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Scientists probe CWD's spread through soils

Katherine (Trina)  McMahon

Katherine (Trina) McMahon (large image)

It is challenging enough to eradicate deer populations in areas thought to harbor Chronic Wasting Disease (CWD), the mysterious neurological deer and elk illness that is cropping up in unexpected places, such as New York, this spring.

But once state officials have killed hundreds of deer, what if infectious prions — the elusive proteins thought to cause CWD — remain in deer carcasses, eventually seeping through garbage landfills and draining uncontrolled into wastewater treatment plants and beyond?

With backing from the Environmental Protection Agency, scientists at the University of Wisconsin-Madison are investigating the potential scenario. They reported early findings during the second International Chronic Wasting Disease Symposium here on July 12-14.

Craig H. Benson

Craig H. Benson (large image)

The symposium comes at a time when the known range of CWD seems to be swelling from its relatively small, endemic home in parts of Wyoming and Colorado toward the east, where deer populations are denser.

Recent evidence suggests that prions have a tendency to latch onto soil particles. Since deer eat soil at certain times of the year scientists have surmised that CWD-inducing prions might piggyback into the animals through ingested earth. But can those prions hop out of a dead deer and back into soil — even if it's the smelly earth of a dumpsite?

To understand the potential risks in disposing of deer carcasses in landfills, environmental biochemist Joel Pedersen, virologist Judd Aiken and geo-environmental engineer Craig Benson are trying to simulate a "mass waste" situation in the laboratory. "[Landfills] are a really, really complicated system, so we are starting out simple, with just a pure column of sand," says Pedersen. The UW-Madison researchers found that prions did indeed leach, or drain down, through the sand particles. "We are currently evaluating leaching through soils commonly used in landfills and will soon be working with synthetic municipal waste and real municipal waste," Pedersen adds.

It's hard tracking prions, not least because researchers know hardly anything about them. Completely unlike bacteria and viruses, prions "don't conform to the paradigm of what we know to constitute infectious diseases," says Trina McMahon, a UW-Madison environmental engineer, who also spoke at the CWD meeting.

McMahon's piece of the puzzle begins where fluids from landfill sites course into wastewater treatment plants. She is exploring how prions would behave if they were to flow from landfills into the wastewater treatment process. "This is a waste containment issue," says McMahon. "If prions remain in solid form, we can probably contain them, but if they get into liquid they would be much more mobile."

When sewage flows into wastewater treatment plants, it runs first into a tank filled with natural bacteria. The organisms essentially break down the incoming waste, which eventually separates into clean water and "waste activated sludge," a combination of bacteria and the sticky, slimy goo it exudes. In early laboratory simulations, McMahon found that prions — which are sticky themselves — latch onto the bacterial goo and remain infectious.

In later stages of wastewater treatment, the sludge is processed into a purified and nutrient-rich organic material known as a biosolids. Scientists have long urged farmers to spread the substance on their fields but the practice remains controversial, says McMahon.

The researcher intends to spend the next year ascertaining whether prions eventually wind up in and remain infectious inside biosolids. Her results could heavily sway the fate of the material's use in agriculture.