Maintaining early equine pregnancies with injectable altrenogest: How often?

Abstract

Altrenogest (ALT) mimics progesterone (P₄) so is widely used for maintenance of equine pregnancies. This is despite limited evidence that inadequate maternal P₄ is a significant cause of early pregnancy loss. Daily dosing of oral ALT is impractical for long-term use and unsafe given ALT carries human health risks. As a result, using longer acting injectable ALT products is becoming increasingly widespread. Unfortunately, while safer and more practical, there is limited evidence to suggest injectable products at the recommended ALT dosage can maintain early pregnancy in mares with insufficient endogenous P₄. Thirteen fertile mares, aged 8.4 ± 1.9 years, were used to assess frequency of administration of intramuscular (IM) ALT for early pregnancy maintenance. Mares were inseminated then checked for ovulation using trans-rectal ultrasound. Once ovulation was confirmed, mares underwent pregnancy diagnosis at d 14 and recruited if pregnant. Pregnant mares were randomly assigned to a treatment group. Treatment A: 2 injections of ALT (0.3 mg/kg IM) at 7-d intervals, n = 10; treatment B: 2 injections of ALT (0.3 mg/kg IM) at 5-d intervals, n = 10; and control: 0.044 mg/kg oral ALT (treatment control) once daily for 14 d, n = 5. All treatments commenced on d 17 of gestation. Mares were administered prostaglandin (PGF_2α; 250 µg Cloprostenol IM) 1 h after treatment to induce P₄ insufficiency, confirmed though commercial P₄ assay. During treatment, mares were blood sampled daily to assess P₄ and ALT concentrations. Mares underwent trans-rectal ultrasound 4 times a week until the embryo was aborted/nonviable or a fetal heartbeat was detected at the end of the treatment period, d 31 (Group A and control) or d 27 (Group B). Aborting mares were rebred. Pregnancy data were analyzed using a chi-squared test and P ≤ 0.05 was considered significant. Following PGF_2α injection, serum P₄ had decreased by 84.7 ± 4.8% within 24 h and declined to <1 ng/mL within 48 h. There were significant differences in pregnancy maintenance between groups (chi-squared 8.51, P = 0.014). Treatment B was better at maintaining pregnancies (9/10) than treatment A (3/10). There was no difference between treatment B and the control group (4/5). Injectable ALT administered every 7 d resulted in reduced pregnancy maintenance when compared with injectable treatment at 5 d intervals and daily oral treatment. If progesterone insufficiency is confirmed, administration of 0.3 mg/kg every 5 d is recommended.