Warmblood fragile foal syndrome: Pregnancy loss in Warmblood mares

Abstract Background Fragile foal syndrome (FFS) or warmblood FFS Type 1 (WFFS) is a recessive, autosomal, hereditary, genetic defect causing late abortions, stillbirths and non‐viable foals. Whether early pregnancy losses occur is unknown. Objectives To investigate how WFFS affects pregnancies and whether early pregnancy losses occur in WFFS matings and if there is a difference in pregnancy success between matings where both parents were allele carriers and those where only mare or stallion were WFFS carriers. Study design Retrospective cohort study. Methods Breeding records from a Warmblood stud farm were evaluated (2016–2019) and 2682 cycles of 177 mares analysed. Matings were assigned to four study groups, depending on the WFFS carrier status of the mare and stallion: N/WFFS × N/WFFS ( n  = 46), N/WFFS (mares) × N/N ( n  = 511), N/N × N/WFFS (stallions) ( n  = 191) and N/N × N/N ( n  = 2149). Results There were no differences in pregnancy rates between matings with only one WFFS carrier and those with non‐carriers. When comparing N/WFFS × N/WFFS matings to those with only one or no WFFS‐carrier, there were no increased pregnancy losses in the embryonic phase (day <42). By contrast, the N/WFFS × N/WFFS group had significantly more abortions (53.8%) than the other three study groups (7.9–9.2%; adjusted p  < 0.0001). The higher losses were evenly distributed between the early (D42–150: 22.7% losses, n  = 5) and late (D150–300: 29.4% losses, n  = 5) foetal phase. The live foal rate per pregnancy for N/WFFS × N/WFFS matings was lowest (34.5%) and differed significantly (other groups 81%–84%; adjusted p  < 0.0001); the peri‐ and postnatal mortality was higher (25%) compared with the other study groups (2.6%–3%). Main limitations Low sample size in N/WWFS × N/WFFS matings, unbalanced study design, outcomes are descriptive, all mares from one stud farm. Conclusions The mating of two WFFS carriers may lead to increased risks of pregnancy and foal loss.