November 15, 2003
USDA Looks at National and Regional Statistics
By Emily Tescher-Johnston
Nov. 2003 -- After a complete year of statistically sound surveillance, the U.S. Department of Agriculture (USDA) announced that the national prevalence number for scrapie in the United States is 2/10th of 1 percent ? or 1 in 500.
?The collective opinion from the industry, states and allied groups was that it would be necessary to gather statistics on disease prevalence so that we had a true picture of scrapie in the United States,? says Dr. Diane Sutton, national scrapie program coordinator for the USDA. ?We determined that the most scientifically sound way to do that was to do a slaughter surveillance study.?
The USDA implemented a Scrapie Ovine Slaughter Surveillance (SOSS) program that would involve random, nationwide testing of cull sheep that showed no sign of disease.
?We targeted cull sheep because they are the age category where we can reliably detect the scrapie prion protein in the tissue,? says Sutton. ?We can?t reliably detect scrapie prion protein in lambs using current diagnostic tests.?
Dr. John Clifford, USDA?s associate deputy administrator for National Animal Health Policy and Programs, says, ?The objective of SOSS was to estimate the national and regional prevalence of scrapie in mature sheep. Prior to the SOSS study, the prevalence of scrapie in the United States was estimated to be 0.07 percent.?
Sheep tested under the SOSS program came from many sources all over the United States, including animals in export channels. Samples were collected at 22 slaughter plants.
Paul Rodgers, deputy director of policy for the American Sheep Industry Association (ASI), explains that there were no trace-back efforts or legal enforcement for sheep that proved scrapie-positive under the SOSS program.
?There were no regulatory consequences for fear of skewing the sample,? says Rodgers. ?We wanted a true sample, and we didn?t want folks keeping their cull sheep out of slaughter because the SOSS program was going on.?
The USDA conducted the SOSS program from April 1, 2002, through March 31, 2003.
?The number of sheep entering the marketplace varies from month to month and season to season,? explains Rodgers. ?That is why we had to designate a beginning date and an ending date.?
A total of 12,508 sheep were tested under the SOSS program. The country was divided into four zones, or regions. Those regions are: West, Mountain, Central and East. Sheep that could not be traced to a region were grouped as Multi-region. (See map for state breakdown). Of the 12,508 sheep tested, 34 were found to be scrapie positive of which there were 27 black face, three mottled face, two white face and two unrecorded face color. The national prevalence was calculated by weighting the test results based on the number of sheep slaughtered at each plant, allowing epidemiologists to calculate the point 2 percent national prevalence number.
Regional prevalence was also calculated. They are as follows: West = , Mountain = .14 percent, Central = .21 percent, East = .52 percent and Multi-Region = .13 percent. (*The sample size in the western region was too small for prevalence estimation; no positives were found in the sheep tested in that region.)
"We have full confidence that this is a statistically valid sample, and it gives us a very good picture of prevalence in mature cull sheep in the United States at the time,? says Sutton.
?We believe the reported prevalence rate of scrapie is relatively low, and knowing the prevalence as well as the regional breakdowns, gives us an opportunity to be aggressive in eradication of the disease,? adds Rodgers. ?Now we need to move forward with regulatory slaughter surveillance and all other methods that are available to help eradicate the disease as soon as possible.?
ASI President Guy Flora says, ?Such a study had never been done and so no one actually knew the rate of scrapie in sheep in the United States. We only knew the rate of scrapie in flocks that had been identified with the disease. The SOSS gave us the first real look at scrapie prevalence.?
Wyoming State Veterinarian, Dr. Jim Logan adds, ?Knowing the prevalence of scrapie along with the regional, breed-type, age and genetic breakdown will provide both the industry and regulatory agencies with information that will be very useful in the eradication program.?
The SOSS program proved very valuable in terms of getting a better picture of scrapie nationwide. It was also costly. Although final figures are not yet calculated, Sutton estimates that the program cost about 1 million dollars.
?This was such a high priority for ASI that in the very early stages, we invested $40,000 to provide the USDA with automated diagnostic equipment necessary to test a large volume of animals for this study,? says Flora.
Adds Rodgers, ?This study is also important for on-going research and test development. The tissues that have been collected will be studied and the findings used to expedite the scrapie eradication process.?
?Additional analysis of the data collected -- region, face color and genetics -- is being completed,? says Clifford. ?The completed information will be available to stakeholders and industry in January 2004.?
Flora adds, "It would be very beneficial if the industry had a statistically sound picture of scrapie internationally as well. The American Sheep Industry Association challenges and urges other countries that want to know the true prevalence of scrapie to conduct a valid, statistically sound slaughter surveillance study.?
Regulatory Scrapie Slaughter Surveillance
The SOSS program ended March 31 of this year. The next day, the Regulatory Scrapie Slaughter Surveillance program (RSSS) was implemented.
?The RSSS differs from the SOSS in that we are no longer conducting a study to determine prevalence,? says Sutton. ?Now, under the RSSS program, our goal is to detect as many scrapie-positive sheep as possible.?
The RSSS program is being conducted in slaughter facilities around the country. A total of 3,210 sheep have been tested under RSSS since April 1, 2003. So far, 12 sheep have been determined to be scrapie-positive or suspect.
?The RSSS target is to expand to 45,000 animal samples for fiscal year 2004,? says Sutton. ?That will allow us to identify the affected flocks that are out there.
?We are going to use this targeted slaughter surveillance until we find no positive animals and then we will adjust our targeting to ensure that no populations have been missed,? she adds. ?Then we will begin a ?monitoring? program. We hope to be able to identify areas where we can regionalize for scrapie such with ?Scrapie-Free Zones? for purpose of export.?
Wyoming?s Dr. Logan says, ?We applaud those slaughter facilities that have cooperated during the study portion (SOSS), and we are urging more slaughter establishments to come forward and work with the RSSS program.
?We need very good partnerships with industry and producers. In the last two years, we?ve made really great strides as far as scrapie eradication. But,? he adds, ?more can, and should, be done. We cannot continue with modified passive surveillance -- that?s where we wait for scrapie to manifest itself clinically. We just can?t wait until we see sick sheep.?
Instead, Logan promotes active surveillance, where strategically targeted groups of animals are tested.
Logan is also concerned about the practice of plucking good ewes from feeder channels and putting them back into breeding herds, if the ewes are not officially identified.
?Unless these ewes are officially identified, we jeopardize the eradication program,? he says.
Programs like the SOSS and the RSSS, as well as other active surveillance efforts, are obviously helping the U.S. sheep industry advance toward the goal of eradicating scrapie by 2010. Continued research and surveillance will bring the industry to that final disease-free status.