Bovine Spongiform Encephalopathy

Introduction:  Bovine Spongiform Encephalopathy (BSE), commonly known as Mad Cow Disease, has become one of the most important issues facing the U.S. cattle industry.  After BSE was found in a Canadian herd, Canada's beef industry lost 5,000 jobs and $11 million per day.  From May-August, 2003, there was a total ban on Canada's exported cattle market.  Subsequently, a single case of BSE was found in the United States.  It was found that the cow had been imported from Canada and did not represent a true "native" infection.  Nevertheless, U.S. trading partners are now demanding proof that we have no other cases of BSE and U.S. beef exports remain at risk.

  BSE is a progressively fatal disease affecting the central nervous system of adult cattle (20 months+).  The exact origin of BSE remains unknown, but it is believed that the cattle were originally infected by eating feed containing sheep meat-and-bone meal that was contaminated with scrapie, a spongiform encephalopathy affecting sheep and goats.  Once BSE became established in cattle, it spread from cow to cow by the feeding of infected cattle origin meat-and-bone meal to other cows and calves.  The first case of BSE was reported in 1986 in the United Kingdom.  Since then, there have been BSE cases reported in 20 different European countries, Japan, Israel, Canada and the United States.  BSE has become an important disease to humans because infection can spread from cattle to humans via ingestion.  This can cause variant Creutzfeldt-Jakob disease (vCJD), which is the human form of BSE.  Although BSE can spread from cattle to humans, the chance of acquiring the disease is low.  The species barrier provides substantial, yet still incomplete, protection against the infection in humans.  However, BSE is potentially fatal to humans and is therefore a major issue in human public health and the cattle industry.

Pathogenesis: The pathogenesis of this disease is not completely known, though there is an emerging concensus among scientists.  The most widely accepted theory suggests that the agent is a transmissible "prion", an abnormal variant of a normal protein.  The new prion is a protein abnormally folded to form a different shape.  When transmissible  prions are ingested through infected food, they are able to convert the body's normal proteins into more abnormal transmissible prions.  The transmissible prions accumulate in the intestines (duodenum to rectum), tonsils, and central nervous tissues such as brain and spinal cord.  As these defective proteins accumulate in the brain, they interfere with normal brain functions, resulting in the neurological symptoms typical of the disease.  Affected animals eventually die of starvation or accidental trauma caused by nervous dysfunction.  Although this prion theory is most commonly accepted, a few researchers still think BSE is caused by a virus-like organism, though no viruses have ever been linked to BSE.

Clinical signs: There are no clinical signs in the beginning phases of this disease.  The incubation period can last for 20 months to 15 years.  Once it becomes clinical, BSE is distinguished by a variety of neurological clinical signs; the most commonly reported is change in behavior or temperament, usually pertaining to increased nervousness or apprehension.  Hypersensitivity to sound and touch are frequently observed, along with head shyness, kicking, excessive ear movement, and nose licking.  Weight loss is also common, even with a continuation in normal eating patterns.  In late stages of this disease, cattle may develop incoordination progressing to paresis or paralysis, recumbency, and death.

Gross lesions:  There are no gross lesions associated with BSE.  Abrasions, lacerations, and contusions may result secondarily from injuries due to incoordination.

Diagnosis: There is no diagnostic test to confirm BSE in a live animal.  Animals suspected for BSE should be euthanized and submitted for postmortem examination.  Histologic examination of the brain stem is the widely accepted way to confirm the presence of BSE, the typical lesion being vacuoles (spongy holes) in the cytoplasm of neurons.  Autolyzed brain tissue can be extracted and examined for scrapie-associated fibrils by electron microscopy.  Recently, immunohistochemistry  has been used for widespread testing of U.S. cattle.  These are enzyme-based tests which utilize antibodies specific for prion proteins.  ELISA testing is also becoming available.

Treatment: Treatment is ineffective.  Infection is best controlled by prevention.

Prevention and control:  Prevention and control of BSE in cattle is difficult since BSE is not a traditional infectious agent (bacteria or virus); therefore, vaccination is not effective.  Because BSE is spread by ingestion, the key to control is eliminating infected material from feed.  Sheep and goat meat-and-bone meal is prohibited in all animal feeds and bovine meat-and-bone meal is prohibited in all ruminant feed.  Also, no cattle may be imported from countries with a history of BSE, although Canada may be the first exception.   Because the BSE agent is a protein and not a living organism, cooking with heat and other traditional food disinfection methods (heat, freeze, UV light, chemical disinfectants, etc) are ineffective.  The key to prevention of BSE in humans (vCJD) is to eliminate infected material from the human food supply.  To accomplish this, the USDA has put stipulations on the U.S. food supply.  No sick or "downer" cows are allowed for human consumption.  Specific risk materials (head, intestines, central nervous tissues, and spinal cord) from all cows and mechanically separated beef are prohibited in the human food supply.  Finally, the USDA has begun comprehensive surveillance of the U.S. bovine population to ensure there is no significant threat of BSE within our domestic beef supply.

-by Andy Reynolds, Center College  Student, Heeke ADDL Intern

-edited by Dr. Duane Murphy,  ADDL Pathologist

Normal prion	Abnormal prion

References:

1. Bovine Spongiform Encephalopathy: 2003.  The Merck Veterinary Manual.

2. Bovine Spongiform Encephalopathy (BSE) and varian Creutzfeldt-Jakob disease: 2005.  The European Food InformationCouncil.  www.eufic.org/gb/safe/safe05.htm#p2

3. Bovine Spongiform Encephalopathy in a Dairy Cow - Washington State, 2003: 2005. MMWR Weekly. Jan 9, 2004  Centers for Disease Control and Prevention. Jan 12, 2005.

4. Bovine Spongiform Encephalopathy: 2002. World Health Organization. www.who.int/mediacentre/factsheets/fs113/en/

5. Commonly Asked Questions about BSE in Products Regulated by FDA's Center for Food Safety and Applied Nutrition (CFSAN). US Food and Drug Administration. www.cfsan.fda.gov/~comm/bsefaq.html

6. Hemming DR: 2005.  Food Safety and Food Animals. South Dakota Public Broadcasting. May 17, 2001.  South Dakota State University

7. Kimberlin RH: 1993.  Bovine Spongiform Encephalopathy. www.rlc.fao.org/prior/ segalim/animal/eeb/pdf/bse.pdf

8. Sheen B: 2005.  Mad Cow Disease.  Farmington Hills: Thompson Gale

9. Timeline of BSE in Canada and the U.S.: 2005.  CBS News. www.dbc.ca/news/ background/madcow/timeline.html