QUEBEC CITY – A genetic link to scrapie susceptibility could help remove the disease from the sheep population.
Countries including Canada and the United States have tried to eliminate the disease but a lack of tests, vaccines and full understanding made it difficult to control, said the Canadian Food Inspection Agency veterinarian in charge of the national scrapie program.
“We started a national program in 1941 when it became a reportable disease. We failed,” Penny Greenwood told the World Sheep Congress in Quebec City held July 17-24.
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Genetics are receiving full attention in countries such as France and the Netherlands where animals are genotyped using blood tests to search for a genetic link. Canada and the U.S are moving in a similar direction by searching for the right amino acid combinations, which indicate susceptibility but do not necessarily mean an animal with the right combinations will sicken under the right circumstances.
Animals receive one copy of each gene from the mother and one copy from the sire. A specific gene has been identified that regulates sheep’s response to scrapie.
Certain combinations of amino acids at three specific locations, known as codons on the scrapie gene, influence the animal’s susceptibility.
The codons are named 136, 154 and 171 and the amino acids are referred to by letters representing their names.
Researcher Francis Barillet said that while more research is needed, scientists know the amino acid sequence can change, which seems to be significant in triggering disease. He works at the genetic breeding station in Toulouse, France, where a major genetic strategy is ongoing to eliminate scrapie by introducing resistant rams to flocks across the country.
France has about 9.4 million sheep, mostly meat breeds.
Sheep producers asked the government for help because there were high levels of scrapie in certain regions in the Pyrenean mountains and Roquefort areas.
France decided in 1998 to breed the problem out of its flocks.
Researchers began by learning if there were genetic differences and then seeing if animals considered sensitive had the same genotype as the control animals.
They learned that among the dairy breeds, the Lacaune seemed the most resistant while the Manech red face was most susceptible.
They found that 80 percent of the animals that die of scrapie had what was called the QQ allele from the parents. Researchers never saw deaths in homozygous RR.
Within five years they learned they could reduce the amount of scrapie in a flock by using rams with the RR sequence.
“The genetic structure changed when we started using resistant rams,” Barillet said.
France decided to embrace the genetic strategy in 2001 with a full launch in 2002. The government provided money to test as many as 100,000 rams per year and by 2003 had a quick increase in the desirable genes. By 2004, 80 percent of dairy ewes were born of RR rams.
In addition, there is active surveillance in abattoirs and fallen stock using rapid tests.
Government compensation helps farms restock with resistant animals bred at specialized breeding stations.
Farms must use resistant rams by breeding naturally or with artificial insemination to eradicate the QQ allele.
