Enzymeimmunoassay of oestrone sulphate concentrations in faeces for non-invasive pregnancy determination in mares

Aim. To determine the suitability of measuring faecal oestrone sulphate (OS ) by enzymeimmunoassay as a means of determining pregnancy status in mares bred under New Zealand conditions. Methods. An antibody-coated microtitre plate-based enzymeimmunoassay was us ed to determine the concentration of OS in faecal and plasma samples obtain ed from pregnant and non-pregnant mares. Results. In non-pregnant mares, the mean faecal OS concentration was 34 ng/ g, and the value three standard deviations above this was 80 ng/g. None of 427 faecal samples collected from 116 non-pregnant mares over a 1-year peri od had an OS concentration > 80 ng/g. Only five samples from three mares ha d an OS concentration > 65 ng/g, the value two standard deviations above th e mean non-pregnant value. Analysis of faecal OS concentrations in 532 faec al samples collected from 39 pregnant mares showed that as pregnancy progre ssed, an increasing proportion of faecal samples had OS concentrations > 80 ng/g. None of the mares 150 days or more pregnant had faecal OS concentrat ions < 50 ng/g, and 204/220 samples obtained from these mares had faecal OS concentrations > 80 ng/g. Following foaling or foetal death, elevated faec al OS concentrations returned quickly to non-pregnant levels. The mean +/-: s.e.m. plasma level of OS in five mares bled daily throughout one oestrous cycle was 1.7 +/- 0.2 ng/ml. Sixty eight blood samples from pregnant mares bled up to five times between 92 days after mating and foaling all had pla sma OS concentrations > 30 ng/ml, with 64/68 being > 50 ng/ml. Conclusions. This study shows that measuring faecal OS concentrations by en zymeimmunoassay offers a convenient, accurate, non-invasive means of determ ining pregnancy status in mares from 150 days after mating onwards. Mares w ith faecal OS concentrations < 50 ng/g can be considered not pregnant, whil e mares with faecal OS concentrations > 80 ng/g can be considered pregnant. Those few mares returning a faecal OS concentration between 50 and 80 ng/g should be retested to obtain a conclusive result. Measuring plasma OS conc entrations allows pregnancy status to be determined earlier (from 100 days after mating). Moreover, the discrimination between "non-pregnant" and "pre gnant" levels is greater for OS in plasma than in faeces. Clinical relevance. Measurement of OS concentrations in faeces provides an alternative and non-invasive means of determining pregnancy status in mares from 150 days after mating.