Soil binding may increase prion infectivity

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Prion particles remain infectious when bound to soil particles, providing strong evidence soil could serve as a reservoir for chronic wasting disease (CWD) and other transmissible spongiform encephalopathies (TSE), a new study shows.

In fact, Dr. Joel A. Pedersen of the University of Wisconsin in Madison and colleagues found, prions bound to the soil mineral montmorillonite may actually be more infectious than unbound prions.

A study published several years ago found that prions could survive in soil for three years, Dr. Pedersen noted in an interview with Reuters Health. Researchers in the older study had extracted prions from garden soil with water, which would not have detected prions that were more strongly bound to soil particles, he explained.

"For management of chronic wasting disease and sheep scrapie it appears that there may be more infectivity in soil than was previously assumed," he said.

Dr. Pedersen and his team conducted the current study to investigate prion binding to the common soil minerals quartz, kaolinite, and montmorillonite, as well as whole soils.

Prions bound to montmorillonite with "surprising" strength, the researchers found. Techniques normally capable of detaching proteins from soil minerals were not effective, and it was necessary for the researchers to boil the prion-bound mineral in a detergent solution before desorption occurred.

Desorbing prions from the mineral caused cleavage resulting in the loss of the protein's N-terminal, which is not required for infectivity.

Hamsters injected intracerebrally with montmorillonite-bound prions developed scrapie symptoms ten days earlier than animals injected with prions alone, an estimated 1-log increase in infectivity.

"Our findings suggest that PrPSc released from diseased animals may be sequestered near the soil surface, maintaining the TSE agent in an environmental medium with which livestock and wildlife come in contact," Dr. Pedersen and his team write in the April issue of PLoS Pathogens.

The findings support current efforts to halt the spread of CWD by removing animals from areas where the disease is present, Dr. Pedersen said. He and his colleagues are currently investigating the infectivity of mineral-bound prions ingested orally, and are also working on developing methods for clearing the infectious agents from pastures