Veterinary Services 

Centers for Epidemiology and Animal Health January 2004

_________________________________________________________________________________________________________________________ 

Scrapie Awareness and 

Prevention on U.S. Sheep 

Operations

Scrapie is a fatal, progressive neurological disorder 

of sheep. Belonging to a group of 

neurodegenerative diseases called transmissible 

spongiform encephalopathies, scrapie is thought to 

be caused by an infectious protein or prion. Once 

infected, an animal is infected for life, and the 

disease is always fatal. Scrapie has a very long 

incubation period. Infected animals rarely show 

clinical signs of scrapie before 2 years of age, with 

the average age being 4 years. Scrapie is difficult 

to eradicate because: transmission of the organism 

is not completely understood; the live animal test 

for scrapie requires a biopsy of the lymphoid tissue 

from the animal’s third eyelid and can result in 

significant numbers of “no tests” and is very labor 

intensive and expensive; it is resistant to most 

disinfectants; and it can be transmitted by sheep 

that display no clinical signs of the disease. 

Therefore, to reduce the risk of scrapie, sheep 

producers should purchase new animals from 

known scrapie-free flocks and focus on 

management practices such as flock certification, 

genetic testing for resistance, and hygienic lambing 

management. 

The USDA’s National Animal Health Monitoring 

System (NAHMS) collected data on sheep health 

and management practices from a stratified random 

sample of sheep production sites in 22 States (see 

map) as part of the Sheep 2001 study. These 

States represented 87.4 percent of the U.S. sheep 

inventory and 72.3 percent of U.S. sheep producers 

with one or more sheep. Overall, 3,210 operations 

participated in the first interview from December 29, 

2000, to January 26, 2001. A second interview was 

completed by 1,101 of these operations with 20 or 

more ewes between February 5 and April 27, 2001. 

Producer Awareness 

Producer awareness of scrapie was high. 

Overall, 92.6 percent of producers had at the very 

least heard of scrapie (Figure 1). 

________________________________________________________ 

Figure 1. Percent of Operations by Familiarity with 

Scrapie 

________________________________________________________

Two-thirds (66.9 percent) of producers who had 

at least heard of scrapie reported being moderately 

or highly concerned about the disease, yet only 3.4 

percent participated in the National Scrapie Flock 

Certification Program in 2000, and nearly half (49.8 

percent) had never heard of the program. 

States That Participated in Sheep 2001 Study

Pacific

West Central

Central

Eastern

Regions

19.5

44.6

28.5

7.4

0

10

20

30

40

50

Very familiar Somewhat familiar Heard of name only Never heard of

Percent 

Familiarity
Breed 

Historically, black-faced sheep have accounted 

for the majority of known scrapie-infected sheep in 

the United States. However, at the time of the 

study, the U.S. sheep population was comprised 

primarily of fine or medium wool, white-faced 

breeds (52.2 percent of sheep). Crossbred sheep 

(which may have black, white, or mottled faces) 

accounted for 29.4 percent of U.S. sheep; blackfaced breeds accounted for only 8.9 percent. There 

also were regional breed differences, with nearly 

one-fourth (23.3 percent) of sheep in the Eastern 

region (see map previous page) being black-faced 

breeds, compared to only 5.5 percent in the West 

Central region (Figure 2). 

________________________________________________________ 

Figure 2. Percent of Sheep and Lamb Inventory by Breed 

Category and by Region.

________________________________________________________ 

Genetic Testing for Scrapie 

If exposed to scrapie, sheep with specific 

genetic variations at codon 171 have been found to 

be either susceptible or more resistant to 

developing clinical signs of scrapie. As part of the 

NAHMS Sheep 2001 study, 13,199 samples were 

genotyped at codon 171. Results are available in 

the NAHMS information sheet Distribution of 

Genotypes at Codon 171 in U.S. Sheep (November 

2003). Overall, 10.8 percent of operations that had 

heard of scrapie used genetic selection to control or 

prevent the disease (Figure 3). 

________________________________________________________ 

Figure 3. For Operations that Had Heard of Scrapie and 

Did Any Genetic Selection for Scrapie Control, Percent of 

Operations that Carried Out the Listed Practices 

_______________________________________________________________

Of those operations, 22.6 percent selected lesssusceptible breeds (low prevalence breeds) of rams 

or ewes; 17.2 percent culled genetically moresusceptible ewes; 27.0 percent selected genetically 

less-susceptible ewes; and 76.8 percent selected 

replacement rams that were genetically less 

susceptible to scrapie (e.g., RR alleles). 

Lambing Management 

Even though infected animals most often do not 

show signs of scrapie until they are 2 years of age 

or older, the highest risk period for infection 

appears to be at birth or soon after. Because 

transmission is thought to occur primarily through 

the environment and because placentas are a 

source of infection, careful management of the 

lambing environment is crucial in preventing 

exposure and infection of neonatal lambs (see 

Table 1 following page). 

10.8%

89.2%

22.6

17.2

27

76.8

0

20

40

60

80

100

 Selected lesssusceptible breeds 

(low prevalence) of

rams or ewes

Culled genetically

more-susceptible ewes

 Selected genetically

less-susceptible ewes

Used genetically less-

 susceptible rams

Practice

 Percent

Operations that Had 

Heard of Scrapie

Operations that Did

 Genetic Selection

52.2

8.9

29.4

9.5

26.9

23.3

42.1

7.7

33.6

13.3

40.4

12.7

67.6

5.5

19.1

7.8

0

20

40

60

80

Pacific

West Central

Central

Eastern

All Operations

Fine/med. wool 

white faced 

Black faced Crossbred Other

36.6

10.0

42.6

10.8

Breed

Percent
Removing placentas from pens or lambing 

areas also decreases the amount of environmental 

contamination. Overall, 75.2 percent of operations 

removed placentas from pens or lambing areas. 

More farm flocks (81.0 percent) removed placentas 

than either herded/open range flocks (49.3 percent) 

or fenced-range flocks (55.5 percent). Of those 

operations that removed placentas from lambing 

areas, most (68.4 percent) removed placentas 

sooner than 6 hours after birth. However, 7.9 

percent of operations left placentas on the ground 

for more than 12 hours, on average. The most 

common way of disposing of placentas was 

composting (29.4 percent of operations), followed 

by landfill/dump (17.9 percent of operations), and 

carnivores (15.4 percent of operations). 

In addition, cleaning lambing areas is very 

important in preventing disease exposure to 

newborn lambs. One-fourth (24.8 percent) of 

operations with a lambing structure cleaned 

manure and waste bedding from the area between 

each ewe. The majority (51.4 percent) of producers 

cleaned lambing areas at the end of the lambing 

season, and an additional 16.2 percent of 

operations cleaned lambing areas between two or 

more ewes, leaving 7.6 percent of operations that 

never cleaned lambing areas. 

Flock Additions

New additions to the flock are a potential 

source of infection. Separating new arrivals 

(quarantining) may allow identification of sick 

animals for many diseases and prevents exposure 

to the rest of the flock. Because of scrapie’s long 

incubation period and the increased risk of 

transmitting scrapie during lambing, lambing new 

additions separately is a more important scrapie 

prevention tool than quarantining. Over one-third 

(34.1 percent) of operations added ewes or ewe 

lambs during the previous year, while 19.8 percent 

had not added ewes or ewe lambs for 10 or more 

years. A total of 14.3 percent of operations that 

added bred ewes separated these new additions 

from the flock until they lambed. 

The rams’ role in scrapie transmission appears 

to be small, since they have not been implicated in 

transmission. Only 4.3 percent of operations had 

not added either rams or ewes to their flocks in the 

last 4 years. 

Knowing the health status of the source flock 

for replacement ewes and rams can reduce the 

risks associated with introducing new animals. 

Flocks that participate in the National Scrapie Flock 

Certification Program are a source of low-risk 

replacement sheep. For operations that had heard 

of scrapie and had acquired ewes or rams during 

2000, 9.6 percent obtained some or all of their 

ewes, and 11.0 percent obtained some or all of 

their rams, from flocks participating in this program. 

______________________ 

For more information, contact: 

USDA:APHIS:VS:CEAH 

NRRC Building B, M.S. 2E7 

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000 

E-mail: NAHMSweb@aphis.usda.gov 

www.aphis.usda.gov/vs/ceah/cahm

#N385.0104 

_______________________________________________________________ 

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, 14th and 

Independence Avenue, SW, Washington, DC 20250–9410 or 

call (202) 720–5964 (voice and TDD). USDA is an equal 

opportunity provider and employer. 

Table 1. Percent of Operations Using the 

Following Lambing Management Practices 

Practice 

 Percent 

 Operations 

Use lambing area as sick ewe 

pen during lambing season 23.0 

Use lambing area as sick ewe 

pen other times 32.3 

Remove placentas 75.2 

Time to removal of placentas 

 Less than 6 hours 68.4 

 6 to 12 hours 23.7 

 More than 12 hours 7.9 

Clean manure and waste 

bedding from lambing areas 

 Between each ewe 24.8 

 Between 2 or more ewes 16.2 

 At the end of lambing season 51.4 

 Never 7.6