Bovine Spongiform

Encephalopathy

Epidemiologic

Process and Testing

Protocols 

Once a cow tests positive for bovine spongiform

encephalopathy (BSE), veterinarians at the U.S.

Department of Agriculture’s (USDA) Animal and Plant

Health Inspection Service (APHIS) immediately begin

an epidemiologic investigation. Its purpose is to 

identify and locate cattle that may have been

exposed to the same source of BSE prions, 

presumably in contaminated feed, as was the 

BSE-positive or index cow. It is unlikely that these

animals have BSE, but by association, they are now

part of the targeted population for testing.

The epidemiologic process requires we determine the origin of the index cow and locate any progeny or birth cohorts to verify there is not more than

one infected animal from the same herd. 

Birth Cohort

Experience in countries that have previously

detected BSE indicates that it is uncommon to have

more than one infected animal in the same herd.

However, BSE epidemiology efforts focus on birth

cohorts—all cattle born on the positive animal’s birth

premises within 1 year, before or after, the date of

birth of the BSE-positive animal—in order to determine a common feed source as the possible mode of

infectivity. (Note: If the precise date of birth of the

index animal is unclear, a potential age range is

used, with 1 year added to each end of that range.)

In most cases, some of the birth-cohort animals

will have moved off the birth premises; others will

have gone to slaughter or died. Investigators can

sometimes pinpoint the current locations of these animals or where they have been in the past by thoroughly examining herd records.

Cattle of Interest

When birth cohorts cannot be definitively identified, a herd inventory and analysis of herd records

are used to identify a group of cattle that includes all

birth cohorts as well as any other cattle that cannot

be ruled out as birth cohorts. All of these animals—

birth cohorts and any additional cattle—will be

defined as “cattle of interest.” Any progeny of the

positive animal born in the last 2 years are also considered cattle of interest.

All cattle of interest are further evaluated using a

number of identification techniques. The goal is to

narrow the group and eliminate all cattle that are not

birth cohorts or progeny. Depending on the specific

circumstances involved, several factors could be useful in identifying cattle of interest. These include age,

gender, breed, color, man-made identification

(eartags, tattoos, brands), and known premises of

birth. Obvious factors, such as age, gender, and

breed, will be used to the extent possible to limit the

size of the group being evaluated. More specific

information from herd-management records and any

other available records will then be compared against

man-made identification from the remaining potential

cattle of interest. Ultimately, cattle that cannot be

eliminated based on any of these factors will be

included in the group of cattle that will be depopulated and tested for BSE.

While APHIS works to locate as many of the cattle of interest involved in a BSE case as possible, it is

important to reiterate the safeguards in place to protect against BSE and protect public health. These

policies include banning nonambulatory (“downer”)

animals at slaughter from the human food and animal

feed chains, removing specified risk materials (tissues known to harbor the prions that cause BSE) in

animals over 30 months of age, banning certain

slaughter techniques, and most importantly implementation of the ruminant-to-ruminant feed ban.

Cattle can become infected with BSE by eating feed

contaminated with the infectious BSE agent. This is

why in 1997 the U.S. Food and Drug Administration

prohibited the use of most mammalian protein in the

manufacture of animal feed intended for cows and

other ruminants.

Testing Protocol

Brain samples from cattle suspected of having

BSE will be tested at any one of the seven USDAAPHIS-approved State veterinary diagnostic laboratories and/or USDA’s National Veterinary Services

Laboratories (NVSL) in Ames, IA, according to the

following testing protocol:

1. The BioRad® enzyme-linked immunosorbent

assay (ELISA) rapid-screening test is performed at

seven State veterinary diagnostic laboratories working on BSE testing under APHIS authority.

2. If a sample tests negative for BSE on the initial

ELISA screening test, it is considered negative and

no further diagnostic testing is performed.

Factsheet

Veterinary Services May 2006

APHIS
3. If the sample is reactive on the initial ELISA

screening test, it is considered an Initial Reactor for

BSE and the ELISA screening test is repeated in

duplicate by the testing laboratory.

4. If either of the repeat tests on the Initial Reactor

sample is reactive, the sample is then considered

“inconclusive” and will be shipped to NVSL for confirmatory testing.

5. NVSL personnel perform an immunohistochemistry

(IHC) test on the samples as one of the confirmatory

tests.

6. Concurrent with IHC testing at NVSL, a SAF

Immunoblot (Western blot) test is run at USDA’s

National Animal Disease Center in Ames, IA.

7. If the sample is negative on both SAF Immunoblot

and IHC, it is considered negative for BSE and for

APHIS regulatory purposes. 

8. If the sample tests positive from either IHC or SAF

Immunoblot, it is considered positive for BSE and for

APHIS regulatory purposes.

9. Further testing on that sample for the purposes of

characterization and/or research may be performed

only after consultation with the Secretary of

Agriculture. Recommendations for further testing will

be conveyed through the APHIS Deputy Administrator

for Veterinary Services. If further testing is requested,

NVSL will provide the Deputy Administrator with a

specific testing protocol together with a written explanation of that protocol. 

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United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture