Sheep producers get a closer look at scrapie and genetic susceptibility

BURTON, Ohio – An otherwise ordinary-looking sheep is lying down when a sudden noise startles her.

As she stands, her back end sways and she breaks into a run that more closely resembles a hopping bunny than a running sheep.

When the farmer rubs her back to calm her, she starts to smack her lips and make a chewing motion.

This is the personification of scrapie, a fatal disease that leaves no surviving infected sheep or goats in its wake.

With persistent research and a program fiercely aimed at wiping out the fatal disease, progress has been made. And, like most of the modern world, this progress has gone in the direction of technology… where it’s all in the genes.

Coming to America. Scrapie entered the United States in 1947, Ohio Department of Agriculture veterinarian Dave Frew told sheep producers March 6 at a scrapie eradication update in Geauga County.

It came from Suffolk sheep from Canada, and in those early years, scrapie was not well understood.

There was a relatively low incidence of the disease until the late 1960s and early 1970s when Suffolk became more popular. The movement and expansion of the sheep caused scrapie to also increase, Frew said.

With this popularity also came higher prices for Suffolk sheep.

When one of these expensive rams died for unknown reasons, it was more likely that the producers would get diagnostic testing to find out what went wrong.

Thus, the parameters of scrapie were being realized.

In the years that followed the arrival of this deadly disease, producers feared having scrapie infect their sheep because it often meant doom for the entire flock.

If a sheep died showing any signs of scrapie, brain tissue was taken from the dead animal and scrapie tests were done.

When scrapie was indeed found, it often meant the death to most of the flock. This was because it was unknown which sheep were infected.

Third eyelid. At this point there weren’t any tests for scrapie that could be conducted on live sheep.

Approximately two years ago, the first live test was developed, Frew said.

This test, called the third eyelid test, was a giant leap because sheep could be tested without being killed.

However, as good as it seemed, there was a problem with the test.

The test is a biopsy of a piece of skin called the third eyelid, a lymphoid tissue where prions are found. If the test comes back positive, the sheep is scrapie positive, but if it comes back negative, it doesn’t confirm anything.

This lack of consistency is because sheep don’t usually test positive until they are showing outward signs. The incubation period ranges from two years to five years, which means a sheep could repeatedly test negative until it showed clinical signs.

Big move. The most recent development in genetics may be the biggest step toward the goal of eradicating scrapie.

Researchers have determined what sheep DNA combinations are most susceptible to scrapie.

This means producers can test, or genotype, their sheep to see which are resistant to scrapie.

This is particularly useful when a scrapie-positive sheep is found on the farm. The rest of the flock can be genotyped. Sheep with the resistant DNA type do not have to culled, thus saving more of the flock.

Before this test, the entire flock would be at risk of depopulation because it couldn’t be determined which sheep were susceptible to the disease. It often left only “a handful of sheep” on the farm, Frew said.

“Now we can eradicate the disease and not eradicate sheep at the same time,” Frew said.

Easily said. “It is estimated that, on average, 60 percent of a flock can be preserved when using a genetics-based plan compared to 25 percent when using a traditional plan,” according to information from the National Institute for Animal Agriculture.

Although the genetic combinations are complicated, what it boils down to is that sheep with a genetic sequence of RR are resistant to scrapie. QQ sheep are the most susceptible, and QR sheep are somewhat susceptible.

In addition, three main codons, 136, 154 and 171, affect scrapie susceptibility.

Who goes, who stays. Finding a scrapie-positive sheep on a farm means RR sheep, the resistant sheep, will not have to culled with the susceptible sheep.

When a scrapie-postive sheep is found on the farm, the producer has a choice. He or she can depopulate the flock or start a genetic-based flock plan. This is when the sheep would be genetically tested for their susceptibility to scrapie.

After the genetic tests, only the QQ sheep would be culled because of their susceptibility. Usually the producer can keep the RR and QR sheep.

This is particularly helpful to producers who have spent years fine-tuning their flocks’ genetics. Without the genetic testing, the entire flock would be depopulated.

Just because a sheep is QQ doesn’t mean it definitely is infected with scrapie, even if it is part of an infected flock. In fact, the most QQ scrapie-positive sheep Frew has found in an infected flock is 20 percent.

Yet, because they are most susceptible, they are culled in this situation.

Marketing tool. There are other reasons for genetic testing, too.

For example, Geauga County sheep producer Craig Fleck has never had scrapie in his flock, yet has still genotyped 30-40 sheep. Instead of using it because of the fear of scrapie, he uses it as a merchandising tool. People purchasing the purebred Dorsets like to know his sheep are scrapie free.

His sheep have tested RR, QQ and QR, but he isn’t worried about scrapie. Fleck and Frew both say that even if a sheep is genetically susceptible, there isn’t anything to worry about as long as scrapie isn’t present on the farm.

Fleck pays $14 for each genetic test, in addition to the cost of having the vet do the test and the blood’s shipping and handling.

Goats. Genotyping is not effective with goats.

Frew says there has been no evidence that the same genotyping for sheep holds true for goats. This means that all goats are killed in flocks with a scrapie-positive sheep or goat.

Long-term goals. Producers are hoping that this genetic testing and a continuing identification program will wipe out scrapie in the United States by 2010, according to the National Institute for Animal Agriculture.

After scrapie is eradicated, there will be a seven-year waiting period until the nation is recognized as scrapie free.

(Reporter Kristy Hebert welcomes reader feedback by phone at 1-800-837-3419, ext. 23, or by e-mail at khebert@farmanddairy.com.)


Related links:

www.animalagriculture.org/scrapie

www.aphis.usda.gov/vs/nahps/scrapie

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Producers’ options with a scrapie-positive animal

SALEM, Ohio – There are many options when scrapie is found in a sheep, according to a flow chart from USDA’s Animal and Plant Health Inspection Service.

If a positive animal is found, the entire flock is quarantined and all sheep are officially identified. After filing information that a scrapie sheep was found, wethers are removed from the quarantine.

Wethers and rams are not considered to be as much of a risk because scrapie is typically spread through lambing. Although a ram may have the disease, so far they have not spread it to another flock, according to Dave Frew, veterinary medical officer with the Ohio Department of Agriculture.

Two choices. After the wethers are removed from the quarantine, producers have a choice.

One choice is to completely depopulate with indemnity. This means all sheep and goats except wethers are euthanized and brain samples are collected from those animals older than 14 months.

The producer then collects payments from the government for his losses.

Since the incubation period for scrapie starts at around two years, sheep less than 14 months old will likely not show signs of scrapie.

Sales of intact animals from the year the scrapie-positive sheep was born are traced and the new owners are contacted.

If the producer plans on restocking with sheep or goats within five years, he or she must sign a post-exposure management and monitoring plan.

After each of these steps and a thorough sanitation process, the quarantine is removed, indemnity is paid and the source, infected or exposed status is removed.

Genetic test. The other choice when a scrapie-positive sheep is found is to do a genetic test on all sheep except wethers. The government is responsible for paying for these tests.

After the testing, all intact QQ sheep, offspring of female-positive sheep, positive sheep and suspects are euthanized. The only sheep kept are QR and RR sheep.

Again, the euthanized sheep are sampled, indemnity is paid and the remaining steps are the same.

The extra step with this process is that any sheep genetically testing QR are retested and the results are sent to another lab to confirm the results.

Starting soon, microchips will be placed in these QR sheep so they can be traced if there is a future problem, according to Frew.

Other situations. Many other situations can also occur, including a trace-exposed female animal and whether she is on the farm or whether she has lambed.

There are also scenarios for trace-exposed rams.

The above information is from a USDA flow sheet. To obtain a copy of the flow sheet, contact USDA/APHIS Vet Services at 12927 Stonecreek Drive, Pickerington, OH 43147 or 614-469-5602.

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Watch for these signs of scrapie

SALEM, Ohio – Scrapie is a slow degenerative disease that affects the central nervous systems of sheep and goats.

It is one of several diseases known as transmissible spongiform encephalopathies. Other examples of TSEs are mad-cow disease and chronic wasting disease.

The disease is most commonly spread from ewes to offspring and other nearby lambs through contact with the placenta and its fluids, according to USDA’s Animal and Plant Health Inspection Service.

Scrapie signs to watch for:

* wasting, thin and unproductive.

* subtle changes in behavior or temperament.

* head and neck tremors.

* lack of coordination, including swaying of the back end and hopping like a rabbit.

* lip smacking or chewing motion.

* scratching and rubbing against fixed objects.

* progressive weight loss.

Not all sheep have the same symptoms nor exhibit each of these signs. Other signs not mentioned may occur in some cases.

Sheep can appear normal when resting, but a sudden noise or stress may startle the infected animal into trembling or falling.

“Signs or effects of the disease usually appear two to five years after the animal is infected but may not appear until much later,” according to the inspection service.

“Sheep may live one to six months or longer after the onset of clinical signs, but death is inevitable.”

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