Potential to Breed for Scrapie Resistance
By Lilian Schaer, reprinted courtesy of Ontario Goat.There is now enough information about goat scrapie susceptibility to begin breeding programs that can build resistance to the disease in the Ontario goat herd. That’s according to Dr. Bradley White of Trent University in Peterborough, who along with Mubrouka Elharram, project coordinator, led a research project into scrapie susceptibility and genotyping of Ontario goats that
wrapped up at the end of 2016.
Scrapie is a slow-moving but fatal degenerative central nervous system disease in sheep and goats that is related to Bovine Spongiform Encephalopathy (BSE) in cattle and Chronic Wasting Disease in deer. To date, testing for scrapie in goats is only possible on dead animals and no treatment or vaccine is available. The disease can be spread by positive animals that don’t show
any symptoms of the disease, and all goats on farms where scrapie is found are destroyed. When the disease was found in two large Ontario goat herds in 2014, White, in collaboration with Dr. Gordon Mitchell of the Canadian Food Inspection Agency (CFIA), began analyzing the DNA extracted from affected animals. This resulted in the discovery of the alleles (different forms of genes) in goats that are associated with an animal’s resistance or susceptibility to developing scrapie.
Scrapie is caused when proteins change shape; the scrapie protein can flip into an abnormal form, known as a prion, which is an infective agent. Prions are shed in bodily fluids and are resistant to degradation. Gradually, prion proteins accumulate in brain and nervous tissue, causing abnormal behaviour in the animals.
Until now, studies into scrapie in goats have been done primarily in Europe and only with small numbers of infected animals. As well, there is a low amino acid variation of the prion protein gene (PRNP) in goat herds in general. That all changed with the discovery of those two scrapie infected Ontario goat herds three years ago. One herd of 331 Saanens over 12 months of age had
66 affected animals, and the other herd of 130 Saanens over 12 months of age contained 13 infected animals, allowing for the complete DNA sequencing of significant numbers of both affected and unaffected animals.
The next step was determining the status of Ontario’s goats, which led to the launch of the Ontario goat herd genotyping study to establish a baseline of scrapie resistance and susceptibility in Ontario’s goat population. Approximately 1,500 animals from 74 farms randomly selected by Ontario Goat were sampled. Nasal swab kits were distributed by Ontario Goat and returned to the Natural Resources DNA Profiling and Forensic Centre at Trent University for testing.
Two amino acid positions in goats, 211 and 222, have been implicated in most studies as indicators of scrapie susceptibility. According to White, ten different alleles showed up in all the goats his team worked with across the province, with some occurring at higher frequency than others. This includes Q (the amino acid glutamine), R (arginine), S (serine) and K (lysine). For example, Q is scrapie resistant and R is susceptible, so researchers looked for the frequency of Qs and Rs in all the Ontario herds they tested.
“We have looked at all positions: when you have Q/Q at 211 you don’t get infected animals, R/Q is rare infection, R/R is animals that are likely to become infected,” White said. “If you have a prevalent herd that is exposed to the infective agent, the proteins will start flipping and you will end up with scrapie disease.”
The genotype of goats on farm determines the scrapie risk level, White said; however, simply having goats with high susceptibility doesn’t mean a herd will end up with the disease unless the disease-causing agent is also present on the farm.
Overall, White’s research found that Saanens had the highest frequency of Q211 (23.6 per cent) followed by Alpines (16.8 per cent), whereas Boers had only one per cent Q211 and Nubians did not have any. However, both Nubians and Boers had a high frequency of S146 (20.9 per cent and 30.2 per cent respectively), which was found either at very low levels or not at all in all
other breeds tested.
The high frequency of resistant Q211 in dairy goats provides the best opportunity to move Saanen and Alpine goat herds to higher resistance. Q211 is at low frequency in meat breeds but S146, which is high in Boers and Nubians, provides the best opportunity to move these herds to more resistance.
“If you have Boers which show no resistance at 211 or 222 and you have a closed herd, you won’t be bringing in any resistance, but you’re also ensuring that you’re not bringing in the disease trigger,” White said. “So you can start breeding with animals that lower susceptibility to increase your chance of resistant animals.”
This means producers can increase the amount of resistance in a herd through breeding R/R at 211 females with Q/Q semen or bucks to end up with R/Q animals that have a higher level of resistance (figure 1).
However, what remains to be seen, according to White, is to determine how resistant those R/Q animals actually are, something that CFIA will be working on. In sheep, amino acid variation at three gene positions (136, 154 and 171) is associated with scrapie resistance and susceptibility. Whether or not an animal in an affected herd is destroyed depends on which amino acids are found in those three positions; CFIA looks at this information when depopulating a sheep flock where scrapie has been found. Sheep breeding programs now use this information to select for highly resistant animals. It’s important to note that in sheep, CFIA only lets fully resistant animals survive on an affected farm and all others are culled.
“We aren’t there yet with goats, but this is the final outcome we will be looking at and are on a pathway towards,” White said, adding that policy decisions regarding goats and scrapie will be based on the outcomes of CFIA’s work, which will also have impact on import and export opportunities for goats. There’s still a tremendous amount of research left to do with goat breeds, according to Ontario Goat, so it is important to maintain scrapie as a research priority to help ensure future growth and sustainability of the industry.
For more information on Dr. White’s research results, or on how to test, please contact Ontario Goat by calling 519-824-2942 or emailing firstname.lastname@example.org.
The Ontario Goat scrapie genotyping project was funded in part by Growing Forward 2, a federal-provincialterritorial initiative, administered by the Agricultural Adaptation Council, and Centre of Excellence for Goat Research and Innovation.
|Look for the following logos on our animals pedigrees to see their Genetic Scrapies Resistance Status if they have been tested.|