Infectious Diseases Spreading
West Nile virus. Hantavirus. Lyme disease. All are infectious diseases spreading in animals, and in humans. Is our interaction with the environment somehow responsible for the increase in incidence of these diseases?
A joint National Science Foundation (NSF) and National Institutes of Health program — ecology of infectious diseases (EID) — supports efforts to understand the underlying ecological and biological mechanisms behind human-induced environmental changes and the emergence and transmission of infectious diseases. Projects funded through the EID program and other NSF programs allow scientists to study how large-scale environmental events””such as habitat destruction, invasions of non-native species and pollution””alter the risks of emergence of viral, parasitic and bacterial diseases in humans and animals.
Researchers supported in the EID program are advancing basic theory related to infectious diseases and applying that knowledge to improve our understanding of how pathogens spread through populations at a time of increasing global change.
The benefits of research on the ecology of infectious diseases include development of theories of how diseases are transmitted, improved understanding of unintended health effects of development projects; increased capacity to forecast disease outbreaks and knowledge of how infectious diseases emerge and reemerge.
“Virtually all the world’s terrestrial and aquatic communities have undergone dramatic changes in biodiversity due primarily to habitat transformations such as deforestation and agricultural intensification, invasions of exotic species, chemical contamination and climate-change events,” said Sam Scheiner, ecology of infectious diseases (EID) program director at NSF. “The coincidence of broad-scale environmental changes with the emergence of infectious diseases may point to underlying and predictable ecological relationships.”
Examples of studies funded by the EID program include research on the origin and spread of the aspergillus-gorgonian coral disease and how climate and environment may have worked as facilitators of the disease; effects of human-induced change on the ecology of human pathogens in North Carolina’s Neuse River estuary, which is polluted by excess nutrients from human activities; the microbial community ecology of tick-borne human pathogens; plague as a model for disease dynamics; ecological reasons for rodent-borne disease outbreaks; and how social organization influences an infectious disease outbreak.
Further information about EID program support is available in the latest program solicitation.
Medical Mystery Solved
When young, otherwise healthy people in the remote Four Corners area of Arizona and New Mexico began dying of a mysterious, acute respiratory disease in the spring of 1993, scientists wondered at the cause.
Tests of the victims’ blood yielded surprising results: the people had become infected with a previously undetected kind of hantavirus. Named for the Hantaan River in Korea, hantaviruses were known to spread from rodents to humans in Asia and Europe, but until the Four Corners outbreak, the microbes had only been seen outside of the United States.
For answers as to how the virus spread in the Four Corners, the U.S. Centers for Disease Control turned to scientists Robert Parmenter of the Sevilleta (SEV) Long-Term Ecological Research (LTER) site in New Mexico, and Terry Yates of the University of New Mexico. Their research at the LTER site revealed that the hantavirus outbreak could be blamed on El Niño, a periodic pattern of change in the global circulation of oceans and atmosphere. Massive rains associated with the 1991-1992 El Niño had substantially boosted plant productivity after several years of drought. A banner year for plants was followed by a banner year for rodents. More mice meant that more humans stood a greater chance of exposure to infected rodents as people moved among barns and did spring cleaning of cabins and trailers.
The deadly hantavirus wasn’t new to New Mexico. The virus had been in the rodents all along. It was the change in climate conditions that triggered the fatal outbreak in humans. Such knowledge likely saved lives in 1998, when another active El Niño prompted health authorities to warn residents in the American Southwest to use caution when entering areas favored by mice.
Further information about the SEV LTER is available at http://www.lternet.edu/sites/sev/
General information about the LTER Network, including links to each of the LTER sites, is available at http://www.lternet.edu
Frogs Vs. Trout
Ecology of infectious diseases data gathered over seven years has played a key role in convincing the National Park Service and the California Department of Fish and Game to remove trout from high-altitude lakes in California’s Sierra Nevada. The trout are causing the disappearance of the mountain yellow-legged frog.
Funded through the EID program, biologist Vance Vredenburg of the University of California at Berkeley showed that introduced-trout have devastated native frog populations over the past 50 years in formerly fish-free, high-Sierra lakes, but that removing the fish can allow the frogs to flourish once more.
“The mountain yellow-legged frog used to be the most common inhabitant of the high Sierra, but frog populations have declined dramatically enough to put it on the endangered species list,” said Vredenburg.
“The worldwide decline in frog and salamander populations is a harbinger of more serious threats posed by the current rapid environmental changes our planet is undergoing,” said Sam Scheiner, EID program director at NSF. “Possible culprits include the spread of disease, increased UV radiation and predation by introduced species. This study helps to tease apart those complex causes and shows that, in these frogs, the decline is due to increased predation. For these populations, removing the trout will save the frogs. Such studies provide hope that we can reverse the large environmental changes we’re causing.”
As part of the research, Vredenburg removed trout from five lakes and documented a rebound in the frog population in all of them. Three years after trout removal, the frog populations in all five lakes were indistinguishable from populations at lakes that had never seen a trout.
“The response was incredibly dramatic and rapid,” Vredenburg said. “Every time you plant hundreds of thousands of fish, you’re hammering a nail in the frogs’ coffins.”
Vredenburg has also teamed up with other researchers to determine the effect of a chytrid fungus, Batrachochytrium dendrobatidis, on the mountain yellow-legged frog. The fungus, which is threatening frog populations around the world, attacks tadpoles as well as adults, and can kill adult frogs. It was discovered in the Sierra Nevada in 2001.
Loss of wetland habitat has also reduced populations of frogs and toads and endangered several species of amphibians with restricted ranges. Alarming new events have added to these trends. For example, frog and toad populations have declined dramatically in the past several years, many in high-altitude places in the United States, Puerto Rico, Costa Rica, Panama, Colombia and Australia. Studies suggest that these population declines may be caused by infections, perhaps promoted by environmental stressors.
For more information on studies of the chytrid fungus, please see Outbreak: Rapid Appearance of Fungus Devastates Frogs, Salamanders in Panama.
Deer Susceptible to Disease
Researchers funded through the EID program recently found that chronic wasting disease (CWD) can be transmitted through environments contaminated by whole carcasses or by excrement of animals infected with the pathogen that causes CWD. CWD is rampant in Western states like Colorado.
“Diseases like CWD are poorly understood and of rising concern,” said Sam Scheiner, EID program director at NSF. “This new knowledge will substantially alter how we manage the disease in wild and domestic animals.”
CWD is a fatal neurological ailment of elk, white-tailed deer and mule deer. Researchers believe the disease is caused by an aberrant prion protein that misfolds in the brain, destroying brain tissue as it progresses. The disease is always fatal and there is no known cure or treatment.
Although live deer and elk still seem the most likely way for CWD to spread geographically, environmental sources could contribute to maintaining and prolonging local epidemics, even when all infected animals are eliminated, said biologist Tom Hobbs of Colorado State University. “Through the EID program, we hope to develop models that will predict the behavior of the disease, shedding light on how potentially complex these epidemics may be in natural populations.”
Further information about CWD is available on the Chronic Wasting Disease Alliance Web site.
Lyme Disease on the Rise
Lyme disease incidence is rising in the United States and is in fact far more common than West Nile virus and other insect-borne diseases. Forest fragmentation could explain the increase.
Areas of patchy woods, which are very common in cities and suburban and rural areas, may have higher populations of Lyme-disease carrying ticks than forest fragments, which generally have fewer species than continuous habitat. This is because some species thrive in smaller places.
White-footed mice, for example, are more abundant in forest fragments in some parts of the country, likely because fewer predators and competitors remain there. These mice are particularly abundant in patches smaller than about five acres, which could spell trouble for people living nearby: the mice are the main carriers of Lyme disease-causing bacteria. In the Eastern and Central United States, Lyme disease is contracted via blacklegged ticks that feed on infected mice, and then transmit the bacteria when the ticks bite people. As a result, says biologist Felicia Keesing of Bard College in Annandale, New York, Lyme disease is concentrated in areas where people live near forests with blacklegged ticks.
Keesing and other scientists found that smaller forest fragments had more infected ticks, which could translate to more Lyme disease. Forest patches that were smaller than three acres had an average of three times as many ticks as did larger fragments, and seven times more infected ticks. As many as 80 percent of the ticks in the smallest patches were infected, the highest rate the scientists have seen.
“Our results suggest that efforts to reduce the risk of Lyme disease should be directed toward decreasing fragmentation of deciduous forests of the Northeastern United States, particularly in areas with a high incidence of Lyme disease,” said Keesing. “The creation of forest fragments smaller than five acres should especially be avoided.”
For more information, visit the Center for Disease Control’s Web page on Lyme Disease http://www.cdc.gov/ncidod/dvbid/lyme/
By Cheryl Dybas
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Image 1: Disease transmission is a complex process that involves the disease organism, disease vectors, disease hosts, and the predators of those hosts. It links relatively pristine areas with human habitations and human-dominated areas. Credit: Nicolle Rager, National Science Foundation
Image 2: Massive rainfall associated with El Niño boosts plant productivity. Feasting on the more abundant plant matter, the rodent population grows. Increased contact with rodents and their waste puts more humans at risk for exposure to hantavirus. Credit: Zina Deretsky, National Science Foundation
Image 3: Mating season (June 2004) in Sixty Lake Basin. The large lake in the foreground is a frog population source and the three lakes in the background are trout removal lakes now colonized by large frog populations. Credit: Rob Bingham, University of California, Berkeley
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