Warmblood Fragile Foal mutation found in thoroughbreds, but not a breakdown cause

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None of the horses in the study had two copies of the Warmblood Fragile Foal mutation and only one of the 22 catastrophic breakdown cases carried the WFFS allele.
None of the thoroughbreds in the study had two copies of the Warmblood Fragile Foal mutation and only one of the 22 catastrophic breakdown cases carried the WFFS allele.

Despite the presence of the Warmblood Fragile Foal mutation in some thoroughbred racehorses, researchers have ruled it out as a cause of bleeding disorders or poor bone formation leading to catastrophic breakdowns.

A study led by Dr Rebecca Bellone, Director of the Veterinary Genetics Laboratory at the University of California, Davis, showed that fewer than four percent of thoroughbreds tested carried the allele for Fragile Foal Syndrome.

Bellone and colleagues have published the results of a study that investigated the frequency of the Warmblood Fragile Foal Syndrome (WFFS) mutation in Thoroughbreds. The study demonstrated that the WFFS mutation is not a genetic risk factor for catastrophic breakdown and is present at only a very low frequency in thoroughbreds.

WFFS is inherited in an autosomal recessive fashion, meaning that a horse needs to have two copies of the mutation to be affected. WFFS is a fatal genetic defect of connective tissue characterized by hyperextensible, abnormally thin, fragile skin and mucous membranes. More than 700 Thoroughbreds were tested for the WFFS mutation, including 22 catastrophic breakdown fatalities. The allele frequency among all samples was 1.2 percent and the carrier rate (or horses with one copy) was 2.4 percent. None of the horses in the study had two copies of the mutation and only one of the 22 catastrophic breakdown cases carried the WFFS allele.

“We have not identified any Thoroughbreds who are homozygous for the Fragile Foal mutation. Based on all of the available data, it is highly unlikely that this mutation has anything to do with bleeding or breakdowns,” Bellone said.

The UC Davis Veterinary Genetics Laboratory offers the WFFS genetic test and recommends testing for all Warmblood breeds since the carrier rate in Warmbloods is estimated to be around 9-11 percent. Since the allele was found to be present in the Thoroughbred population, albeit at a low frequency, genetic testing could eliminate the possibility of breeding two carriers with the potential of producing an affected offspring.

Warmblood fragile foal syndrome type 1 mutation (PLOD1 c.2032G>A) is not associated with catastrophic breakdown and has a low allele frequency in the Thoroughbred breed. Bellone, R. , Ocampo, N. , Hughes, S. , Le, V. , Arthur, R. , Finno, C. and Penedo, M. (2019), Equine Vet J. Accepted Author Manuscript. doi:10.1111/evj.13182.

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