Scrapie is a fatal, progressive neurodegenerative disease that primarily affects the central nervous systems of sheep and goats. It is one of the earliest recognized transmissible spongiform encephalopathies (TSEs). The name “scrapie” comes from a prominent symptom where affected animals compulsively rub their bodies against objects.
The Nature of Scrapie
Scrapie is caused by an abnormal form of a protein known as a prion. Prions are misfolded versions of a normal protein, called cellular prion protein (PrPᶜ), which is naturally found in the cell membranes of neurons. When these misfolded prions (PrPˢᶜ) come into contact with normal PrPᶜ, they induce the normal proteins to also misfold into the abnormal, disease-causing shape. This accumulation of misfolded prions in the brain leads to the death of nerve cells, causing a characteristic “spongy” appearance in brain tissue, which is why these conditions are also called transmissible spongiform encephalopathies (TSEs).
Recognizing Scrapie’s Symptoms
Clinical signs of scrapie are progressive, typically appearing two to five years after an animal is infected. Early symptoms can be subtle behavioral changes, such as increased apprehension, nervousness, or a tendency to lag behind the flock. As the disease advances, more pronounced neurological signs develop.
Animals may exhibit intense itching, leading them to rub or scrape their bodies against fences and other objects, often resulting in significant wool or hair loss and skin lesions. Other common signs include tremors, particularly of the head, and coordination problems like stumbling, a high-stepping gait, or general ataxia. Affected sheep might show a characteristic nibbling reflex when scratched over the rump. Despite maintaining a normal appetite, animals often experience severe weight loss. Affected animals eventually become unable to stand and die, usually within one to six months of symptom onset.
How Scrapie Transmits
Classical scrapie spreads through oral exposure to contaminated environments. The most common route of natural transmission is from an infected ewe to her lamb. High concentrations of the scrapie agent are shed in placental tissues and birth fluids, which lambs can ingest.
Environmental contamination is a significant factor, as prions shed in feces, saliva, and urine can persist in the environment for years. Direct animal-to-animal contact is not the primary mode of spread; however, susceptible adult animals can become infected through repeated exposure to a contaminated environment. In contrast, atypical scrapie occurs spontaneously and is not naturally transmissible between animals.
Detecting and Controlling Scrapie
Diagnosing scrapie definitively requires a post-mortem examination of brain tissue. Veterinarians look for the accumulation of abnormal prion protein (PrPˢᶜ) in the central nervous system. Specialized laboratory tests such as enzyme-linked immunosorbent assay (ELISA) and immunohistochemistry are used to detect PrPˢᶜ in these tissues.
Control and prevention strategies focus on managing affected flocks and reducing transmission. These include:
Culling infected animals and their offspring to remove the source of infection.
Genetic selection, as certain genotypes of the prion protein gene (PRNP) are associated with resistance to classical scrapie, while others increase susceptibility. For instance, the ARR/ARR genotype in sheep is highly resistant, whereas VRQ/VRQ is highly susceptible.
Regulatory measures in many countries, such as animal identification programs, movement restrictions for at-risk animals, and surveillance programs.
Biosecurity measures, including ensuring clean lambing environments and avoiding contaminated feed.
Scrapie and Human Health
A common concern is whether scrapie can be transmitted to humans. Unlike Bovine Spongiform Encephalopathy (BSE), there is no scientific evidence to suggest that scrapie prions can naturally infect humans. Extensive research, including studies on humanized transgenic mice, has indicated a low potential for scrapie to cross the species barrier to humans.
While some laboratory experiments have shown that certain classical scrapie isolates can propagate in humanized mice, the resulting prions were confirmed to be different strains from those causing human Creutzfeldt-Jakob disease (CJD). This research suggests a rare phenotypic convergence but does not provide evidence of natural transmission under field conditions. Therefore, consuming lamb or goat products is considered safe, as no consistent link has been established between scrapie and CJD or any other human prion disease.