Wednesday, 30 November 2011

Scrapie Resistance in Sheep

I was assigned a genetics assignment that allowed me to pick  a topic realted to livestock genetics. I could go anywhere with this, some of my classmates choose teacup pigs, minaure Herefords, and sexed semen. I decided to go with something that our Sheep Student Managed Farm team had to test for this year, Scrapie Resistance in sheep.

Scrapie is a nervous system disease that affects sheep. It is one of several transmissible spongiform encephalopathies (TSEs), which are related to bovine spongiform encephalopathy (BSE) in cattle and chronic wasting disease of deer. Like other spongiform encephalopathies, scrapie is caused by a prion. Scrapie has been known since the 18th century and does not appear to be transmissible to humans.

Clinical Signs

The name scrapie is derived from one of the clinical signs of the condition, where an affected animal will compulsively scrape off their fleece against rocks, trees or fences. The disease apparently causes an itching sensation in the animals. Other clinical signs include excessive lip-smacking, altered gaits, and convulsive collapse.

This sheep has Scrapies

Testing for scrapies resistance in sheep is done through DNA testing the blood of the sheep. The cost of testing is approximately $40 per animal, and is done in Saskatchewan and Ontario. 

Why Test?
Our biggest international trade partners in the sheep industry, United States and Britain, are trying to eradicate scrapie from their flocks. In order to do so, they must ensure anything they buy has the resistant genotype.   However, you must get the DNA tested at a certified testing facility. To find one in Canada, visit the Scrapie Canada website.
Many people believe that if your sheep have the resistant genotype, they can and do not have the disease. This is false, they only way you can truly test to see if the sheep has the disease is to test their brain tissue. This test is simply to see if it is genetically resistant to the disease.

The plan aims to reduce and eventually erase small ruminant-TSEs (transmissible spongiform encephalopathies) from the national sheep flock by selecting on PrP genotypes. However, a focus on scrapie genotype alone is risky due to the chance that it leads to losses in genetic variablility.

 The SFCP [Scrapies Farm certification Program] National Standards were developed by the Canadian Food Inspection Agency (CFIA), in collaboration with the sheep industry, as the basis for Canada’s on-farm, voluntary scrapie control program. Its goal is to be an internationally recognized flock/herd scrapie control program for the sheep and goat industries. This program is unique as a CFIA approved disease control strategy. The CFIA's involvement is merely to guide the program to maintain international stnadards.

Scrapie may go undectected in a flock for serval years. . As well, with no reliable live animal test for detecting the disease in individual animals. Working with these restrictions, the SFCP is designed to gradually assess whether flocks are currently infected and to minimize the risk of contracting scrapie in the future. Flocks advance through the various levels of the program as the risk of scrapie infection decreases. Producers have the option of following one of three pathways under the program. In Pathway 1, the assessment of risk (level the flock has achieved on the program) is determined by the number of years that the producer has followed program requirements and scrapie has not been detected. In this pathway, flocks/herds advance one level for every year that the requirements are met. In Pathways 2&3, other technologies, such as genotyping for resistance to scrapie, are used in combination with disease surveillance to reach certification in a fewer number of years.

Requirements include:
  • Surveillance for the disease is made by submitting brain samples from all adult sheep and goats that die on-farm. If no animals die on farm during a 12-month period, a sample from at least one cull animal over 24 months must be submitted.
  • Producers must work with a veterinarian accredited with CFIA to deliver the SFCP.
  • Producers must make an annual, vet supervised inventory their flocks/herds and maintain documentation throughout the year on animals entering and leaving the premises.
  • The flock/herd must be closed to additions of female animals, except from flocks/herds on the same or higher program level. The source of rams, bucks and/or semen is not as restricted, although some conditions do apply in Pathways 2 & 3.

PrP is the genotype a breeder is looking for when try to eradicate scrapie from their herd. If you animals DNA test comes back with this geneotype, it means it is scrapie resistant. This DOES NOT mean that your animal is scrapie free, simply that they are resistant to this nervous system disease.

In conclustion, why not test for this disease in your breeding stock? It may cost a little bit of money to do so, but it will save you in the long run. If you ram has the genotype of non-resistance, it could pass this on to all of his offspring, forcing you to cull these animals. As well, it will help you to broden your market, and be able to sell international sheep meat.



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