Plants are known to take up prions, but it is still unclear whether plants can serve as significant vectors of prion diseases such as mad cow, scrapie, and chronic wasting disease. Especially concerning is the possibility that plants might spread prion diseases across species, not just in laboratory settings, but also under environmental conditions.
Test animals, it has been established, become sickened with prion disease if they are injected with contaminated plant material. But what if animals merely feed on prion-containing plants? This has remained an open question, even though some research suggests that grasses, for example, may take up prions from contaminated soil without carrying them to above-ground structures.
A new study, however, is less encouraging. It indicates that grass plants bind prions. Also, stems and leaves from grass plants grown in infected soil contain prions. Finally, when prion-contaminated plant samples are fed to hamsters, the animals develop prion disease.
These findings appeared May 14 in Cell Reports, in an article entitled, “Grass Plants Bind, Retain, Uptake, and Transport Infectious Prions.” The article’s authors, who are based at the University of Texas Health Science Center at Houston, emphasized that there is no proof of transmission from wild animals and plants to humans. However, according to senior author Claudio Soto, Ph.D., the possibility “needs to be explored and people need to be aware of it.” Prions, he added, have a long incubation period.
Prions are the protein-based infectious agents responsible for a group of diseases called transmissible spongiform encephalopathy, which includes bovine spongiform encephalopathy (mad cow disease) in cattle; scrapie in sheep; variant Creutzfeldt-Jakob disease in humans; and chronic wasting disease in deer, elk, and moose. All are fatal brain diseases with incubation periods that last years.
Dr. Soto's team analyzed the retention of infectious prion protein and infectivity in wheat grass roots and leaves incubated with prion-contaminated brain material and discovered that even highly diluted amounts can bind to the roots and leaves. When the wheat grass was consumed by hamsters, the animals were infected with the disease.
The team also learned that infectious prion proteins could be detected in plants exposed to urine and feces from prion-infected hamsters and deer. “The prion-plant interaction occurs with prions from diverse origins,” the researchers noted, “including chronic wasting disease.” In addition, the researchers found that plants can uptake prions from contaminated soil and transport them to different parts of the plant.
“These findings,” the researchers concluded in Cell Reports, “demonstrate that plants can efficiently bind infectious prions and act as carriers of infectivity, suggesting a possible role of environmental prion contamination in the horizontal transmission of the disease.”
To minimize the risk of exposure to chronic wasting disease, the Centers for Disease Control recommends that people avoid eating meat from deer and elk that look sick or test positive for chronic wasting disease. Hunters who field-dress deer in an affected area should wear gloves and minimize handling of the brain and spinal cord tissues.
“This research was done in experimental conditions in the lab,” Dr. Soto noted. “We’re moving the research into environmental contamination now.”
