Veterinary Services 

Centers for Epidemiology and Animal Health March 2004

_________________________________________________________________________________________________________________________ 

Highlights of Phase II: Scrapie: 

Ovine Slaughter Surveillance 

Study 2002-2003 

The purpose of the SOSS study was to estimate

the regional and national prevalence of scrapie in

mature cull sheep in the United States. 

Phase I of SOSS was conducted from February 

2001 through March 2002 and included refinement 

of the study design and sample collection training. 

The purpose of Phase I was to develop and modify 

the sample collection and testing processes,

without emphasizing statistical results. 

SOSS Phase II is similar to Phase I in that 

sample collection procedures and testing were 

used, along with a representative sample 

allocation. Beginning April 1, 2002, and continuing 

through March 31, 2003, Phase II included the 

collection of tissue samples from 12,508 sheep 

from 22 slaughter plants throughout the United 

States (21 FSIS inspected, 1 State plant) and 1 

large livestock market in Texas. The 21 FSIS plants 

represented approximately two-thirds of the total

FSIS mature sheep slaughtered during the study 

period. The livestock market represented 

approximately one-half of the live sheep exported 

to Mexico. All sample data were statistically 

weighted to reflect the population from which the 

sample was selected. The number of samples 

collected from each plant on a specific day was 

statistically weighted to represent the volume of 

mature sheep slaughtered (sold) through each 

plant (market) that specific day. This weight was 

adjusted for the total volume of mature sheep 

through the plant (market) from April 2002 through 

March 2003. Within each facility sample collectors 

were instructed to collect samples using systematic 

sampling. Overall, the samples collected from the 

22 plants and the livestock market represented 

299,000 sheep (54 percent of the cull sheep 

population, estimated at 550,000 head). 

Sheep were traced to State of origin based on 

ear tags and/or other information obtained by the 

collector at the plant or market. For analysis 

purposes, samples identified to individual States 

were assigned to one of four defined regions. 

Sometimes only a listing of multiple States could be 

obtained for a group of sheep (e.g., market animals 

accumulated across numerous States). These 

samples were assigned to the Multiregion category 

if the States they came from were not all in the 

same region. In cases where a trace State was not 

identified by the collector (n = 2,020), a region was 

assigned based on their official identification 

information. The 2001 NAHMS Sheep study 

showed that at least 95 percent of cull sheep 

movement was within the region of origin. Out of 

the 12,508 samples submitted, all but 2,127 were 

identified to a unique region (Table 1). 

Table 1. Number of Samples Submitted, by Face Color 

and By Region. 

Samples Submitted

Region

Face 

Color West Mountain Central East

Multiregion Total 

Black 100 535 680 1,023 453 2,791

White 493 2,997 1,993 1,283 1,472 8,238

Mottled 71 305 413 404 194 1,387

Unknown 6 32 4 42 8 92

Total 670 3,869 3,090 2,752 2,127 12,508

Obex, tonsil, and lymph-node tissues from each 

sheep were tested using the immunohistochemistry 

(IHC) technique at the National Veterinary Services 

Laboratory. A positive case was defined as having 

a positive test result on any tissue. 

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 
Prior to the SOSS study, the estimated 

pre as 0.07

p ’96

spected 

’96 

hlights were excerpted from 

Pha

revalence estimates

Of the 12,508 mature sheep sampled, valid (at 

itive

0.20

s

.

Three tissue types (obex, tonsil, and 

llected

in

s

valence of scrapie in the United States w

percent (based on unpublished data from the 

NAHMS Sheep ’96 study). However, the Shee

estimate was based on a mail-in survey of 

producers who reported the presence of su

or confirmed cases of scrapie in their flock over a

period of 5 years, including lambs and mature 

sheep. The flock estimate was then expanded 

based on flock size to generate the animal-leve

prevalence estimate. The results of the SOSS 

study cannot be directly compared to the Sheep

prevalence estimate because of differences in 

study design, reference population, and data 

collection methods. 

The following hig

l

se II: Scrapie: Ovine Slaughter Surveillance

Study 2002-2003. 

P

•

least one testable tissue) test results were 

obtained from 12,491 (99.9 percent). A pos

result was recorded for any animal that tested 

positive by IHC on one or more of the tissues

sampled. The overall weighted national 

prevalence of scrapie in mature sheep is

percent. Estimates could not be made in the 

West region due to the low number of sample

obtained. However, national estimates include

samples collected in the West region (Figure 1)

•

retropharyngeal lymph node) were co

from each sheep head for IHC testing. As 

expected, each tissue type differed slightly

the number tested as well as the number of 

positive results; however, the prevalence wa

similar for the three tissue types (Figure 2). 

0.0

0.2

0.4

0.6

Figure 2. Percent of Sheep That Tested Positive for Scrapie, 

by Tissue Type

Percent

Tissue Type

Obex Tonsil Retropharyngeal 

lymph node

0.18 0.19 0.20

Scrapie prevalence (one or more tissue 

lackot

•

samples tested positive) was highest in b

faced sheep (0.84 percent). White-faced sheep

were far less likely to test positive for scrapie 

(less than 0.01 percent). Some animals were 

presented for sample collection with the skin 

removed. Therefore, face color could not be 

determined on these animals and they were n

included in these estimates (Figure 3). 

West Mountain Central East

United States Department of Agriculture • Animal and Plant Health Inspection Service • Safeguarding American Agriculture 
0.0

0.2

0.4

0.6

0.8

1.0

Percent

Face Color

Figure 3. Percent of Sheep That Tested Positive for Scrapie,

by Face Color

0.00

0.84

0.12

White Black Mottled

p

t of

,

Percent of Sheep That Tested Positive for Scrapie,

by Face Color

0.00

0.84

0.12

White Black Mottled

Age was determined based on the number of 

p

t of

,

• Age was determined based on the number of 

visible permanent incisors. Four-year-old shee

tested positive (one or more tissue samples 

tested positive) most frequently (0.49 percen

sheep tested). Scrapie prevalence increased 

with age until the animals reached 4 years old

then decreased (Figure 4). 

visible permanent incisors. Four-year-old shee

tested positive (one or more tissue samples 

tested positive) most frequently (0.49 percen

sheep tested). Scrapie prevalence increased 

with age until the animals reached 4 years old

then decreased (Figure 4). 

0.0

0.5

1.0

1.5

2.0

Black-faced sheep

National

Figure 4. Percent of Sheep That Tested Positive for Scrapie

(National and Black-faced Sheep), by Age

Percent

Age

0.11

0.00

0.21

0.54 0.49

1.77

0.13

0.69

1 and 2 years 3 years 4 years 5 years or more

enetics

ue samples from the 33 sheep that tested 

pos

e at 

______________________________ 

or more information, contact:

E7

G

 

Tiss

itive for scrapie were submitted for genetic 

testing. All 33 samples were of the QQ genotyp

codon 171. This genotype has been characterized 

as the least resistant to scrapie. 

__

F

USDA:APHIS:VS:CEAH

NRRC Building B, M.S. 2

2150 Centre Avenue 

Fort Collins, CO 80526-8117 

970.494.7000

E-mail: NAHMSweb@aphis.usda.gov 

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

#N421.0304

____________________________________ 

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M

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