Part 2: Progress toward eradicating a fatal disease
The eradication of scrapie is becoming closer to a reality, thanks to the efforts of numerous sheep and goat owners along with veterinary partners working with state and federal government.
Dr. Linda Detweiler, Veterinary Medical Officer at USDA-APHIS, said parties with a vested interest in eliminating scrapie – breed and industry associations, American Farm Bureau, veterinarians and other organizations – were brought together to develop a program in response to the problem.
While this was happening, in 1986 the UK reported the first case of bovine spongiform encephalopathy (BSE). “This made an impact on the whole process because the first theory for BSE was that it was scrapie in sheep made into meat and bone meal and fed back to cattle,” said Detweiler.
In 1988, the UK banned the feeding of ruminant protein to ruminants. “There was worry in the U.S. among renderers and the feed industry,” said Detweiler. “It was easy to point the finger at sheep but we know now that probably wasn’t the right story.”
The negotiated rule-making process took over a year. “It was really intense to come up with a solution that would work for the sheep industry,” she said. “We came up with regulations to establish identification of sheep from scrapie-infected and scrapie source flocks. We raised the indemnity to find infected and source flocks and established a voluntary scrapie flock certification program.”
Because there was no live animal test to determine whether a non-clinical animal had scrapie, establishing a voluntary program was important to purebred producers who wanted to locate scrapie-free breeding stock. At the time, the disease was found predominantly in black-faced breeds.
Detweiler noted that BSE drove funding for scrapie research. “We see through these cycles not only in UK but also in the U.S. that if the emphasis was off scrapie, funding went down,” she said. “This happened in a number of countries.”
Finding the abnormal protein led to other diagnostics, including finding the prion protein in tissue other than the brain. More reliable tests helped diagnose cases that would have otherwise gone undetected.
As to whether scrapie is transmissible or genetic, Detweiler said it’s a combination. “Scrapie is an infectious, transmissible disease with a genetic influence,” she said. “There’s infectivity in the placenta, birth fluids, colostrum and milk. Transmission usually occurs at or near birth, with an average of three to four years’ incubation until the animal develops clinical signs. Once an animal is clinically ill, it’s always fatal.”
An important research finding was identification of the prion protein gene in the 1990s. Most purebred sheep producers are familiar with codon 171, which makes sheep susceptible to scrapie. The discovery of the prion protein in the lymphoreticular system led to the development of live animal tests of the rectal mucosa and the third eyelid.
Dr. Diane Sutton, DVM, assistant director, Ruminant Health Center, USDA, discussed improved scrapie testing. The accelerated scrapie testing program established in 1998 began with another Advanced Notice of Proposed Rulemaking.
“It proposed to bring back the regulatory program and proceed with accelerated eradication of scrapie from the U.S.,” said Sutton. “In 2000, as a result of the 201 Trade Action initiated by the industry, we were able to get an emergency declaration and funding that allowed us to kick off the program.”
The key changes included mandatory identification of animals upon change of ownership of sheep and goats. The industry recognized they wouldn’t find all scrapie-infected animals through surveillance, which resulted in the preliminary scrapie ovine slaughter surveillance study in 2002-03. The study showed that slaughter surveillance was an effective means of identifying infected animals.
An ongoing program in 2003 involved sampling as many sheep and goats as possible ages 18 months to 72 months at slaughter. 2003 was also the year live animal testing via lymph tissue was initiated. In 2004, a genetic-based program reduced the number of animals that had to be destroyed by 60% because they were only removing the genetically susceptible animals. State programs were expanded to a national program.
The primary components of today’s Accelerated National Scrapie Eradication Program include education and prevention, animal ID at markets, market compliance, surveillance (slaughter and on-farm), tracing positive and exposed animals to herd or flock of origin, testing exposed animals, cleanup of infected and source flocks through genetic susceptibility testing, indemnification of susceptible exposed animals, a switch to rectal biopsy to replace third eyelid testing and monitoring previously infected and exposed flocks.
“We found reinfection of cleaned up flocks was very rare and generally explained by purchasing of genetically susceptible animals that had been exposed in other herds,” said Sutton. “We also continued to manage the scrapie-free flock certification program and upgraded the requirements to ensure that when a flock was certified as ‘free,’ it truly was free. Since we have moved to higher standards, we have not had a Scrapie Certified Flock break with infection.”
Since the inception of the Scrapie Free Flock Certification program, an oversight committee in the early days provided producer education, direction and guidance and general support.
Dr. Stephanie Ringler, DVM, USDA-APHIS Veterinary Services, Sheep and Goat Team, said programs and cooperation over the past 30 years have shaped today’s progress.
The current status of scrapie in the U.S. starts with the surveillance goal of testing at least 40,000 animals annually for scrapie. From May through July 2023, “we have collected over 15,000 sheep and goats from slaughter and over 1,000 animals on farm,” Ringler said. Collections are rising each year thanks to cooperative efforts. In the U.S., 18 labs are approved to test scrapie samples.
In a recent project, over 70% of sheep genotypes from 2019 to 2022 were found to be resistant to scrapie. Another notable statistic is that 44 of 50 states have been free of classical scrapie for longer than seven years.
Since Jan. 1, 2021, only one in 50,000 sheep sampled was positive on-farm or at slaughter. “Our last case of classical scrapie was found in 2021,” said Ringler, “which means if we don’t have another scrapie case, scrapie will be eradicated in the U.S. in 2028. Achievements through surveillance are paying off.”
by Sally Colby