Pregnancy diagnosis in the mare by immunoassay of estrone sulfate in serum and milk

Abstract

Concentrations of estrone sulphate (ES) were determined by radioimmunoassay in serum and milk samples from mares at various stages of gestation and lactation and from non-pregnant cycling and noncycling mares. Serum ES concentrations began to increase within a few weeks post-breeding in pregnant animals, but showed a very sharp rise after 60 days; concentrations generally reached over 30,000 pg/ml at 80 days and were maintained at that level thereafter throughout pregnancy. Differences between pregnant and non-pregnant non-estrous mares were highly significant (P<.001) from 20 days post-breeding onwards. Mean serum ES concentration in non-pregnant mares was approximately 750 pg/ml; however, there were transient peaks of from 2 to 7 times this amount during estrus. By 60 days post-breeding, the majority of pregnant mares have serum ES levels high enough to distinguish them from estrous non-pregnant mares. There was an abrupt fall in serum ES at parturition or after abortion. Concentrations of ES in milk were substantially lower than in serum, but rapidly increased in pregnant mares following a pattern similar to that in serum. By 90 days all pregnant mares had milk values of 2,000 pg/ml or more. In milk from non-pregnant mares the concentration was consistently low (100 pg/ml or less). Determination of ES in serum and milk provides evidence of the persistence of a viable conceptus, and should serve as a valuable adjunct to other means of assessment of the reproductive status of the mare.