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Genetics of Scrapie

Scrapie is a 100% fatal, degenerative disease affecting the central nervous system of sheep and goats. The disease has been reported in countries throughout the world with few notable exceptions (Australia and New Zealand). The first case of scrapie was discovered in the United States in 1947. The current incidence is 2/10 of 1 percent or 1 in 500 U.S. sheep. The incidence of scrapie in the U.S. goat population is not currently known.

While bovine spongiform encephalopathy (BSE or “mad cow disease”) is believed to be caused by the consumption of contaminated feed (meat and bone meal), scrapie is transmitted during lambing when lambs come into contact with infected placenta and birth fluids from infected ewes . Rams can get scrapie, but are not known to transmit scrapie.

Scrapie is NOT caused by genetics, but the genetic make-up (DNA) of an animal determines whether it will get scrapie if it is exposed to the infective agent. In other words, if a genetically susceptible lamb is exposed to a scrapie-infected placenta, it will develop scrapie. It takes from 2 to 5 years after exposure for an animal to show clinical signs of scrapie. If a genetically resistant lamb is exposed to a scrapie-infected placenta, it will not develop scrapie. No resistant genotypes have been identified in goats.

How the Genetics Work
Sheep have one pair of genes that affects scrapie susceptibility. The pair of genes are known as PRNP genes (PrioN Protein genes). Each sheep has two copies (one pair) of the PRNP – one copy from each parent. PRNP exists in all animals with small differences between species. All genes are made up of codons. Each codon instructs cells to put a specific amino acid at a particular location when building a protein molecule. The prion protein molecule (produced by PRNP gene) has 254 amino acids. The locations of the 254 amino acids are numbered 1 through 254. In the PRNP gene, three codons affect scrapie susceptibility: 171, 154, and 136.

Codon 171 is the major determinant of scrapie susceptibility in the U.S. Codon 136 affects susceptibility in sheep exposed to some scrapie types. Codon 154 plays a minor role and is often not used in the U.S. Codon 136 programs for the amino acids Valine (V) and Alanine (A). Codon 171 programs for the amino acids Glutamine (Q), Arginine (R), Histidine (H), or Lysine (K). Q, H, and K are considered to have the same susceptibility and are reported as Q by most labs.
The genotypes of sheep in the U.S. are written in two ways: 1) letters of the amino acids, AA QR, AV RR, etc.; or 2) Codon number followed by the corresponding amino acids: 171QR, 171RR, etc.

Ewe Genotype
Ram Genotype
AQ
VQ
AR
AQ
AA QQ
AV QQ
AA QR
VQ
AV QQ
VV QQ
AV QR
AR
AA QR
AV QR
AA RR
  Highly susceptible
  Rarely susceptible
  Resistant

Genotyping (DNA testing) can be used to determine a sheep’s susceptibility to scrapie. A simple blood (or tissue) test is all that is required. A sheep’s genotype never changes so only one test should be needed. Genotype can be determined at any age. It is important to note that genotyping only measures susceptibility to scrapie, not whether the animal has scrapie.

Producers should consider using scrapie genetics (genotyping) as a management tool if:

• They have a breed in which scrapie is prevalent and they have purchased ewes of unknown scrapie status.
• They have purchased ewes from an infected flock.
• They have observed signs of scrapie in their flocks in the past.
• Customers are requesting scrapie-resistant breeding stock.
• They wish to provide scrapie-resistant breeding stock to their customers.
• They are forward-thinking!

Scrapie Genotypes

AA RR - resistant
AA QR - rarely susceptible
AV QR - much less susceptible
AA QQ - highly susceptible
AV QQ - highly susceptible
VV QQ - highly susceptible
APHIS APPROVED LABS FOR SCRAPIE GENOTYPING

Gene Check
800-822-6740
www.genecheck.com

GenAlysis
888-300-4363
www.genalysislaboratory.com

Genmmark (Infigen)
877-766-3446
www.genmarkag.com

Source: Scrapie Control: Genotyping – A New Tool For Producers. A PowerPoint presentation produced by the National Scrapie Education Initiative (available online at http://www.animalagriculture.org/scrapie).