Researchers examine inheritance behind heart condition in Standardbreds

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Researchers in Canada have identified Standardbred bloodlines that contributed to a regional spike of a debilitating heart condition among the breed.

Atrial fibrillation is a condition in which the heart’s upper two chambers beat rapidly and out of coordination with the lower chambers. This commonly causes poor blood flow and has a major effect on horse performance.

Researchers from the University of Guelph said admissions of Standardbred racehorses to the Ontario Veterinary College Teaching Hospital requiring treatment for atrial fibrillation began to increase in the early 1990s.

They set out to determine the marginal genetic contributions of ancestors to Standardbreds affected by the condition, and whether those ancestors contributed significantly more to the affected cohorts than to the control horses used in the study.

In total, the researchers identified 168 Standardbreds born between 1993 and 2007 that were admitted to the hospital for treatment for the condition.

Five randomly selected racing contemporaries for each admitted Standardbred comprised the control group, all of whom were assumed to not suffer from the arrhythmia.

The study team, writing in the journal PLOS ONE, found evidence through their pedigree analysis that several stallions contributed significantly more highly to groups of Standardbreds affected by the condition than to unaffected racing contemporaries.

“A number of these sires are related, being descendants of ancestors that contributed significantly more to groups of affected Standardbreds,” Flávio Schenkel and his colleagues reported.

“The numerous appearances of many of these stallions in the five-generation pedigrees of affected horses demonstrate the current popularity of these bloodlines.”

They found that the marginal contributions of 26 ancestors were significant, with 11 contributing significantly more to affected Standardbreds than the corresponding controls.

They said the arrhythmia appeared to be particularly prevalent in the descendants of one sire family. One broodmare also featured prominently.

The majority of the stallions that contributed significantly to affected lines were from pacing bloodlines, but one was from trotting bloodlines.

The close relationships of ancestors significant to affected Standardbreds indicated that there was a genetic background to atrial fibrillation in the breed. However, there was no indication of a simple mode of inheritance.

“Gathering more information on affected and healthy horses within these families, conducting genetic evaluations and genome-wide genotyping of horses, including influential sire lines, is the logical next step in determining the genetic background of atrial fibrillation in the Standardbred and better understanding how to manage genetic liability to the arrhythmia.”

Because atrial fibrillation has both economic and welfare implications, efforts should be made to decrease the number of Standardbreds with a potential to develop the disease, they said.

They described their results as simply a starting point, saying the findings provided researchers with a definitive direction for future studies of this disease.

It was possible that strategic breeding practices and avoiding one particular stallion’s bloodlines may help to decrease the incidence of the arrhythmia, but further investigation was required, they said.

Individual horses were not identified in the published paper.

Kraus M, Physick-Sheard P, Brito LF, Sargolzaei M, Schenkel FS (2018) Marginal ancestral contributions to atrial fibrillation in the Standardbred racehorse: Comparison of cases and controls. PLoS ONE 13(5): e0197137. https://doi.org/10.1371/journal.pone.0197137

The study, published under a Creative Commons License, can be read here

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