Bovine Spongiform Encephalopathy

Bovine spongiform encephalopathy (BSE) is a progressive 

neurological disease of cattle. It is caused by prions, which are 

infectious agents made up of protein material. The prion proteins 

affect the brain structure of infected animals, causing the animal 

to lose motor skills and eventually causing death. Clinical signs 

include behavioral changes, coordination problems, weight 

loss, and decreased milk production. Once clinical signs appear, 

the animal’s condition deteriorates until it either dies or is 

euthanized. There is no treatment or vaccine to prevent BSE.

There are two types of BSE—classical and atypical. Neither 

form is contagious.

Classical BSE

Classical BSE spreads through the ingestion of certain materials 

(brain, spine, etc.) from infected animals. The main way classical 

BSE spread was through contaminated animal feed containing 

meat or bone meal from infected cattle. Because of BSE, the 

Food and Drug Administration (FDA) banned the use of ruminant 

protein in feed for ruminants in 1997. FDA additionally prohibited 

the use of certain high-risk cattle tissues in feed for all animals 

in 2009. The feed ban is proven to keep animals from becoming 

infected with classical BSE.

The incubation period for classical BSE from the time of infection 

until the onset of clinical signs averages 3 to 6 years.

The first diagnosis of classical BSE occurred in 1986 in the 

United Kingdom, where most of the worldwide cases occurred. 

Since then, many other countries have confirmed cases. The 

first case of classical BSE in the United States was confirmed 

by the U.S. Department of Agriculture (USDA) in 2003. The 

animal was imported from Canada and born before feed bans 

were established.

Atypical BSE

Atypical BSE occurs rarely and spontaneously, typically in 

cattle that are 8 years old or older. There is no association of 

atypical BSE with contaminated feed or ingestion of infected 

materials. Instead, a spontaneous change in the prion proteins 

in older adult cattle cause atypical BSE. Because changes in 

prion proteins are sporadic and very rare, scientists do not 

fully understand what causes prions to change from normal to 

abnormal forms.

Scientists are currently studying whether genetics or 

environmental factors may contribute to atypical BSE. As part of 

this research, scientists typically examine the offspring of animals 

diagnosed with atypical BSE and animals born in the same 

location at the same time as affected animals.

Six of the seven cases of BSE identified in the United States have 

been diagnosed as atypical BSE. In most cases, the animals were 

10 years of age or older. Two of the six detections of atypical BSE 

involved animals aged approximately 5 years or older.

BSE and Trade

The World Organisation for Animal Health (WOAH) officially 

recognizes a country’s BSE risk status after evaluating the 

country’s history with the disease, the implementation and 

enforcement of their feed ban, and their BSE surveillance. 

WOAH guidelines outline criteria for safe trade in cattle and their 

products according to their risk status. The United States’ import 

requirements follow these guidelines, and USDA encourages 

other countries to do the same regarding U.S. imports.

After the 2003 BSE finding, many countries stopped importing 

U.S. beef. This caused substantial losses for the cattle industry. 

In 2013, WOAH upgraded the U.S. BSE-status to negligible risk, 

the best status available. In 2015, WOAH determined that atypical 

BSE occurred spontaneously at a low rate in all cattle populations 

and would be excluded when determining BSE risk. Today, most 

countries that prohibited U.S. beef due to BSE have removed 

those restrictions.

www.aphis.usda.gov/animalhealth

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APHIS 23-026 | May 2023
BSE Surveillance 

USDA began conducting BSE surveillance testing in 1990 to 

detect any cases that might be present in the United States. 

If no cases are found, surveillance results also prove the absence 

of the disease in the country. The current U.S. surveillance 

program, in place since 2006, far exceeds the current surveillance 

guidelines provided by WOAH. The testing program focuses on 

cattle populations that are at higher risk for BSE, including:

 Animals 12 months of age and older that display central 

nervous system signs

 Animals over 30 months of age that are excluded from 

slaughter due to poor health status (non-ambulatory, 

unhealthy, or dead)

Producers can support USDA’s efforts to identify BSE, especially 

cases of atypical BSE that fall outside of normal surveillance 

efforts, by:

 Immediately reporting any unusual or suspicious signs of 

disease to your veterinarian, State or Federal animal disease 

control officials, or to your county agricultural agent;

 Watching cows that show nervous or aggressive behavior, 

difficulty with coordination, trouble standing up, decreased 

milk production, and weight loss; and

 Reporting the location of any potential offspring for 

affected cows.

Related Diseases

BSE is part of a group of diseases known as transmissible 

spongiform encephalopathies (TSEs). Other TSE diseases include 

scrapie in sheep and goats; chronic wasting disease in deer, elk, 

and moose; and classic and variant Creutzfeldt-Jakob disease 

(CJD) in people.

BSE is considered a public health concern because cases of variant 

CJD in people are linked to the consumption of food containing 

ingredients derived from classical BSE- infected cattle.

More Information 

For more information about BSE, email USDA’s Ruminant 

Health Center at VS.SP.Cattle.Health.Center@usda.gov. Current 

information on BSE detections in the United States is available 

online at www.aphis.usda.gov/animalhealth.