Persistent prions: soilbound agents are more potent.

Deformed proteins called prions cause fatal brain-destroying disorders, such as chronic wasting disease in deer and elk and mad cow disease, which can infect people. Evidence suggests that prions make their way into animals' nervous systems through ingestion, but scientists aren't sure.

A new study shows that prions become more infectious when they latch on to soil particles that animals eat, suggesting that ingestion is a primary route of disease transmission. "Our study points us in one direction that explains how these animals are getting infected," says study author Judd Aiken of the University of Wisconsin-Madison.

Prions enter the environment from the remains of infected animals, and, to some degree, from body fluids such as urine and saliva. Prions linger in soil for at least 3 years (SN: 2/11/06, p. 93) by binding tightly to clay and other minerals. Aiken had hypothesized that soil would hinder the action of the clingy prions, making them less infectious. He was surprised to find the opposite.

"The binding of infectious agents in soil actually greatly enhances the infection," Aiken says. "It makes the disease more transmissible."

Wild and farm animals often swallow up to several hundred grams of soft per day when eating plants, drinking muddied water, and licking the ground to get minerals. In doing so, they may consume prions. The relationship between ingestion and infectivity is unclear, though, because previous experiments showed that prions are inefficient at infecting animals that eat diseased tissue.

Aiken and his team fed each of three groups of hamsters a different soil type containing prions. Other hamsters were given an equivalent dose of a prion mixture derived from the brains of infected animals. All soil-eating hamsters were at least as likely to contract the prion disease as those that had ingested the prion-brain mixture, which has been considered an efficient transmitter of prions.

Two of the three soils had an even more dramatic effect. Hamsters that ate either of those soils had a higher rate of prion disease than did animals that ate the prion-brain mix. Animals that ate the third soft, which contained more organic matter than the other two did, had the same infection rate as hamsters that ate the prion-brain mix.

Researchers hypothesize that soil might protect prions from the destructive environment of the digestive system. Alternatively, Aiken says, soil particles might break up clumps of prions into smaller, more numerous clusters. Or, the particles could change the way in which prions enter nervous system tissues.

The study, in the July PLoS Pathogens, yielded "very fascinating findings," says Michael Miller, a wildlife veterinarian at the Colorado Division of Wildlife in Fort Collins. "It ties together observations that people have made throughout the years." He suggests that the different infectivity rates of prions in the three soils may also explain why the disease afflicts animals in some areas more than in others.