Scrapie is the oldest known prion disease and the one most people have never heard of, even though it has been in sheep flocks for centuries.
What it does to sheep and goats
It is a fatal degenerative disease of the nervous system. Most animals are infected as young lambs or kids, though adults can be infected too.
Signs take two to five years to appear, which is part of why it spreads before anyone notices. They include loss of coordination, weakness, tremors, head pressing, star gazing, an abnormal gait, changes in behaviour and heightened sensitivity to noise. Some animals lose weight while still eating normally, or rub their wool off. Affected animals usually live one to six months after signs begin. There is no treatment, and the available tests need brain or lymphoid tissue.
How it spreads
Apparently healthy infected animals can pass it on, which is the central difficulty. Classical scrapie passes from the mother to her offspring around birth and through weaning, and before birth, and it can also spread to unrelated sheep and goats. Fetal membranes and milk from affected animals can transmit it. The agent persists for years on pastures and in buildings, binds tightly to surfaces and resists most disinfectants.
Atypical scrapie behaves differently. WOAH describes it as not considered transmissible under field conditions, behaving as a sporadic, non-contagious condition, which is why it tends to turn up in older animals through routine testing rather than as an outbreak.
The part genetics plays
Susceptibility depends on the animal’s own prion protein gene. In sheep, codon 171 is the main determinant, with one form linked to susceptibility and another to resistance, and codon 136 also plays a part. About 30 percent of United States sheep are genetically susceptible, and APHIS reports that breeding resistant animals has significantly reduced scrapie there.
All goat breeds are susceptible. Three goat prion gene variants give meaningful resistance, but for regulatory purposes all exposed goats are treated as susceptible.
Eradication programmes
The United States runs a National Scrapie Eradication Program, a joint state, federal and industry effort aiming at the criteria for freedom from disease set by the World Organisation for Animal Health. APHIS reports scrapie eliminated in 99 percent of United States sheep and goats, with effort now directed at the remaining higher-prevalence groups, and prevalence substantially down since 2002. It pays for testing up to 30 animals per flock per year, and asks owners to submit samples from sheep or goats over 18 months old that die or are euthanised on farm, which is needed before the country can be declared free of the disease. A voluntary flock certification programme also exists.
Is it a risk to people?
WOAH states that there is no evidence of a causal link between classical or atypical scrapie and human prion diseases. Scrapie has been recognised in sheep for well over two hundred years without a human disease being traced to it.
For context on the human side, the 2026 review of human prion disease describes about 85 percent of human cases as sporadic CJD, with no known genetic or environmental trigger, and acquired forms, meaning those traced to an outside source, as exceedingly rare.
Research on species barriers continues, as it does for every animal prion disease, and that work is covered in scrapie in the research section.
Related pages
BSE, chronic wasting disease, the animal prion diseases hub and safety.