Resistant prions

A flock of sheep at pasture – a seemingly idyllic scene. But appearances can be deceptive: If the animals are suffering from scrapie, entire flocks may perish. Scrapie is an infectious disease in which prions destroy the animal’s brain, rather like BSE. The brain becomes porous, the sheep lose their orientation, they suffer from strong itching sensations and scrape off their fleece. Eventually, the infected animals die.

It is difficult to contain the disease – all too often, scrapie will break out again on the same farm several months or years after it has apparently been eradicated. Are the prions transmitted not only by direct contact, but also by the environment – perhaps by the pastures? How long do prions that get into the pasture via the saliva and excrements of the sick animals, persist in the ground?

Together with fellow-scientists from the Robert Koch Institute in Berlin and the Friedrich Loeffler Institute (Federal Research Institute for Animal Health) on the island of Riems, research scientists from the Fraunhofer Institute for Molecular Biology and Applied Ecology IME in Schmallenberg investigated these questions on behalf of the German Ministry for Environment, Nature Conservation and Nuclear Safety BMU. “We mixed soil samples with scrapie pathogens to find out how long the pathogens would survive,” says Dr. Björn Seidel, who headed the investigations at IME. “Even after 29 months, in other words more than two years, we were still able to detect prions in the soil.”

But are these prions still infectious? “The soil actually seems to increase the infectiousness of the pathogens. The incubation period – the time it takes for the disease to break out – is exceedingly short even after the prions have persisted in the soil for 29 months. All of the animals that were given contaminated soil became sick within a very short time. These results indicate that fresh incidences of scrapie among sheep are due to contaminated pastures,” says Seidel in summary. The results of the study reveal that sheep may even become infected from the surface water, though the risk of infection is much lower in this case. There is no danger to humans, however: scrapie pathogens seem unable to affect them.

Another cause for concern is chronic wasting disease (CWD). Like BSE and scrapie, this is caused by prions, but it mainly affects deer. The numbers of infected animals in North America are rising steeply. How long do BSE and CWD prions survive in the ground? “To find this out, we urgently need to carry out further tests. The appropriate research applications have already been submitted,” says Seidel.