Botswana. Rainfall is the primary driver of malaria in Botswana, because it provides breeding habitat for the parasite’s mosquito vectors. “Our system makes predictions with as much accuracy as a system that makes forecasts 1 month in advance based on observed precipitation at the end of the rainy season,” Doblas-Reyes says. The forecasts will help public-health officials garner national and international funding for medicine, bed nets, and pesticides far in advance of outbreaks.
By showing that malaria cases respond to climate signals and by developing a methodology for forecasts, Doblas-Reyes and his colleagues are building the foundation for eventually connecting long-term climate change to effects on human health, says Rita Colwell, a molecular microbial ecologist at the University of Maryland. By combining remote-sensing and satellite imagery with details on parasites and their hosts, scientists can now make robust short-term forecasts of risk for other human diseases, she says.
Predicting cholera epidemics
“We can now predict how serious cholera epidemics will be and when they will occur in Bangladesh,” Colwell says. Infamous for causing deadly diarrhea in humans, cholera is a marine bacterium that is normally found in the gut and on the surface of copepods, microscopic animals that eat algae. People pick up the disease when they drink untreated brackish water in rivers and at the heads of estuaries, she says.
Colwell and her colleagues have found that cholera populations soar when copepods gorge themselves on algal blooms. This relationship has allowed researchers to craft a model that accurately predicts cholera incidence rates 4 months in advance on the basis of temperature and height of the sea surface and chlorophyll concentrations.
The cholera model also performs well off the western coast of South America, where outbreaks are associated with El Niņo years, Colwell says. El Niņo weather patterns are predicted to increase in intensity and frequency as the globe warms. As with malaria in Botswana, the forecasts will help public-health teams target medicines and preventive measures.
Although diseases are sensitive to climate, a host of other factors also affect the prevalence of disease. A good example is mosquito-borne dengue fever, which made a dramatic global reemergence in the 1970s and 1980s, raising suspicions that climate change is largely to blame. But research presented at the Forum on Climate and Disease at Yale University on December 9 indicates that climate is not a major factor and that demographics, culture, economy, and environment are influencing transmission.
Eradication measures begun in the 1940s, including spraying of pesticides, virtually eliminated dengue from the tropical Americas by 1970, Gubler says. Shortly afterward, the eradication programs ended, and the mosquito species that transmits dengue re-invaded the whole region from Argentina to Florida and Texas.
“Dengue is closely associated with population growth and urbanization,” Gubler says. However, sound housing construction, screens, air-conditioning, and television—which keeps people indoors while mosquitoes are feeding—can dramatically reduce disease incidence. For instance, from 1980 to 1999, Texas reported only 64 cases of dengue, while Mexican states directly across the border reported 62,514 cases.
Relatively low human population density and aggressive mosquito control make large outbreaks unlikely in the U.S., Gubler says. In addition, a second mosquito species that is an inefficient epidemic vector is out-competing the more efficient dengue-transmitting species in the U.S., he says.
Like dengue, cases of Lyme disease are unlikely to grow significantly in North America, but for different reasons. The distribution of the deer ticks that carry the Lyme disease bacterium is regulated by climate, says Durland Fish, an epidemiologist at Yale University. Fish and his colleagues used the Canadian global-warming model to examine how climate change would alter the distribution of the ticks and of potential cases of Lyme disease. “We found that [a warmer climate] would move distribution of the vector further north, into Canada, and there would be less of it in the south,” he says. Because the ticks will be moving out of densely populated areas and into more sparsely populated ones, the number of cases is not expected to rise, he says.
“Although progress awaits better and more field surveillance data and modeling methods, a significant amount of ecological theory and empirical data exist that link anthropogenic environmental change and pathogen emergence,” Colwell concludes.
