- 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
- 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
- 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