Scrapie is a fatal brain disease that affects sheep and goats. It belongs to a family of diseases called transmissible spongiform encephalopathies, the same category that includes mad cow disease in cattle and chronic wasting disease in deer. Scrapie has been recognized for over 250 years, making it the oldest known disease in this family, and it remains a concern for livestock producers worldwide.
How Scrapie Damages the Brain
Every healthy sheep or goat naturally produces a normal protein on the surface of its brain cells. In scrapie, this protein gets physically reshaped into an abnormal, misfolded version. The misfolded protein then acts like a template, forcing neighboring normal proteins to refold into the same distorted shape. This chain reaction is sometimes described as “autocatalytic” because it feeds itself, spreading without any virus or bacteria involved.
As more misfolded proteins accumulate, they clump together into tough, fibrous deposits called amyloid fibrils throughout the central nervous system. These deposits are insoluble and resist the body’s normal cleanup enzymes, so they simply build up over time. The result is progressive nerve cell death that riddles the brain with tiny holes, giving it a sponge-like appearance under a microscope. Once this process starts, there is no way to stop or reverse it.
Signs and Symptoms
After infection, an animal can appear perfectly healthy for 2 to 5 years, sometimes even longer, before any outward signs emerge. When symptoms do appear, they generally fall into two overlapping patterns: a “scratching syndrome” and a “nervous syndrome.”
The scratching syndrome is where scrapie gets its name. Affected animals develop intense, unrelenting itchiness. They rub against fences, posts, and any fixed object they can find. They nibble at their own wool or hair and scratch themselves with their hooves or horns. A classic diagnostic clue is the “scratch test”: when a veterinarian scratches along the animal’s back, it responds with rhythmic lip movements or head bobbing, sometimes called the nibble reflex. Over time, the constant rubbing leads to patches of lost wool or hair and visible skin damage.
The nervous syndrome involves progressive loss of coordination. Animals develop an unsteady, wide-based gait, particularly in the hind legs. Tremors affecting the whole body or limbs become noticeable, and some animals start high-stepping with their front legs. Postural abnormalities are common: standing with a lowered head, an extended neck, or hind legs pushed far back. Animals also become hypersensitive to sounds, touch, and movement around them, startling easily. Vision deteriorates, and many affected animals lose their normal blink reflex when something approaches their eyes. Behavioral changes during routine handling, such as nervousness, trembling, or kicking during milking, can be early warning signs. Once clinical symptoms are clearly established, the disease progresses to death within weeks to months.
How Scrapie Spreads
Classical scrapie is highly contagious, and the most significant transmission event happens at birth. Misfolded prion proteins accumulate heavily in the placenta, so lambs and kids are exposed to concentrated infectious material the moment they enter the world. Other animals in the flock that come into contact with birthing fluids or the placenta are also at risk. This makes lambing season the peak period for new infections spreading through a herd.
The environment itself becomes a long-term source of infection. Prions shed during birth, through saliva, or in other body fluids bind tightly to soil particles and remain infectious for years. Research has confirmed that prion infectivity persists in soil for at least 29 months, and likely much longer. There are currently no effective methods to decontaminate naturally contaminated pastures or barns. Composting infected tissue for several months can reduce detectable prion levels, but standard disinfection doesn’t reliably eliminate infectivity. This environmental persistence means a flock can become reinfected from contaminated ground even after sick animals are removed.
A separate form of the disease, called atypical scrapie, appears to arise spontaneously in individual animals without the same pattern of contagious spread. Unlike classical scrapie, the atypical form does not accumulate prions in peripheral tissues like the placenta, which helps explain why it doesn’t seem to transmit naturally between animals.
Genetics and Susceptibility
Not all sheep are equally vulnerable to scrapie. Susceptibility is strongly influenced by the animal’s genetic makeup, specifically by variations at three positions in the gene that codes for the prion protein: codons 136, 154, and 171. Of these, codon 171 is the most important. Sheep carrying certain variants at this position are highly resistant to classical scrapie, while others are far more susceptible. Codon 136 also plays a role, though its influence can shift depending on how heavily a flock is infected.
This genetic link has practical consequences. Breeding programs that select for resistant genetic profiles have become one of the most effective tools for reducing scrapie in flocks. By genotyping rams and choosing those with resistant variants, producers can shift the genetic makeup of their flock toward greater resistance over several generations.
Diagnosis
Diagnosing scrapie in a living animal is difficult. The long incubation period means infected animals look healthy for years, and early clinical signs like mild tremors or subtle behavioral changes are easy to miss. Clinical screening that checks for tremor, scratch test response, hair loss, coordination problems, and vision loss can identify advanced cases, but this approach has very low sensitivity for catching animals in early stages of infection.
Definitive diagnosis typically requires examining brain tissue after death, looking for the characteristic spongy damage and the presence of misfolded prion proteins. In some cases, biopsy of lymphoid tissue in living animals can detect prion accumulation before clinical signs appear, but this is mainly used in surveillance programs rather than routine veterinary care.
Risk to Humans
Despite centuries of scrapie existing in sheep populations around the world, and widespread human exposure to potentially infected animals and meat, no credible evidence has ever linked scrapie to any human prion disease. Laboratory studies from the CDC’s Emerging Infectious Diseases journal tested whether scrapie prions could convert normal human prion protein into the disease-causing form. Classical scrapie failed to convert human prion protein to any measurable extent. By contrast, the mad cow disease prion, which is the only animal prion known to cause disease in humans, efficiently converted human prion protein in the same test. Atypical scrapie also showed no ability to convert human prion protein. The molecular barrier between sheep prions and human prion protein appears to be robust.
Why Eradication Is So Difficult
Scrapie has proven remarkably stubborn to eliminate. The combination of a years-long silent incubation period, transmission at birth before anyone knows an animal is infected, and prions that persist in soil for years creates a cycle that’s hard to break. Infected animals can spread the disease through an entire flock before a single case is identified. Even after removing all animals, contaminated pastures remain a risk because no practical decontamination method exists for outdoor environments.
The USDA has maintained a National Scrapie Eradication Program that requires identification and traceability of sheep and goats. These efforts, combined with genetic selection for resistant animals and removal of infected flocks, have significantly reduced scrapie cases in the United States. But the biological properties of prions, particularly their environmental durability and resistance to standard disinfection, mean that complete eradication remains an ongoing challenge.