A newly published review in the Journal of Equine Veterinary Science explores the science, prevalence and policy implications of Fragile Foal Syndrome, FFS, raising important questions about the future of sport horse breeding.
Titled Fragile Foal Syndrome: Manifestations, Heterozygous Advantage and the Future of Breeding Policies, the paper examines how advances in genetic testing have reshaped our understanding of this lethal recessive condition and how breeding decisions may influence its persistence.
What is Fragile Foal Syndrome?
Fragile Foal Syndrome is a lethal autosomal recessive genetic disorder caused by a single nucleotide mutation in the PLOD1 gene. Foals must inherit two copies of the mutated allele, one from each parent, to be affected.
Clinical signs in affected foals include:
- Extremely fragile, thin skin that tears under minimal stress
- Hyperextendable joints
- Spinal and connective tissue abnormalities
Recent research suggests that while some affected foals are aborted during gestation, others are born alive but die shortly after birth due to incompatibility with life outside the uterus.
Before the development of a genetic test in 2013, many cases were likely misdiagnosed as other neonatal disorders. Since testing became available, the scale of the issue has become clearer.
How common is it?
The review reports that the mutated allele is present in:
- 11 to 30 percent of Warmbloods
- 2 to 3 percent of Thoroughbreds
Originally referred to as Warmblood Fragile Foal Syndrome, the condition was renamed after confirmed cases were identified in other breeds.
Despite the lethal consequences in homozygous foals, a relatively high number of actively breeding stallions are heterozygous carriers.
The heterozygous advantage hypothesis
One of the most controversial aspects explored in the paper is the suggestion that horses carrying a single copy of the mutated gene may have an athletic advantage.
Studies cited in the review report a positive correlation between FFS carrier status and Estimated Breeding Values (EBVs, particularly in elite-level dressage horses. This has led to the hypothesis of a heterozygous advantage, where carriers may benefit from traits that enhance performance.
If true, this could help explain why the allele has persisted at relatively high frequencies in certain sport horse populations, potentially as a result of balancing selection driven by human breeding choices.
A breeding crossroads
The authors highlight the dilemma now facing studbooks and breeding organisations. On one hand, there is demand for elite bloodlines and proven performance. On the other, there is an ethical and welfare responsibility to reduce the incidence of a lethal genetic disorder.
The paper calls for consideration of amendments to studbook regulations and breeding policies, aimed at:
- Promoting routine genetic testing
- Avoiding carrier-to-carrier matings
- Protecting equine welfare while maintaining genetic diversity
As equestrian sport continues to prioritise both performance and welfare, the review suggests that transparent, evidence-based breeding policies will be critical in preventing further proliferation of FFS.
The full paper is available in the Journal of Equine Veterinary Science. Click here to read.
