May 01, 2002

 

 The rationale for ridding U.S. of scrapie - May 1, 2002

Posted on April 15, 2002

 

 

Scrapie is uncommon in the United States. The disease has been diagnosed in only about 1,600 sheep and goats since it was first found here in 1947. So, why is the federal government accelerating the scrapie eradication program and pumping millions of new dollars into the campaign?

Courtesy of Dr. Michelle L. Crocheck, USDA-APHIS-VS-NVSL
The dramatic wool loss over the neck and back of this scrapie-positive Rambouillet-Cheviot cross ewe developed over the course of a single week. She had a hard time placing her hind feet (proprioceptive deficits) and reacted with the classic lip-smacking response when her back was rubbed.

For one thing, scrapie costs U.S. producers $20 to $25 million every year, much of it attributable to loss of sales from abroad.

Dr. Diane Sutton, national scrapie program coordinator, said, "We want scrapie eliminated by 2010 so we would be officially declared free by 2017. The primary reason for doing this is to minimize losses in the sheep industry and to make us more competitive in the world market—primarily to prevent such markets from being cut off or to stop it from damaging the industry in the future. It's also to mitigate any negative impact that scrapie may have on other ruminant products being exported."

Scrapie is classified as a transmissible spongiform encephalopathy. Even though there is no evidence that humans who consume sheep or goat meat or milk are at risk of contracting scrapie, countries are showing increasing concern over all TSEs as a result of bovine spongiform encephalopathy in cattle, feline spongiform encephalopathy, chronic wasting disease in deer and elk, mink encephalopathy, TSEs of exotic ruminants, and new variant Creutzfeldt-Jakob disease and other TSEs in humans.

According to Dr. Linda Detwiler, senior staff veterinarian with the Emergency Programs Staff of the Department of Agriculture, APHIS Veterinary Services, the concern is whether scrapie could lead to BSE in cattle and then put the sheep population at risk for BSE.

There's another worry. If naturally occurring BSE were diagnosed in sheep in the United Kingdom or elsewhere in Europe, would it spread the way BSE does in cattle—or the way scrapie does in sheep?

The scrapie prion is thought to be spread most commonly from the ewe to her offspring and to other lambs in contemporary lambing groups through ingestion of the placenta and placental fluids or cutaneously through abraded skin. Scrapie spreads to other flocks through the movement of infected or exposed sheep, primarily if they lamb in a new flock, so identifying those animals and restricting their movement helps prevent transmission.

Transmission of BSE in cattle is quite different. It is not spread by environmental exposure to the placenta and placental fluids of infected cattle, but by an infected animal being slaughtered, rendered, and fed back to other cattle or other species.

With BSE, the movement of incubating animals is important only if they have been slaughtered, rendered, and incorporated into feed. Then, the problem becomes the movement of BSE-contaminated feed.

Dr. Diane Sutton
Dr. Diane Sutton

In sheep experimentally infected with the BSE prion protein, the agent has been found in the brain, spinal cord, and spleen; throughout the intestine; and in the lymph nodes. Given this distribution of infected tissue, researchers are making the assumption that BSE in sheep would likely be transmitted similarly to scrapie (through ingestion of the placenta or placental fluids or cutaneously), not like BSE in cattle (through contaminated feed).

For that reason, Dr. Detwiler noted, the United Kingdom's government Web site recently invited comments as to whether—if BSE were confirmed in sheep there—the entire national flock should be depopulated.

"Why would they even suggest that?" she hypothesized. "In a worst-case scenario—if BSE were in sheep and it spread between sheep like scrapie does, a feed ban wouldn't control it like it controls BSE in cattle.

Dr. Linda Detwiler
Dr. Linda Detwiler

"Second, the distribution of infectivity is similar to scrapie. Hence, it would be in many tissues, from the lymph nodes to the peripheral nerves. So, what would be safe to eat from that sheep? Also, scrapie infectivity has been found soon after infection, so even in young lambs, some of these tissues may not be safe."

Although the incubation period is typically two to five years, the scrapie agent has been found in the visceral tissue of lambs as young as four months when tissue from a suspect lamb was injected into the brain of a mouse.

So far, BSE has not been found to occur naturally in sheep. Researchers have induced it in sheep orally with just a half gram of infected brain tissue, in both the genetic line that does and the line that doesn't get scrapie naturally.

Clinically and histologically the BSE agent when induced in sheep is like the scrapie agent. The only way to conclusively differentiate between scrapie and BSE in sheep is the mouse bioassay sequence, which takes two to three years.

In 1998, the Scientific Steering Committee of the European Union announced that the sheep and goat populations in the E.U. were most likely fed with the same meat and bonemeal as were the cattle that had contracted BSE. The meat and bonemeal was also incorporated into rations, especially for sheep with high protein requirements, such as milk sheep.

The U.K. has begun to collect brains from animals whose owners report an illness as scrapie and to put them into the mouse bioassay system. They must wait the two to three years to see whether the disease was scrapie or BSE.

The European Union instituted a ban effective in October 2000 on "specified risk materials." Now, when sheep and goats go to slaughter, the brain, spinal cord, and spleen can no longer be used for human or animal consumption, even if it appears healthy.

And in March, a French government agency issued an opinion that it is an urgent priority to be able to distinguish scrapie from BSE in small ruminants. The French agency called for additional resources for strain typing in mice and for research support into large-scale differential diagnostic tests for scrapie and BSE. The agency also recommended additional precautionary measures "in view of the potential presence of the BSE agent in sheep and/or goat flocks."

Dr. Roger Perkins
Dr. Roger Perkins

From her travels around the world, Dr. Detwiler knows firsthand that TSEs are a stigma. "Because of BSE and because of the of BSE in sheep, there's a push in the world to eradicate scrapie and other TSEs. For us to have scrapie and chronic wasting disease is a big stigma for our trade. Our meat and bonemeal and even pharmaceutical products are called into question."

Because of scrapie, some countries do not want certain U.S. exports, according to Dr. Roger W. Perkins, a senior staff veterinarian for the National Center for Import and Export, USDA-APHIS-VS.

Europe and the Eastern bloc counties ban live sheep and goats, embryos, and semen from the United States. New legislation is expected in the European Union that will restrict trade in pet foods and rendered products. The exporting country would have to certify that no sheep were ever rendered in a plant that developed the exported commodity.

In March 2001, China imposed a ban on U.S. sheep and goats and rendered products such as tallow and bonemeal. Around the same time, Japan—a country that itself has BSE problems—banned milk replacer containing tallow, presumably on the basis of BSE potential. Israel has prohibited the importation of U.S. goats in what was previously an active market. Argentina will not import U.S. sheep because of scrapie.

Mexico, one of the United States' biggest trading partners, requires heat treatment of rendered U.S. products. Each year, the United States exports to Mexico $270 million to $295 million in rendered products, $50 million to $70 million in pet food, and $45 million in feeder lamb and sheep.

"These are opportunities denied us," Dr. Perkins said. "We should be able to access some of these markets and let them grow, but [the TSEs] are barriers that prevent us."

To eliminate scrapie from the United States, Dr. Sutton said the USDA must have a commitment of funding from the administration. In fiscal 2001 and again this year, scrapie eradication was a line item funded at $2.9 million. Emergency funding was given in FY 2000 of $10; of that amount, the USDA spent $2.11 million in FY 2001, leaving $6.76 million for this year. The agency can request an additional $5 million this year if needed. For FY 2003, the USDA has requested $19.1 million, which will become available this October.