The discovery that prions stay deadly despite sticking to soil comes as a surprise, because while many proteins can bind to soil, that binding usually changes their shapes and activities.
In a paper published in the journal PLoS Pathogens (April 14), scientists at the University of Wisconsin-Madison suggest that certain soil types serve as natural prion repositories in the wild. As animals regularly consume soil to meet their mineral needs, it's possible that prion-laden soil particles contribute to the transmission of prion disease such as CWD among animals.
CWD is a fatal, incurable condition that belongs to a family of prion-inflicted neurological disorders known as transmissible spongiform encephalopathies (TSE). Other TSEs include "mad cow" disease, sheep scrapie and Creutzfeldt-Jacob disease in humans.
After a long incubation period, deer and elk infected with CWD suffer neurological and behavioral problems such as staggering, shaking, and excessive salivation and urination. Over time, the animals literally waste away, often dying in woods and fields. Originally detected in the 1960s in Colorado and Wyoming, CWD is now present in 14 states and two Canadian provinces.
Prions are an incorrectly folded variation of a protein normally found in mammals, including humans.
"Prions most likely enter soil via excretion or from the carcasses of infected animals," says lead author Christopher Johnson, a UW-Madison doctoral student in the department of animal health and biomedical sciences. "Our results suggest that reducing the number of infected animals -- as has been done in the recent outbreak of CWD in Wisconsin -- could limit the potential for further (disease) spread. These results also suggest that other species that share ranges with CWD-infected deer may be exposed to soil-bound prions, increasing the potential of CWD transferring to other species."
This may just be the case. Last year, Colorado authorities documented the first known instance of CWD in a wild moose, a species previously not known to be susceptible to CWD. How the moose became infected is unknown. The known range of the disease continues to spread eastward from where it was first discovered in Wyoming and Colorado to deer-rich states such as West Virginia, where CWD was first detected last September.
Using a variety of laboratory procedures, the UW-Madison team measured the affinity of prions to three common soil minerals: quartz, kaolinite and montmorillonite. The infectious prions, they found, bind tightly to montmorillonite, a type of clay found in soil.
"We also wanted to determine how difficult it is to remove prions from clay," says senior author Joel Pedersen, a UW-Madison assistant professor of soil science. "It turned out to be extremely difficult."
In fact, prions could be released from clay only when the scientists boiled the clay-bound proteins in a detergent solution.
To ascertain whether prions remain infectious in soil, the researchers also injected clay-bound prions into laboratory animals. The animals began to show TSE symptoms at approximately the same time as animals injected with only prions.
"That result indicates that interactions with the clay mineral do little to reduce prion potency," says co-author Debbie McKenzie, a senior scientist in UW-Madison's School of Veterinary Medicine. "Knowing that prions could be maintained in the environment makes it important to continue removing as many CWD-infected deer as we can," she says.
"While injecting clay-bound prions into experimental animals has shown that they remain infectious, more environmentally relevant exposure routes need to be examined," says Pedersen. Experiments examining oral infectivity are under way. The researchers also plan to determine how long prions remain infectious in soils.
The Wisconsin team indicated that while their results may provide some new insight into how prion diseases such as CWD are transmitted, there are many questions that have yet to be answered about how animals acquire the disease.
Judd Aiken, a professor in the UW-Madison School of Veterinary Medicine, and Kristen Phillips and Peter Schramm of the UW-Madison Molecular and Environmental Toxicology Center, also participated in the new study.
The article is available in PLoS Pathogens; unlike other scientific publications, a subscription is not required to access this journal at: http://www.plos.org/press/plpa-02-04-pedersen.pdf
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