Fragile foal syndrome (FFS), also known as “warmblood” fragile foal syndrome type 1 (WFFS), is a condition that causes late-term abortions, stillbirths, and nonviable foals, primarily in warmbloods. This recessive, autosomal hereditary genetic defect affects connective tissue. The carrier frequency is 11-17% in warmbloods and 2.4% in Thoroughbreds. The disorder does not appear to affect embryonic death in the first 42 days of pregnancy.
Matings of homozygous individuals result in foals that are unable to survive outside the uterus. Notably, affected foals have very fragile skin and missing areas of skin on the lower limbs and flanks. Their joints are hyperflexible, making them unable to stand.
Interestingly, heterozygous carriers are favored for their gaits and rideability; hence, the genetic disorder persists in warmblood populations, especially in dressage horses.
How Does WFFS Affect Equine Pregnancies?
A German retrospective cohort study investigated how WFFS affects pregnancies. The researchers examined whether WFFS matings result in early pregnancy loss and explored the differences in pregnancy success when both parents are allele carriers compared to a single parent.
The researchers examined breeding records from 177 mares on a stud farm through 2,682 estrous cycles. The mares were healthy and had more than three live foals through embryo transfer in at least one year between 2016-2019. Matings were assigned to four study groups, depending on the WFFS carrier status of the mare and stallion:
- 46 N/WFFS mares x N/WFFS stallions leading to homozygous foals.
- 511 N/WFFS mares x N/N stallions.
- 191 N/N mares x N/WFFS stallions.
- 2,149 N/N mares x N/N stallions.
Of 177 mares, 31 (18%) were heterozygous for the WFFS allele. Of the 114 stallions, 14 (12%) were carriers of the WFFS allele. The study reports:
- WFFS impacts pregnancy only when both parents carry the WFFS allele (N/WFFS x N/WFFS).
- There “were no significant differences in reproductive outcome” if only one parent carried the WFFS allele.
- WFFS has no effect on the embryonic phase. Embryonic loss in this study was 6-10.5%, which is comparable to Thoroughbred embryonic losses (up to 15.5%).
- The highest embryo recovery and pregnancy rates occurred when both mare and stallion possessed the allele; however, nearly two-thirds of these fetuses/foals perished.
- Higher fetal losses and mortality within 24 hours following birth occurred when both parents carried an allele (N/WFFS x N/WFFS). Between Days 42-300 of gestation, nearly 54% of these fetuses perished compared to 7.9-9.2% in the other study groups.
- Fetal losses occurred in both early and late gestation. Inbreeding is a known contributor to mid- and late-term pregnancy loss and could factor into high fetal loss in N/WFFS x N/WFFS matings.
- Foal mortality in the peri- and postnatal periods was 25% for N/WFFS x N/WFFS matings compared to 2.6-3% in the other groups.
- N/WFFS x N/WFFS matings resulted in a 35.5% live foal rate. The other groups had live foal rates of 81-84%. The authors note that the high fetal losses came from recipient mare pregnancies and not the breeding mares. However, the other groups going through embryo transfer showed “no significant difference in pregnancy success between recipient and breeding mares where neither or only one parent carried the WFFS allele.”
Final Thoughts
The authors concluded that matings of two WFFS allele carriers increase fetal loss rates and risk peri- and postnatal mortality in warmbloods.
Reference
Kehlbeck A, Blanco M, Venner M, et al. Warmblood fragile foal syndrome: Pregnancy loss in Warmblood mares. Equine Veterinary Journal Oct 2024; DOI: 10.1111/evj.14435
Related Reading
- Factors Affecting Pregnancy Rates in Mares Bred With Frozen Semen
- UC Davis First to Produce Equine Embryos by In Vitro Fertilization With Frozen Sperm
- Disease Du Jour: Contagious Equine Metritis (CEM)
Stay in the know! Sign up for EquiManagement’s FREE weekly newsletters to get the latest equine research, disease alerts, and vet practice updates delivered straight to your inbox.
