Location: Livestock Bio-Systems
Title: Sheep GEMS update: Using genotypes to reduce Ovine Progressive Pneumonia (OPP) susceptibiltyAuthor
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Submitted to: Sheep Industry News
Publication Type: Trade Journal Publication Acceptance Date: 8/16/2024 Publication Date: 8/30/2024 Citation: Freking, B.A. 2024. Sheep GEMS update: Using genotypes to reduce Ovine Progressive Pneumonia (OPP) susceptibilty. Sheep Industry News. ASI Weekly Newsletter. August 30, 2024. p.2-3. Interpretive Summary: Technical Abstract: In the United States, Ovine Progressive Pneumonia (OPP) causes extensive financial losses due to decreased ewe productivity of about 20 percent and eventual culling or death of breeding stock. Surveys of U.S. sheep have shown that 36% of sheep operations have infected animals and 24 percent of all animals tested were seropositive. Once infected, sheep remain infected for life, and there is no effective treatment or vaccine. Sheep producers can eradicate OPP from a flock in two ways. First, is by serological testing and removing all infected sheep with the obvious reduction in genetic diversity and reduced opportunity for making gains for other economically important traits. Second, is creation of a second separate negative flock by testing and isolating all young ewe lambs after weaning until serological testing at 9 months of age. As time goes on, the second flock ultimately replaces the infected flock, but with less impact on loss of genetic diversity or gains on other traits. While effective, these management procedures are time consuming, expensive, and do little to improve your flock in terms of genetic resistance to OPP. Thus, OPP-free flocks may continue to remain genetically predisposed to the virus causing OPP infection, with risk of infection if exposed to infected sheep. Genetic variation within the ovine gene known as TMEM154 is associated with reduced susceptibility - although not complete resistance - to OPP infection. The testing for this relative susceptibility requires consideration of the entire protein rather than evaluating a single genetic marker. Those markers are more formally known as nucleotide polymorphisms (SNP). Variations at multiple SNP define the form of the TMEM154 gene expressed in each animal. The four most common versions of the TMEM154 protein, called haplotypes, in American sheep were designated “1” through “4” and represent 97 percent of the animals tested. The occurrence of one or two copies of either haplotype “2” or “3” is associated with increased susceptibility to OPP virus infection. Conversely, animals with homozygous combinations of haplotype “1” and/or “4” have decreased susceptibility. In fact, such animals are nearly 10 times less likely to become infected with OPP than those carrying either haplotype “2” or “3”. The haplotype “4” version - while favorably impacting susceptibility to OPP results in a deletion of the THEM154 protein. When homozygous, it results in animals with a complete knock-out and loss of function of the THEM154 protein. The genetic markers used to identify TMEM154 haplotypes are drawn from the genotyping platform used in Sheep GEMS. That entire platform has 50,000 SNP. The reliability of these TMEM154 haplotypes is being assessed in two ways. First, they were compared with known TMEM154 genotypes on a set of animals. They matched perfectly to previously recorded genotypes. Second, the SNP used to determine an animal’s genetic status for OPP susceptibility appear multiple times on the genotyping platform. When determining an animal’s TMEM154 haplotype, the consistency of those repeated SNP is checked. This second strategy resulted in 92.7 percent of the animals having genetic status determined and reported for TMEM154 for 15,586 sheep evaluated in the four breeds contributing to Sheep GEMS. Genotypic status at TMEM154 differs among Katahdin, Polypay, Rambouillet, and Suffolk breeds. Nearly 60 percent of Katahdins and Suffolk sheep contained at least one of the highly susceptible alleles relative to OPP. On the other hand, in Polypay is at 8 percent, and Rambouillet, 15 percent, were highly susceptible. It might be tempting to use the TMEM154 genotype as a primary selection criterion. However, we recommend a more balanced approach, where this information is incorporated into an existing breeding program. This allows breeders to continue to make genetic p |
