The deadly brain diseases known as prion diseases might pass from one species to another more easily than previously thought, a new study from France finds.

Prions, such as those that cause “mad cow” disease, spread via exposure to infected lymphoid tissues — such as the spleen or lymph nodes — more easily than they can through exposure to infected brain tissues, the study showed.

The finding suggests that the number of animals or people that are “silent carriers” of prion diseases may be higher than previously thought.

Unlike diseases caused by viruses or bacteria, prion diseases are caused by infectious protein molecules that pass from animals to humans. Although differences between species makes it difficult for prion diseases to spread from one species to another, transmission does occur.Bovine spongiform encephalopathy, as mad cow disease is properly called, can be transmitted to humans by eating meat infected with the disease.

This study might help researchers better determine the risk of exposure to animal prions and silent carriers, said study co-author Vincent Beringue, a scientist at the French National Institute for Agricultural Research.

The findings were published today (Jan. 26) in the journal Science.

Prion diseases are rare

The cause of prion diseases is unknown, but experts believe that somehow, normal proteins may transform into abnormal, harmful prions.

The most common human prion disease, Creutzfeldt-Jakob disease, occurs worldwide. The disease is rare, striking about one out of 1 million people per year, according to the Centers for Disease Control and Prevention (CDC).

It leads to a rapid decrease of mental functioning and movement and ultimately, death, about a year after diagnosis.

Is the species barrier as strong we think?

In the new study, researchers studied transgenic mice that were infected with prions from either sheep or humans, and found that their lymphoid tissues, including the spleen, tonsils, appendix and lymph nodes, had more prions than their brain tissue.

Researchers found, on average, 7 percent of mice's prions in their brains, whereas 65 percent were in their spleens.

“Prion proteins can multiply quite efficiently in the lymphoid tissue,” Beringue said. “This was totally overlooked until now, most likely because studies were focused on brain tissue,” he said.

The study has significant implications for public and animal health, according to ChristinaSigurdson, who researches prion diseases at the University of California at San Diego, and was not involved with the study.

For example, Sigurdsonsaid, chronic wasting disease is a prion disease found in deer and elk, and is currently spreading in these animals throughout the United States, according to the CDC.

“Although there is evidence of a species barrier for human infection with this disease, we don't know if there is a population of people that may be currently infected with deer or elk prions,” she said.

“This is a major concern, because the infection could remain undetected for long periods of time,” she added.

Pass it on: Brain diseases may pass from one species to another a lot easier than experts thought.

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