Scrapie
Scrapie is a fatal neurodegenerative disease that affects sheep and goats, characterized by significant central nervous system degeneration. This disease is part of a broader class of illnesses known as transmissible spongiform encephalopathies (TSEs), which also includes mad cow disease and Creutzfeldt-Jakob disease. Symptoms typically emerge between two to five years post-infection, beginning with behavioral changes, weight loss, and coordination issues, eventually leading to severe neurological damage. The disease is transmissible only among ruminant species, primarily through bodily fluids during birthing, and contaminated environments. Diagnosis can be challenging due to the long incubation period and initial symptomlessness. Control measures include quarantining infected animals and eradicating them to prevent further spread. Monitoring and testing programs have significantly reduced the incidence of scrapie in the United States, with federal agencies implementing tight regulations to manage outbreaks. Despite advancements, scrapie remains a concern for sheep producers, resulting in economic losses primarily due to trade restrictions related to the disease.
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Scrapie
Scrapie (pronounced SKRAY-pee) is a fatal disease that causes the degeneration of the central nervous system in sheep and goats. Although it is transmissible among different animals of the same species, no evidence exists to suggest that it can be passed to humans. As animals remain symptom-free for several years, it can be very difficult to diagnose. The only known method of controlling transmission is to quarantine and destroy all infected animals. It is far more common among sheep than goats.
The disease makes infected animals feel an intense itching sensation, which causes them to bite, scratch, or scrape themselves compulsively against any available surface. This scraping behavior, which can lead to bare patches of reddened skin, is one of the most obvious signs of the disease and the origin of the name scrapie. The disease is part of a class of progressive illnesses called transmissible spongiform encephalopathies (TSEs) that infect the brains and nervous systems of their victims. So-called "mad cow disease" (bovine spongiform encephalopathy, or BSE) and Creutzfeldt-Jakob disease are perhaps the best known forms of TSE.
Background
Scrapie was first identified in 1732 in sheep flocks in the United Kingdom. It then spread rapidly across Western Europe. It has subsequently been recognized as the first known form of TSE. Since its initial identification, it has been discovered in flocks around the world. The first identified case of scrapie in the United States in 1947 in a flock of sheep in Michigan that had been imported from Great Britain. Australia and New Zealand are the only countries in the world that are classified as free of the disease.
Although scientists have known about scrapie for more than two hundred years, they were unable to identify the source of transmission until the late twentieth century. In 1962, scientists learned that scrapie could be artificially spread by injecting brain matter from infected sheep into lambs. Over time, the transmission source for TSEs was found to be a type of particle called a prion that is made up almost entirely of proteins. Prions are not fungal, bacterial, or viral in origin. They are not considered living organisms at all as they lack genetic material, a metabolism, and the ability to reproduce outside a host cell. They are the smallest infectious particles known to science. The proteins that form the basis of prions are found throughout the bodies of all living animals. However, the structures of prions are different from normal proteins. Their unusual protein structure enables them to resist enzymes called proteases that typically break down proteins in the body. The initial development of these abnormal prions may occur spontaneously.
Prions are capable of altering the structure of normal proteins, although the exact method of replication is unknown. When they multiply in this manner, the prions build in the central nervous system and attack tissue structures, particularly in the brain and nerve tissues. Ultimately, affected tissues begin to develop holes and assume a characteristic spongy appearance. Victims of prion diseases such as TSEs develop symptoms such as encephalitis, which is a swelling of the brain; ataxia, a condition that causes patients to lose the ability to stand or walk properly; and loss of cognitive functions.
Several strains of prions have been discovered that cause scrapie. Scientists have been able to deliberately infect several species of laboratory animals using scrapie prions, though these methods of infection required bypassing normal barriers of transmission. As a result, it is believed that scrapie is contagious only among such ruminants as sheep, goats, and other species of related animals. This category of ruminants includes wild animals such as mouflon and big-horned sheep.
In sheep, rams are not known to transmit the disease. Instead, a primary method of exposure occurs through the transmission of bodily fluids between ewes and lambs in the afterbirth or milk. Lambs delivered through Caesarean section do not seem to contract the disease if they are not fed milk from their mothers. Contaminated soil or pastures may constitute other methods of transmission.
Overview
Infected animals typically begin to show signs of the disease between two and five years after transmission. The earliest symptoms of scrapie are associated with changes in behavior. Infected sheep demonstrate significant weight loss, loss of coordination, tremors, lip smacking, sensitivity to light, and increased episodes of scratching or biting themselves. Over time, infected animals adopt a hunched appearance, an unusual gait, and, eventually, an inability to stand. Firm identification of scrapie may be made through the testing of living lymphatic tissues or postmortem tests.
Any animals demonstrating symptoms should be quarantined from the flock immediately and watched to see if they develop other signs of the disease. Any cases of infected animals should be reported to federal agencies such as the Animal and Plant Health Inspection Service (APHIS) of the United States Department of Agriculture (USDA). Deceased animals should be carefully destroyed through incineration to prevent further spread of the disease. The flock's quarters must be disinfected with a strong agent shown to be effective in the elimination of abnormal prions. Regular cleaning of the animals' quarters (particularly in the birthing area) with bleach or lye is thought to be an effective measure of prevention. Afterbirth should be disposed of immediately.
Federal agencies in many countries have established eradication programs to eliminate the further spread of the disease. In the United States, the USDA has established a control program that requires sheep and goat producers to test animals while they are alive and again at the point of harvest. When infected animals are identified, the animal's original herd is located and studied. All sheep and goats have tags, which allow officials to more easily determine the origins of animals. Identifying flocks that have remained scrapie-free for extended periods and culling sheep that are more genetically susceptible to scrapie are other forms of control.
Ninety-six percent of known cases in the United States occur in blackface sheep breeds, such as Suffolks and Hampshires. Studies of harvested sheep demonstrated an incidence rate of 0.015 percent in 2013. Between 2003 and 2017, the number of sheep that tested positive for scrapie declined by 90 percent. Despite these advances, scrapie is responsible for losses to US producers of $20–25 million annually, primarily as a result of trade restrictions that have been established to prevent the spread of the disease to other countries.
Bibliography
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