Evaluation of salivary progesterone profiles as an indicator of reproductive status in equines

Diagnostic assays of reproductive hormones are usually performed by veterinarians on mares after inconclusive findings from trans-rectal palpation or ultrasound genital examination, or on intractable mares. Of the reproductive hormones, progesterone (P4) is one of the most commonly measured hormones in the field of equine reproduction. Serum concentration of P4 from mares is usually evaluated using an extracted radioimmunoassay (RIA). The use of RIA for serum P4 measurement usually involves an extraction process to dissociate the steroids in blood from carrier proteins, which bind more close to 90 to 95% of the steroid hormone present. Estimation of salivary steroid hormones presents an attractive alternative, since the steroids are not protein bound and represent the free fraction which appears to represent the biologically active form. The objective of this study was to develop and validate a sensitive and specific RIA for estimation of salivary P4 concentration and evaluate its usefulness in identifying the stage of the estrous cycle and early pregnancy in mares. Serum and saliva samples were collected on selected days of the estrous cycle and early pregnancy (up to day 65) from mares and were validated using a liquid phase RIA. Saliva samples did not require solvent extraction and were run without this processing step. The inter-assay coefficient of variation (CV) for low and high controls was 6.91% ± 0.81 (Mean ± S.E.) and 5.06 % ± 0.57 (Mean ± S.E.), while the intra-assay CV averaged 13.19% for saliva and 11.71% for serum. Serum P4 showed statistical significance with Friedman’s One Way ANOVA (p 0.05). Saliva: Serum ratio was elevated during the follicular phase of the cycle (35%), remained between 8 to 12% for the remaining duration of the cycle and showed a statistical significance (p > 0.05). As anticipated, there was a significant difference in the serum P4 levels by day of observation in the pregnant mares (p 0.05), only for days 1 and 3. Sa: Se ratio for pregnant mares showed significance (p > 0.05) but with only day 1 contributing to the significance. Pearson’s correlation analysis showed that the correlation between salivary and serum progesterone levels in cycling mares was not significant except on day 5 for the cycling mare group (p >0.05), whereas for pregnant mares only days 5 and 8 (p < 0.05) were seen to correlate significantly. The study shows that salivary P4 concentration in mares was not statistically correlated with serum values, but nevertheless can be utilized to monitor the luteal phase of the estrous cycle as well as for estimating P4 concentrations during early pregnancy