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Sheep Color Genetics
Sheep Health Issues
Please note: This information is compiled from a large number of websites, programs, and other sources of information. Every effort has been made to ensure it is up-to-date and accurate, including review by Dr Joe Rook, sheep extension veterinarian at the Michigan State University (MSU) College of Veterinary Medicine, yet clearly explained so that laymen can understand the issues involved. If you have any questions regarding this presentation, be sure the visit the websites listed at the bottom of the page. Consult with your vet if you are uncertain about how to proceed with your own flock.
Scrapie is a degenerative neurological disease of sheep that is always fatal. It one of a group of diseases including "Mad Cow Disease" or Bovine Spongiform Encephalopathy (BSE) in cattle and new variant Creutzfeldt-Jakob disease (nvCJD) in humans. These diseases are called Transmissible Spongiform Encephalophathy (TSE) diseases.
The infectious agent is a prion (pronounced "pree-on"), or small particle of protein, present in the nervous system of its victims. When these prions are consumed or otherwise contracted by people or animals, the prions can infect the victims with TSEs, although it can take many years for the effects to become noticeable.
Scrapie is the most common TSE, and is found in sheep and goats. It has been known for about 250 years, being first described in 1732, with the first known case in the US occurring in 1947. The disease got its name from one of the later symptoms of the disease, when the infected animal scratches itself against fences, poles, etc. There is more than one strain of scrapie known in Europe. In the US, however, only type A scrapie has occurred.
We don't yet know for sure how scrapie is transmitted from one sheep to another. Nor is there yet a widely-accepted (or USDA approved) live-animal test--right now, all that can be done is slaughter suspected sheep and examine their brains to look for the tell-tale holes, or culture their tissues and, a year or more later, be able to see if mice innoculated with the culture develop their own scrapie-type illness. The 'third eyelid' test is being used, but is not yet approved as an official live animal test in the US.
Why is scrapie important?
With so many unknowns, why be bothered about scrapie? The problem is that there are so many unknowns. How do sheep get scrapie? How do they pass it to other sheep? Can they get other TSEs? Have scrapie-infected sheep parts in feed caused other animals to come down with TSEs? And the bottom line for human safety: Can any form of animal TSE cause TSE in humans? These are serious problems that must be researched. In the meantime, the best protection is prevention.
A related problem is that sheep who are infected with scrapie don't usually show symptoms until they are several years of age. In the meantime, particularly if passed from ewe to lamb before the lamb is born or immediately afterwards, many additional sheep could have become infected. Lambs don't show symptoms yet, but when slaughtered, their infection may enter the food chain.
Important: Prions are resistant to most standard forms of sterilization, including cooking heat and ultraviolet light.
Most often, scrapie symptoms begin to show when the sheep is between two and five years of age. It may have the disease prior to that time, and be able to infect other animals, but not show symptoms. Some cases have been known in older and younger animals.
A few of the early symptoms may include nervousness, jumpiness, or hypersensitivity. Over a period of months (typically 1-6 months) these symptoms may become more and more obvious. Eventually the sheep may develop tremors, may lose weight even though its appetite is normal, and may lose wool due to rubbing (scraping) itself against objects.
The sheep may also show an unusual gait when walking or running. It may have a high-stepping gait, hop like a rabbit, or sway in the back end. It may also have what appear to be convulsions when startled.
Because some of these symptoms may mimic other problems--sheep commonly scratch themselves against fence posts, for example--it may be necessary to test for and eliminate several other diseases before a diagnosis of scrapie can be made.
As with many problems, an ounce of prevention is worth a pound of cure. Sheep can be bred for genetic resistance to scrapie. Here's a brief summary of how this works:
Every sheep has pairs of chromosones in its body, which are part of its DNA. Chromosones determine how the animal will grow and develop. In the case of scrapie, one chromosone has been identified which helps to tell us how resistant to scrapie that animal may be.
There are three main codons, or specific locations on that chromosone, that are tested for scrapie resistance. They are:
Every codon has exactly two pieces of information present. But there can be more than two different bits of information that potentially could locate at that codon. These are called alleles. The alleles may both be the same at that codon, or they may be different from each other, but there are always two. One comes from the sire, one from the dam, of the animal.
Each allele tells the sheep's body to do make a specific amino acid. For reasons not yet understood, these patterns of amino acids appear to prevent or allow infection with scrapie when the animal is exposed. Here's a chart to show what each allele is believed to do:
It should be noted that these genes are inherited in groups. For example, R at 171 is always found with A at 136. But this ideal scrapie resistant genotype has not yet been found together with the 154 H allele which delays the onset of symptoms. As far as is known, A R H at 136, 154, 171 is not found in sheep.
Genotypes and Risk Levels
US and European scrapie programs use slightly different systems of noting the genotypes of tested animals. The reason is that in North America there is only one strain of scrapie. In Europe, there is more than one type of scrapie, and codon 154 is important in identifying sheep resistant to those types. For the purpose of reference and comparison, these systems are listed here.
No sheep with RR at codon 171 have been known to have scrapie. Only one sheep has ever been found to have scrapie who also had QR at codon 171, and in that one case, the test was conducted before hitidine (H) was known to exist at 171, so the results are uncertain. Many factors concerning susceptibility and resistance are still not understood. But in general, RR genetics is believed to provide protection from the disease.
In the Michigan Scrapie Risk Reduction Program (MSRRP), DNA testing is done for the genes at codons 136 and 171. The process of testing is quite simple, and can be done by any vet; the shepherd does not have to be in any scrapie program either to have the sample collected or the test processed by a lab. The cost per test is about $25 when done at the Michigan State University Animal Health Diagnostic Lab (MSU AHDL).
Other labs may require different procedures for DNA testing of scrapie resistance genetics. Talk with your vet to learn more.
Genotyping in our flock
This policy has been quite successful. In 2000 we already had two ram lambs born with RR AA genotypes, as well as one ewe lamb. In 2001, five RR AA ram lambs were born, and two ewe lambs. In fact, the lamb shown above was born to two QR parents, but is RR himself. At the same time, we are not sacrificing the quality of the sheep by this program of focusing on genetic resistance. See our flock goals for more information on all our priorities.
One restriction applied by the MSRRP is that we cannot move our QQ (codon 171) animals off the farm except directly to slaughter. We evaluate all our QQ animals for their potential contributions to our breeding goals. Those who don't make up for QQ by other positive attributes are slaughtered for meat when they are about one year old.
Again, it's important to note that a sheep with a QQ genotype at codon 171 does not necessarily have scrapie! It is simply more likely to contract the disease if exposed to it. At this point, we have no reason to believe there is scrapie in our flock, so our QQ animals are safe to breed with and eat. When slaughtered, the central nervous system (brain, spinal cord, etc) are removed and destroyed so no contamination will accidently occur.
Status of Monitored Flocks
The Voluntary Scrapie Certification Program (run by the USDA) and the MSRRP both also monitor the health of the animals in enrolled flocks. The theory is that if the flock has no incidence of scrapie in five years of looking for it, that the flock is scrapie-free.
There are levels of certification in the federal program (and some state programs), starting with 0 and working up to 5, which indicates the flock is officially free of scrapie. When new animals are brought in, the entire flock drops to the level of the farm the new sheep came from. So, if your flock is at level 3, and you buy a sheep from a farm which is at level 2 or lower, your flock status drops to that same level. On the other hand, if you buy a sheep from a flock with level 3 status, you remain at level 3; while if you buy a sheep from a level 4 or 5 flock, you also remain at level 3.
In our opinion, using this method of determining whether a flock is free of scrapie works only if the flock is completely closed during the five-year period, and afterwards brings in stock only from other closed flocks. As the number of flocks enrolled in the VSCP is quite limited as yet, this is difficult to do.
The Proposed Federal Scrapie Program
Although the current federal scrapie program, the Volunteer Scrapie Certification Program, places no emphasis on resistant genetics, the proposed program which is expected to become law does. For your own copy of the proposed rules, see the snail mail address below. A copy is available online in PDF format on the federal scrapie site. Be aware that it's 112 pages and takes quite a while to download. Sending for a free hard copy might be easier.
To get your own copy of the way the federal scrapie rules will be implemented when the time comes, snail mail to:
Dr. Diane Sutton
Resources on the Internet
Following is a selection of resources that have helped us reach this understanding of scrapie. We recommend you read these carefully to inform yourself of the issues. Then consult with your vet and state authorities to decide what is best for your flock.