Introduction

Bovine spongiform encephalopathy (BSE), widely

referred to as “mad cow disease,” is a chronic degenerative disease affecting the central nervous system 

of cattle. The disease was first diagnosed in 1986 in

Great Britain.

BSE has substantially damaged the livestock industry

in the United Kingdom. The disease has also been

confirmed in native-born cattle in Belgium, the Czech

Republic, Denmark, France, Germany, Greece, Italy,

Ireland, Liechtenstein, Luxembourg, the Netherlands,

Northern Ireland, Portugal, Spain, and Switzerland.

BSE has not been diagnosed in the United States, 

and the U.S. Department of Agriculture’s (USDA) Animal

and Plant Health Inspection Service (APHIS) is enforcing

import restrictions and conducting surveillance for BSE 

to minimize the risk of this disease becoming established

here. In addition, the Food and Drug Administration (FDA)

has prohibited the use of most mammalian protein in

ruminant feed.

Clinical Signs

Cattle affected by BSE experience progressive

degeneration of the nervous system. Affected animals

may display nervousness or aggression, abnormal

posture, difficulty in coordination and rising, decreased

milk production, or loss of body weight despite continued appetite. Affected cattle will die. There is neither 

any treatment nor a vaccine to prevent the disease.

The incubation period (the time from when an animal

becomes infected until it first shows disease signs) is

from 2 to 8 years. Following the onset of clinical signs,

the animal’s condition deteriorates until it either dies or

is destroyed. This process usually takes from 2 weeks 

to 6 months. Most cases in Great Britain occurred in

dairy cows between 3 and 6 years of age.

Currently, there is no test to detect the disease in 

a live animal; veterinary pathologists confirm BSE by

postmortem microscopic examination of brain tissue 

or by the detection of the abnormal form of the prion

protein. BSE is so named because of the spongy

appearance of the brain tissue of infected cattle 

when sections are examined under a microscope.

Rico have been examined with no evidence of BSE

or any other transmissible spongiform encephalopathy (TSE) detected.

APHIS monitors the remaining cattle imported

from Great Britain and other European countries

before the bans on imports from those countries

went into effect. As of July 1, 2001, of the 496 

cattle imported from the United Kingdom and

Ireland between 1981 and 1989, 3 animals are 

still alive. The animals are quarantined and

observed regularly. To date, no evidence of BSE 

or a TSE has been detected. APHIS continues to

attempt to purchase those three live animals for

diagnostic research purposes. The six European

cattle imported in 1996 and 1997 that are still alive

are currently under quarantine, and APHIS is

attempting to buy these animals as well.

All cattle presented for slaughter in the United

States are inspected before slaughter by FSIS 

for signs of central nervous system disorders.

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 APHIS’ National

Veterinary Services Laboratories for analysis.

Cattle affected by BSE experience progressive degeneration of 

the nervous system. Changes in temperament (e.g., nervousness 

or aggression), abnormal posture, incoordination and difficulty in

rising, decreased milk production, and/or loss of weight despite

continued appetite are followed by death.

APHIS supports the FDA regulation (effective August 4, 1997)

prohibiting the use of most mammalian protein in the manufacture 

of animal feeds given to ruminants. In addition, the final regulation 

also requires process and control systems to ensure that ruminant

feed does not contain the prohibited mammalian tissue.

History

Between November 1986 and July 2001, more than

178,000 head of cattle in over 35,000 herds were diagnosed with BSE in Great Britain. The epidemic peaked in

January 1993 at approximately 1,000 new cases reported

per week. Agricultural officials in Great Britain have taken

a series of actions to eliminate BSE, including making it a

reportable disease, prohibiting the inclusion of mammalian

meat-and-bone meal in feed for all food-producing animals, prohibiting the inclusion of animals more than 30

months of age in the animal and human food chains, and

destroying all animals showing signs of BSE and other

potentially exposed animals at high risk of developing the

disease. As a result of these actions, most notably the

imposition of feed bans, the rate of newly reported cases

of BSE has decreased and continues a downward trend.

USDA Actions in Response to BSE

Implementation of Stringent Measures

BSE has not been diagnosed in the United States. The

USDA policy has been to be proactive and preventive.

In cooperation with USDA’s Food Safety and Inspection

Service (FSIS), APHIS has implemented stringent measures in prevention, education, surveillance, and response.

Entry Ban—To prevent BSE from entering the country,

since 1989 APHIS has prohibited the importation of live

ruminants from countries where BSE is known to exist in

native cattle. Other products derived from ruminants, such

as fetal bovine serum, bonemeal, meat-and-bone meal,

bloodmeal, offal, fats, and glands, are also prohibited from

entry except under special conditions or under USDA

permit for scientific or research purposes.

On December 12, 1997, APHIS extended these

restrictions to include all of the countries in Europe 

owing to concerns about widespread risk factors for BSE.

As of December 7, 2000, USDA prohibited all imports of

rendered animal protein products, regardless of species,

from Europe. This decision followed the recent determination by the European Union that rendered products of nonruminant origin were potentially cross-contaminated with

the BSE agent. The restriction applies to products originating, rendered, or processed in Europe or otherwise associated with European-rendered animal protein products.

APHIS leads an ongoing, comprehensive interagency

surveillance program for BSE in the United States to ensure

the health of America’s cattle herd.

APHIS photo library

APHIS photo library

APHIS photographer Dr. Art Davis, a veterinarian at the National Veterinary Services Laboratories (NVSL) in Ames, IA.

USDA has taken this emergency action to prevent potentially cross-contaminated products from entering the United States. The same type of rendered product from

ruminant origin has been prohibited from BSE-infected

countries since 1989.

Education, Training, Outreach—APHIS educates

veterinary practitioners, veterinary laboratory diagnosticians, industry, and producers about the clinical signs 

and pathology of BSE. Videotapes of cattle showing

clinical signs of BSE and BSE factsheets, risk assessments, and reviews have been widely distributed to State

and Federal veterinarians, private practitioners, other

industries, and producers. Microscope slides showing

typical BSE lesions have been distributed to Federal and

State diagnostic laboratories, and Federal foreign animal

disease (FAD) diagnosticians have been trained in Great

Britain in BSE recognition. More than 250 Federal and

State veterinarians throughout the United States have

been trained to recognize FADs, including BSE.

APHIS veterinary pathologists and field investigators 

have received training that has included instruction from

their British counterparts in diagnosing BSE. APHIS is continuing an education effort to inform U.S. cattle producers

and veterinarians about this disease. Numerous briefings

have been held for industry groups. In addition to press

releases and factsheets, a videotape on BSE and an information packet were distributed to all APHIS field offices,

State veterinarians, extension veterinarians, colleges of

veterinary medicine, and industry groups.

Surveillance and Monitoring—APHIS leads an ongoing,

comprehensive, interagency surveillance program for BSE

in the United States. Samples of BSE are obtained from

five sources: field cases of cattle exhibiting signs of neurological disease, cattle condemned at slaughter for neurological reasons, rabies-negative cattle submitted to public

health laboratories, neurological cases submitted to veterinary diagnostic laboratories and teaching hospitals, and

aged cattle that are nonambulatory (downer cattle and

fallen stock).

APHIS’ surveillance program is based on laboratories’

histopathologically examining brains and using a technique to test brain tissues for the presence of the abnormal prion protein. As of mid-August 2001, more than

14,600 bovine brains from the United States and Puerto
Bovine Spongiform

Encephalopathy:

An Overview

The U.S. Department of Agriculture (USDA) prohibits discrimination in 

all its programs and activities on the basis of race, color, national origin,

sex, religion, age, disability, political beliefs, sexual orientation, or marital

or family status. (Not all prohibited bases apply to all programs.) Persons

with disabilities who require alternative means for communication of

program information (Braille, large print, audiotape, etc.) should contact

USDA’s TARGET Center at (202) 720–2600 (voice and TDD).

To file a complaint of discrimination, write USDA, Director, Office of 

Civil Rights, Room 326–W, Whitten Building, 1400 Independence

Avenue, SW, Washington, DC 20250–9410 or call (202) 720–5964

(voice and TDD). USDA is an equal opportunity provider and employer.

Issued October 2001

United States Department of Agriculture

Animal and Plant Health Inspection Service

Program Aid No. 1705

Cooperation With Other Agencies

In cooperation with FSIS, APHIS has also drafted

an emergency response plan to be used if BSE is

identified in the United States. The plan specifies a

step-by-step series of actions to be taken if BSE is

detected here. In addition, APHIS' TSE Working

Group monitors and assesses all ongoing events

and research findings regarding TSEs. APHIS continually revises and adjusts prevention and diagnostic measures as it receives new information and

knowledge.

APHIS supports the FDA’s regulation (effective

August 4, 1997) prohibiting the use of most mammalian protein (with certain exceptions) in the manufacture of animal feeds given to ruminants. In

addition, the final regulation also requires process

and control systems to ensure that ruminant feed

does not contain the prohibited mammalian tissue,

thus preventing the possibility of the transmission of

BSE to cattle.

Origins of the Disease

On the basis of epidemiological data, researchers

believe that the source of BSE in Great Britain was feed

containing meat-and-bone meal. There are different scientific hypotheses concerning the origins of BSE. BSE in

Great Britain may have been caused by feeding cattle rendered protein produced from the carcasses of scrapieinfected sheep, or cattle with a previously unidentified

TSE. The practice of using products such as meat-andbone meal as a source of protein in cattle rations has

been common for several decades. Changes in rendering

operations in the late 1970s and early 1980s may have

played a part in the appearance of the disease.

There is no evidence that BSE spreads horizontally, that

is, by contact between unrelated adult cattle or from cattle

to other species. Limited research suggests that maternal

or vertical transmission may occur at a very low level. This

low level most likely would not perpetuate the epidemic

under British farming conditions. Research on this issue

continues.

BSE is classified as a TSE. The agent responsible for

BSE and other TSEs is smaller than the smallest known

virus and has not been completely characterized.

There are three main theories on the nature of the

agent: (1) the agent is a virus with unusual characteristics,

(2) the agent is a prion—an abnormal form of a normal

protein known as cellular prion protein, and (3) the agent

is a virino—an "incomplete" virus composed of nucleic

acid protected by host proteins. The BSE agent is

extremely resistant to heat and to normal sterilization

processes. It also does not evoke any detectable immune

response or inflammatory reaction in host animals.

In cattle naturally infected with BSE, the BSE agent has

been found only in brain tissue, in the spinal cord, and in

the retina. In experimentally infected cattle, the distal

ileum, bone marrow, dorsal root ganglion, and trigeminal

ganglion also were found to be infective.

The presence of the BSE agent in tissues is determined

by inoculating animals, usually mice, with material believed

to be infected with BSE. Mouse inoculation studies take a

long time (up to 700 days) to detect the agent, and failure

to identify it in tissues may indicate either true absence of

the agent or simply the limited sensitivity of current diagnostic methods.

Related Diseases

TSEs

The TSE family of diseases includes scrapie, which

affects sheep and goats; transmissible mink encephalopathy; feline spongiform encephalopathy; chronic wasting disease of deer and elk; and in humans, kuru, both classic

and variant Creutzfeldt–Jakob disease (CJD), Gerstmann–

Straussler–Scheinker syndrome, and fatal familial

insomnia. TSEs have also been reported in captive 

exotic ruminants. The strain of agent isolated from the

exotic ruminants and cats is indistinguishable from BSE 

in cattle, suggesting that the occurrence of TSEs in these

species resulted from BSE-contaminated feed.

On March 20, 1996, the United Kingdom’s Spongiform

Encephalopathy Advisory Committee (SEAC) announced

the identification of 10 cases of variant CJD. The disease

process in 10 patients had a characteristic clinical and

pathological phenotype differing from other routinely

diagnosed cases of classic (sporadic) CJD. The 10

individuals experienced the onset of symptoms at a

younger age, exhibited behavioral changes, were sick 

for longer than patients with classic CJD, displayed a

nondiagnostic or normal electroencephalogram, and

experienced brain lesions that were, under microscopic

examination, different from lesions seen in brain tissue

from patients with classic CJD.

SEAC concluded that, although there has been no direct

scientific evidence of a link between BSE and variant CJD,

on the basis of current data and in the absence of any

credible alternative, the most likely explanation is that the

cases resulted from exposure to BSE before the introduction of a specified bovine offal (SBO) ban at slaughter in

1989. The SBO ban excluded brain, spinal cord, and other

organs with potential BSE infectivity from human consumption. As of August 6, 2001, 106 cases of probable or

confirmed variant CJD had been identified in the United

Kingdom, 1 in Ireland, and 3 in France.

Difference Between Variant CJD and Classic CJD

It is important to clarify the difference between classic

CJD and variant CJD further. Classic CJD occurs each

year at a rate of 1 to 2 cases per 1 million people throughout the world, including in the United States and other

countries where BSE has never occurred and among vegetarians and meat eaters alike. Classic CJD occurs sporadically (about 90 percent of cases), iatrogenically (less

Vacuoles—microscopic holes in the grey matter—give the

brain of BSE-affected cows a spongelike appearance when

tissue sections are examined in the lab.

NVSL veterinarian Dr. Al Jenny

than 1 percent), or genetically (about 10 percent). According

to the U.S. Centers for Disease Control and Prevention

(CDC), no cases of variant CJD have been identified in the

United States.

Current evidence suggests that variant CJD is a clinically

and pathologically new condition. The epidemiologic evidence

is consistent with BSE, and the causal agent and recent laboratory evidence provide strong support for the hypothesis of a

causal link between BSE and variant CJD.

Contacts for More Information About BSE

For general information about BSE, contact USDA, 

APHIS, Veterinary Services, Emergency Programs 

at (301) 734–8073.

For information about importing animals or animal products, contact USDA, APHIS, Veterinary Services, National

Center for Import/Export Animals Program at (301)

734–8170 or the National Import/Export Products Program

at (301) 734–7885.

For questions related to food safety, meat and meat

products, or meat inspection, contact the USDA’s FSIS at

(202) 720–9113.

For questions related to human health or Creutzfeldt–

Jakob disease, contact CDC at (404) 639–7292.

For questions related to science or BSE research, 

contact the National Institutes of Health at (301) 496–5751.

For questions related to food, feed, drugs, cosmetics, or

biological products, contact the FDA at (301) 443–1130.

Current information on animal diseases and 

suspected outbreaks is also available on the Internet at

http://www.aphis.usda.gov. Specific information on BSE 

can be found at http://www.aphis.usda.gov/oa/bse.