Sep. 17, 2003 MADISON - Dirt may help scientists answer a question that has baffled them for decades: How does chronic wasting disease (CWD) in deer and elk spread from animal to animal?
By turning to the land, University of Wisconsin-Madison researchers show that prions - infectious proteins considered to be at the root of the disease - literally stick to some soil types, suggesting that the landscape may serve as an environmental reservoir for the disease.
The findings will be discussed during a poster presentation on Wednesday, Sept. 10, in New York City at the 226th national meeting of the American Chemical Society.
Extraordinarily resistant to a range of environmental conditions and decontamination measures, prions are abnormally folded proteins that can make an animal's brain as holey as a sponge. They've been implicated as the cause of diseases such as mad cow and scrapie in sheep.
Once infected, deer and elk, for example, experience a number of neurological and behavioral problems - staggering, shaking and excessive salivation, thirst and urination - until they waste away, many times dying in fields or woods. The disease is always fatal, and, to date, there is no cure.
Even though chronic wasting disease was first detected in free-ranging deer and elk in Colorado and Wyoming during the mid-1980s, it received a charge in scientific and public interest in February 2002, when the first evidence of the disease in Wisconsin appeared.
"The route by which CWD is transmitted from animal to animal is not understood," says Joel Pedersen, an environmental chemist and lead investigator on the soil study. "Strong circumstantial evidence suggests an environmental reservoir exists." Reports show, for instance, that healthy elk placed in pens where animals infected with CWD had once lived developed the fatal disease.
With funding from a recently awarded five-year, $2.4 million grant from the Department of Defense's National Prion Research Program, Pedersen and his colleagues are examining the ability of the infectious agent to associate with or be absorbed by certain soil particles.
"Soil is a candidate [as an environmental reservoir] because grazing animals ingest it both inadvertently, as part of feeding, and on purpose, as part of certain deer behaviors," explains Pedersen.
To begin to understand how the disease stays in the environment, Pedersen and his colleagues turned to sand and clay - common components found in soils. Because of differences in surface area and mineral composition, Pedersen says sand and clay represent different ends of the spectrum in the ability to absorb proteins.
From the study's results, the capacity of sand and clay to take up abnormally folded proteins, says the lead researcher, "differs dramatically."
Pedersen and his colleagues determined this by taking samples of sand and clay and adding infectious prions taken from hamsters, as well as a water-based solution representing one found naturally in soils. After removing the water and doing further analysis, they noticed that many of the prions in the sand mixture remained in the water solution, whereas those in the clay mixture stuck to the particles' surface.
"Almost all the prions in the clay mixture associated with the clay, not the water," says Pedersen, adding that this finding suggests that the movement of prions through the landscape depends on the soil environment.
Understanding how the infectious agent moves - or, in the case of soils with high clay concentrations, stays put - could lead to new information on disease transmission or techniques for managing CWD. For instance, Pedersen says, "If we decide to bury infected carcasses, a clay liner underneath the landfill may be a good idea."
But while clay soils may work to contain infection, they may also help spread it. Whereas prions in sandy soils either may wash away or travel deeper into the ground, says Pedersen, those in clay soils may remain near the surface. "Because the material may be more available for ingestion by animals," he explains, "the rate of infection may be greater."
Analyzing the absorption capacity of sand and clay is just the first step, says Pedersen. In addition to quantifying the ability of prions to bind to these two soil components, they'll consider other soil materials, additional soil minerals and organic matter. Also under way are studies to determine the degree to which prions in different soil types remain infectious.
"What we'll be getting at is if prions are more likely to persist in some environments," says Pedersen, adding that results from all these studies will help natural resource managers and other experts perform risk assessments for the spread of CWD and similar diseases across the landscape. "Understanding the role of soil in the spread of CWD is critical in designing and implementing effective disease strategies."
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