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From the USDA's Animal and Plant Health Inspection Services (APHIS):

Bovine Spongiform Encephalopathy (BSE)

Overview

Bovine spongiform encephalopathy (BSE), widely known as "mad cow disease," is a chronic, degenerative disease affecting the central nervous system of cattle. Worldwide there have been more than 180,000 cases since the disease was first diagnosed in 1986 in Great Britain. BSE has had a substantial impact on the livestock industry in the United Kingdom. The disease has also been confirmed in native-born cattle in Austria, Belgium, Czech Republic, Denmark, Finland, France, Germany, Greece, Ireland, Israel, Italy, Japan, Luxembourg, Liechtenstein, the Netherlands, Northern Ireland, Poland, Portugal, Slovakia, Slovenia, Spain and Switzerland. However, over 95% of all BSE cases have occurred in the United Kingdom. 

BSE belongs to the family of diseases known as the transmissible spongiform encephalopathies (TSE's). These diseases are caused by a transmissible agent which is yet to be fully characterized. They share the following common characteristics

  1. a prolonged incubation period of months or years; 
  2. a progressive debilitating neurological illness which is always fatal;
  3. when examined by electron microscopy, detergent treated extracts of brain tissue from animals or humans affected by these diseases reveal the presence of scrapie associated fibrils (SAF);
  4. pathological changes appear to be confined to the CNS and include vacuolation, and astrocytosis;
  5. the transmissible agent elicits no detectable specific immune response in the host which has inhibited the development of a preclinical live animal diagnostic test to date.

The Causative Agent of BSE

The causative agent of BSE as well as other TSE's is yet to be fully characterized. Three main theories on the nature of the agent have been proposed: 

  • An unconventional virus. 
  • A prion or abnormal partially-proteinase K-resistant protein, devoid of nucleic acid, capable of causing normal prion protein in the host to change and form more abnormal protein. 
  • A virino or "incomplete" virus composed of naked nucleic acid protected by a host protein.

The BSE agent (1) is smaller than most viral particles and is highly resistant to heat, ultraviolet light, ionizing radiation, and common disinfectants that normally inactivate viruses or bacteria; (2) causes no detectable immune or inflammatory response in the host; and (3) has not been observed microscopically.

How BSE Is Currently Diagnosed

There is no test to detect the disease in a live animal. Currently there are two laboratory methods to confirm a diagnosis of BSE: 1. microscopic examination of the brain tissue to identify characteristic changes; 2. techniques to detect the partially-proteinase resistant form of the prion (PrPres) protein. These techniques are immunohistochemistry, immunoblotting and ELISA.

Transmission of BSE

There is no evidence that BSE spreads horizontally, i.e., by contact between unrelated adult cattle or from cattle to other species. Some evidence suggests that maternal transmission may occur at an extremely low level. Results of British research show that there is approximately a 9-percent increase in the occurrence of BSE in offspring of BSE-affected dams as compared to calves born to dams where BSE was not detected. The study did not ascertain if this was the result of genetic factors or true transmission. The research did however point out that at this level if maternal transmission does occur it alone will not sustain the epidemic (Wilesmith et al. 1997). 

A recently published study found no evidence of disease transmission via embryos collected from cows with BSE. The embryos were collected and handled in accordance with international health standards (Wrethall et. al., 2001).

USDA Actions

What is the USDA policy in regard to BSE, and what actions has USDA taken?

The USDA policy has been to be proactive and preventative. APHIS has taken measures in surveillance, prevention, education, and response. Import restrictions have been in place since 1989, and active surveillance efforts began in 1990. The USDA continually monitors and assesses all ongoing events and research findings regarding spongiform encephalopathies, as new information and knowledge may lead to revised conclusions and prevention measures. APHIS has also created a Transmissible Spongiform Encephalopathy (TSE) Working Group to analyze risks of BSE to the United States, disseminate accurate information about the TSE's, and act as a reference source for responding to questions about TSE's. 

Is APHIS working with other agencies and groups to coordinate efforts?

Yes. APHIS has actively shared information and met with State and Federal agencies, including the Centers for Disease Control and Prevention(CDC), the Food and Drug Administration (FDA), the Food Safety and Inspection Service (FSIS), the National Institutes of Health (NIH), and stakeholders to assure we are taking the proper actions in response to changing knowledge and information concerning BSE. 

Is BSE a notifiable disease in the United States?

Yes. Under Title 9 Code of Federal Regulations, Parts 71 and 161, BSE is a reportable disease by accredited veterinarians. 

What types of BSE surveillance are we doing?

USDA-APHIS, in cooperation with USDA-FSIS and State diagnostic laboratories, has a surveillance program which targets the segment of the cattle population where the disease would most likely be found if it were to occur. 

  • APHIS educates veterinary practitioners, veterinary laboratory diagnosticians, industry and producers on the clinical signs and pathology of BSE. 
  • APHIS monitors the remaining cattle imported from countries known to have BSE or have high risk factors for BSE. 
  • Since 1990, more than 60 veterinary diagnostic laboratories across the United States and USDA's National Veterinary Services Laboratories continue to examine hundreds of cattle brains each year submitted from adult cattle displaying neurologic signs either at slaughter or on the farm. FSIS performs antemortem slaughter inspection at all federally-inspected slaughter establishments, and inspectors are alert for central nervous system (CNS) disorders. Any CNS suspect animals are condemned and tested. Public health laboratories also submit to APHIS any samples that have tested negative for rabies. 
  • The network of private veterinary practitioners that refers unusual cases to veterinary schools or State diagnostic laboratories around the United States provides an extensive informal but important surveillance system. 
  • USDA has trained more than 250 State and Federal field veterinarians located throughout the United States in the recognition and diagnosis of foreign animal diseases, including BSE. 
  • Veterinary pathologists at zoos in the United States routinely conduct postmortem examinations on the brains of zoo animals exhibiting neurologic signs since BSE-like encephalopathies have been diagnosed in seven species of exotic Bovidae at zoos in England.

What type of adult cattle do we test?

  1. Neurologically ill cattle found on farm
  2. Neurologically ill cattle presented at veterinary diagnostic labs or hospitals
  3. Rabies-negative cattle
  4. Cattle condemned at slaughter for neurologic disease
  5. Non-ambulatory (down/fallen stock)
  6. Adult cattle dying on farms from an unknown cause

Does the United States still permit the feeding of ruminant protein to ruminants?

On August 4, 1997, the Food and Drug Administration (FDA) established regulations that prohibit the feeding of most mammalian proteins to ruminants. 

What proactive initiatives are underway to educate farmers, veterinarians, extension agents, etc.?

An important part of the USDA's active surveillance program is the training of veterinary practitioners in the clinical signs, diagnosis and sample submission for BSE. Videotapes of cattle showing clinical signs of BSE have been distributed to veterinarians in Federal and State governments, veterinary diagnostic laboratories, and pathology departments of veterinary colleges. Microscope slides showing typical BSE lesions have been distributed to the above diagnostic laboratories, and Federal Foreign Animal Disease (FAD) diagnosticians have trained in Great Britain in BSE recognition. BSE factsheets, risk assessments, and reviews have also been sent to State and Federal veterinarians, private practitioners, other industries, and to producers. In addition, APHIS personnel have given numerous presentations to various animal health groups. Finally, over 250 Federal and State veterinarians throughout the US have been trained in the recognition of FAD's including BSE. 

What actions are taken at USDA-inspected slaughter establishments to ensure that cattle with neurological disease would not enter the human food supply?

All cattle presented for slaughter in the United States are inspected before slaughter by FSIS for signs of CNS impairment. Any animals exhibiting neurologic signs during this inspection are condemned, and the meat is not permitted for use as human food. The brains from these animals are submitted to USDA's National Veterinary Services Laboratories for analysis. 

Does USDA have a response plan in the event a case of BSE or TSE is diagnosed in US cattle?

In 1990, APHIS developed a plan to respond to a confirmation of BSE in the US In August 1996, a joint APHIS-FSIS working group updated this BSE response plan. The purpose of the plan is to provide a step-by-step plan of action in the event that a case of BSE is detected in the United States. The plan outlines those events that should take place, including identification of a suspect animal, confirmation, the epidemiologic investigation, animal and herd disposition activities, and communication of information.  The plan has been shared with other government agencies that have developed their own plans to coordinate with those of USDA.

BSE Response Plan Summary