Comparison of the merits of measuring equine chorionic gonadotrophin (eCG) and blood and faecal concentrations of oestrone sulphate for determining the pregnancy status of miniature horses

Abstract

The relative merits of measuring blood concentrations of equine chorionic gonadotrophin (eCG, previously known as pregnant mare serum gonadotrophin (PMSG)), or oestrone sulphate (OS), or faecal OS concentrations for determining pregnancy status in miniature horses were investigated. Pregnant mares between 40 and 140 days after mating had serum eCG concentrations >1 I.U. mL^–1, with the highest concentrations occurring between days 50 and 120. However, eCG measurements were susceptible to returning a ‘false positive’ diagnosis of pregnancy. Plasma OS concentrations ranged from 0.1 to 3.6 ng mL^–1 in non-pregnant mares, whereas pregnant mares beyond 100 days post-mating all had plasma OS concentrations >30 ng mL^–1. Faecal OS concentrations ranged from 4 to 89 ng g^–1 in non-pregnant mares. For faecal samples collected from pregnant mares 150 days or more after mating, 97% of samples had OS concentrations >85 ng g^–1, the value 3 standard deviations above the mean non-pregnant value. None had values below 67 ng g^–1, the value 2 standard deviations above the mean non-pregnant value. These results show that measurement of eCG is suitable for determining pregnancy status in miniature mares between 40 and 100 days post-mating. However, mares returning a ‘pregnant’ diagnosis should undergo a blood OS test 100 or more days after mating to eliminate the possibility of a ‘false positive’ diagnosis. Measuring blood OS is recommended as the method of choice for determining pregnancy status in miniature mares 100 or more days after mating. Faecal OS measurements provide a non-invasive alternative to blood OS testing from 150 days post-mating. However, the discrimination between ‘pregnant’ and ‘non-pregnant’ levels of OS is not as great in faeces as it is in blood.