Both “mad cow disease” (bovine spongiform encephalopathy, or BSE) and scrapie are forms of transmissible spongiform encephalopathy (TSE). Bovine spongiform encephalopathy (BSE), commonly known as mad-cow disease, is a fatal, neurodegenerative disease in cattle, that causes a spongy degeneration in the brain and spinal cord. BSE usually has a long incubation period, about four years, usually affecting adult cattle at a peak age onset of four to five years, all breeds being equally susceptible. In the United Kingdom, the country worst affected, more than 179,000 cattle have been infected and 4.4 million slaughtered during the eradication program.
The brain, central nervous system tissue, and the distal ileum (portion of the small intestine) can carry the infective agent, and measures have been taken to exclude those parts known to carry the infective agent (primarily brain and central nervous system tissue) from the food and feed supply. The disease is most easily transmitted to human beings by consuming food that is contaminated with the brain or spinal cord of infected carcasses. However, the infectious agent, although most highly concentrated in nervous tissue, can be found in virtually all tissues throughout the body, including blood. In humans, it is known as new variant Creutzfeldt–Jakob disease (vCJD or nvCJD), and by October 2009, it had killed 166 people in Britain (the most recent being of a different genotype than other sufferers, and 44 elsewhere with the number expected to rise because of the disease’s long incubation period). Between 460,000 and 482,000 BSE-infected animals had entered the human food chain before controls on high-risk offal were introduced in 1989.
On the other hand, scrapie is a fatal, degenerative disease that affects the nervous system of sheep and goats. It is one of several transmissible spongiform encephalopathies (TSEs), which are related to bovine spongiform encephalopathy (BSE, or “mad cow disease”) and chronic wasting disease of deer. Like other spongiform encephalopathies, scrapie is caused by a prion. Scrapie has been known since the 18th century (1732) and does not appear to be transmissible to humans. The name scrapie is derived from one of the clinical signs of the condition, wherein affected animals will compulsively scrape off their fleece against rocks, trees, or fences. The disease apparently causes an itching sensation in the animals. Other clinical signs include excessive lip-smacking, altered gaits, and convulsive collapse.
Scrapie is infectious and transmissible among similar animals, and so one of the most common ways to contain scrapie (since it is incurable) is to quarantine and destroy those affected. However, scrapie tends to persist in flocks and can also arise apparently spontaneously in flocks that have not previously had cases of the disease. The mechanism of transmission between animals and other aspects of the biology of the disease are only poorly understood, and these are active areas of research. Recent studies suggest that prions may be spread through urine and persist in the environment for decades.
The common characteristics between BSE and scrapie are that they both have long incubation periods, they have the presence of scrapie-associated fibrils in the brain, and the ability to transmit the disease to laboratory animals by an injection into the brain or brain tissue from a diseased animal.