Prions' dirty little secret.

Fifteen years ago, scientists at the National Institutes of Health reported that malformed prions--proteins that can trigger lethal illnesses including mad cow disease--remain on soil surfaces for at least 3 years. Now, scientists report why rain doesn't flush away the prions: The proteins bind almost irreversibly to clay.

In fact, clay can "retain up to its own mass of ... prion proteins," says Peggy Rigou of the National Institute of Agronomic Research (INRA) in Jouy-en-Josas, France.

Her team added sheep prions to pure clay, sandy soil, and loam. Positively charged parts of the protein molecules bound to the negatively charged surface of the clay that was present in all the soft samples. Extensive washing failed to dislodge the prions. However, when the chemists treated the mixtures to make the proteins negatively charged and then ran an electric current through each mixture, the prions migrated off the clay particles.

Freeing the prions was a major achievement, Rigou notes, because it enables scientists for the first time to measure prion concentrations in soil. Until now, no technique could confirm that intact prions were present in soil. In an upcoming Environmental Science & Technology, her team reports that the new procedure permits detection of concentrations as low as 0.2 part per billion.

Soils might acquire prions from animal wastes or carcasses. Scientists' concern is that livestock might ingest infected day par-tides while eating grass or drinking from mud puddles, Rigou says.--J.R.