A researcher with the University of Veterinary Medicine, Vienna, has found that a form of hormone treatment improved fetal development in older pregnant mares.
The fetuses of mares treated with altrenogest grew at a normal rate, researchers in Austria have found. Christine Aurich says horses, like humans, are prone to miscarriage. with about one in 10 pregnancies results in miscarriage at a very early stage.
Some horses have a history of early miscarriages and it has become customary to treat them with a type of progestin known as altrenogest, although, until now, no studies had assessed whether this actually improved the chances of the pregnancy running to term.
Researchers led by Aurich investigated altrenogest treatment on the development of the fetus and on the horses' hormone levels.
The researchers found fetuses developed significantly slower in older mares compared with younger animals.
The difference disappeared in horses treated with altrenogest, showing for the first time that the progestin has a positive effect on fetal development.
The results are published in the 75th issue of the journal Theriogenology.
Most miscarriages in horses result at very early stages of pregnancy, within about three weeks. It is generally believed the primary cause is that the fetus grows or develops too slowly - smaller embryos have a higher chance of being lost then normally sized ones.
It is not clear whether low concentrations of progesterone leads to slower embryonic development, but, nevertheless, the progestin altrenogest is routinely used to treat mares that frequently suffer miscarriages.
Aurich's group has found altrenogest treatment has no effect on the levels of luteinizing hormone (LH) or progesterone - hormones known to be important in maintaining pregnancy.
Furthermore, treatment does not influence the ease with which the mares became pregnant, nor does it affect the size of the vesicles housing the embryos, at least for the first 22 days after conception.
However, the researchers did notice that, at 20 days after conception, the embryonic vesicles are smaller if the mares are older.
They also found that the fetuses of older mares grew significantly more slowly after this period, although if the mares are treated with altrenogest their foetuses grow at the normal rate.
The smaller size of the fetuses in older mares provides a nice explanation for the higher rate of pregnancy losses as horses grow older.
Smaller fetuses in these animals may result from a reduced quality of the eggs as the horses age, making the mares more susceptible to miscarriage.
Encouragingly, treatment with altrenogest appeared to enable the smaller fetuses to recover and to grow at a normal rate during the second crucial period in the animals' development, when the embryonic organs are formed and the mare's placenta is generated (from 35 to 45 days after conception).
It is possible altrenogest encourages the formation of the placenta.
The results show altrenogest treatment reduces the risk of miscarriage in horses, although not in a way that might have been expected.
It does not seem to prevent miscarriages in early pregnancy, but instead compensates for later problems in fetal development that are more frequently encountered as mares grow older.
Aurich says: "We are now well used to the idea of a hormonal therapy in humans to prevent osteoporosis. Perhaps horses could also benefit from the same type of treatment to help them avoid miscarriages."
The paper Effect of Age and Altrenogest Treatment on Conceptus Development and Secretion of LH, Progesterone and eCG in Early-pregnant Mares by Conrad Willmann, Gerhard Schuler, Bernd Hoffmann, Nahid Parvizi and Christine Aurich is published in the 75th issue of the journal Theriogenology (2011, Vol. 75(3), 421-428). The work was performed at the Graf Lehndorff Institute for Equine Science in Neusdadt (Dosse), Germany in collaboration with groups from Giessen and Mariensee.