Bovine Spongiform Encephalopathy, or Mad Cow Disease

Filed Under: Cows, Diseases

Are you a member of the meat-eating public? Then you might want to familiarize yourself with “Mad Cow Disease,” or Bovine Spongiform Encephalopathy (BSE). First described in 1986 in Great Britain, this disease is a fatal, slow-onset encephalopathy, or disease of the brain, in cattle. “Why should I be concerned?” you might ask. Mad cow disease is the only known form of a transmissible spongiform encephalopathy to be transmitted from animals to humans, which makes it a lethal zoonotic disease.

Bovine spongiform encephalopathy is one of a group of diseases that results in vacuolization (holes in the brain matter) as well as characteristic protein deposits in the brain and central nervous system, or CNS. The vacuolization is most noticeable in the medulla oblongata of the brain. The causative agent of the disease is a prion protein. A prion protein is able to replicate without the need for nucleic acid. Any 9th grade biology class will teach you that RNA or DNA is the necessary roadmap for the replication of life. For the prion, this facilitation is not necessary. The mere ingestion of tissue containing these prions will transmit the disease to a new host. These prions are nearly indestructible. Cooking meat to a crisp will not inactivate the prion.

Normal prion proteins do exist and are a normal component of host cell membranes of vertebrates, especially in nerve tissue. Disease-causing prions and normal prion proteins have identical amino acid sequences and will differ only in their folding pattern or conformation and are therefore called isoforms. These disease-causing isoforms accumulate as prion deposits in the brains of infected animals, leading to the characteristic clinical signs and eventual death of the animal.

Mad cow disease has a long incubation period that will often exceed the typical age at which cattle are slaughtered. The average age of onset is 5 years, but the disease may occur at any age. The incidence of the disease is therefore suspected to be far higher than the number of cattle exhibiting clinical signs. Since 1980, it has been estimated that at least 2 million head of cattle have developed bovine spongiform encephalopathy in the United Kingdom alone. The disease has also been seen in at least 25 additional countries including countries in Asia and North America.

Bovine spongiform encephalopathy is thought to have originated as a man-created problem. Transmission occurs through the feeding of sheep or cattle meat and bone meal, called sheep offal, to cows. There is also some maternal-associated risk for calves born to affected cows. Dr. Juergen Richt of Kansas State University’s College of Veterinary Medicine reports that they have found a genetic predisposition to mad cow disease. Disease caused by this genetic mutation is believed to be a rare occurrence. This finding is significant, however, since the disease could potentially be seen everywhere in the world, regardless of a so-called BSE-free status. This genetic mutation occurs within the gene responsible for the production of prion proteins. Then these prions are abundantly expressed in the brain and immune systems of affected cattle.

Clinical signs of bovine spongiform encephalopathy are typically vague and nonspecific. The first signs are often a decrease in milk yield and a loss of body weight. Common neurologic signs include apprehension, pelvic limb ataxia, and hyperesthesia (increased sensitivity) to sound, sight, or touch. A low heart rate (bradycardia) and reduced rumination are often observed in affected animals indicating a disturbance of the autonomic nervous system. A common sign seen shortly before death is the downer cow, or an animal that is physically unable to get up.

The diagnosis of bovine spongiform encephalopathy may only be confirmed by the analyses of brain tissue (histopath) collected after death. Evaluation of cerebral spinal fluid or CSF will not reveal any abnormalities and will not exhibit an inflammatory response. The recent discovery of a genetic mutation involved in mad cow disease will soon allow for the possibility of genetic screening prior to the breeding of affected cattle. In early 2006, a group of scientists from Texas A&M University College of Veterinary Medicine engineered a prion-deficient goat fetus -- perhaps prion-deficient cattle are next.

References:

Harman, Jane. “Bovine Spongiform Encephalopathy.” JAVMA, Vol. 234, No. 1, January 1, 2009. Pp. 59-72.

Nolen, Scott. “Beef Recall, Sparks Hearing, Tightened Inspections.” JAVMA, Vol. No. 10/NEWS. PP./ 1112-1115.

“Researchers Create, Evaluate Prion-Free Cattle.” JAVMA, Vol. 230, No. 4. Feb. 15, 2007. P. 483.

“Researchers Find Genetic Mutation That can Cause BSE.”  DVM. October 2008. Pp. 17.

Topics: mad cow disease

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