March 15, 2004
Mar/Apr 2004 -- Katherine Marshall, veterinary medical officer and sheep specialist with the U.S. Department of Agriculture's Center for Animal Health Monitoring in Fort Collins, Colo., reported that the Scrapie Ovine Slaughter Surveillance (SOSS) study is estimating the regional and national prevalence for scrapie in mature and cull sheep in the United States.
Scrapie, a transmissible spongiform encephalopathy, costs American sheep producers around $20 million a year in lost sales abroad, increased costs and lost animals. Many cases go undetected, but efforts like SOSS and the National Accelerated Scrapie Eradication Program are proving effective at monitoring the disease.
Marshall said that from April 1, 2002, to March 31, 2003, SOSS evaluated brain, tonsil and lymph node tissue in 12,508 sheep, of which 12,491 had at least one testable tissue sample. The survey, conducted at 21 high volume slaughter plants in 12 states, was broken into four regions, west (which did not have enough samples to draw any conclusions), mountain, central and east.
The highest incidence of scrapie was found in black-face sheep, with 0.84 percent testing positive. As for age, four-year-old sheep had the highest number testing positive. Combining age and color, the survey found that four-year-old, black-face sheep had the highest prevalence of scrapie, with 1.77 percent testing positive.
The east was the highest among regions with 0.52 percent testing positive - compared with an overall rate of 0.2 percent. Only one white-face sheep tested had scrapie.
Diane Sutton, national scrapie program coordinator with APHIS, said the surveillance goal for fiscal year 2004 is to test around 4,000 sheep and goats a month. Surveillance will be targeted at the groups with the highest prevalence, namely mature animals that are dead, down, weak or have wool loss as well as mature black- and mottle-face sheep.
Samples will be collected from these targeted groups at ewe slaughter plants, diagnostic and public health labs, at markets or farms or wherever scrapie suspects might be found. In addition, third eyelid testing will be conducted on sheep in flocks known to be at increased risk.
Sutton said that in the fiscal year ended Sept. 30, 2003, the Accelerated Eradication Scrapie Program had confirmed 351 new cases of scrapie. As of Dec. 31, a total of 1,842 flocks were participating in the scrapie flock certification program, of which 113 had been certified, 1,722 completely monitored and seven selective monitored.
Twenty-five state and university diagnostic laboratories have now been approved for testing as have five genotype labs.
"Competition in the marketplace should bring prices down," said Sutton.
She noted the Animal and Health Inspection Service in November 2003 approved a test on a lymph node or tonsil, which should increase by 24 percent the number of sheep found through slaughter surveillance.
The program is also cranking up producer education programs, and should have a summary guide for veterinarians completed early this year.
Katherine O'Rourke, a microbiologist with the USDA-ARS Animal Disease Research Unit in Pullman, said the third eyelid test is currently the only practical live animal test for scrapie -- although it does have its drawbacks. (Adding histamine to the eye drops does aid third-eyelid sampling.)
O'Rourke said a live animal blood test would give producers tremendous capabilities in testing for scrapie, but researchers have yet to find a marker in the blood.
"We know there's something in the blood, but no data have confirmed it despite considerable effort. There's no shortage of people working on this," she said. "I think the blood test is going to be helpful to us, but I can't predict what else is going to pop out of the box when we open it."