Study shows prions stick around in certain soils

Dirt may help scientists answer a question that has baffled them for decades: How does chronic wasting disease in deer and elk spread from animal to animal?

By turning to the land, UW–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 were discussed during a Sept. 10 poster presentation at the 226th national meeting of the American Chemical Society in New York City.

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, without a cure.

Even though CWD was first detected in Colorado and Wyoming during the mid-1980s, it received a charge in 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.

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.