Breeding: Regumate - what is it and what does it do?

The active ingredient in Regumate is a drug called altrenogest. It mimics the action of progesterone, which is the hormone that prepares the mare’s reproductive tract for pregnancy. Mares are long day breeders so during winter they have very little activity on their ovaries. As the days lengthen into spring, new follicles start to grow on the ovaries, but it may take several weeks before one of these ‘transitional follicles’ is mature enough to ovulate. Once the mare has had this first ovulation of the breeding season, she will typically come into heat every three weeks until she either becomes pregnant or the ovaries start to shut down again as the days shorten into autumn.

Commercial breeders generally aim for early foals, so a long transitional period at the start of the breeding season is undesirable. Regumate was initially developed to help mares achieve a fertile cycle early in the season. A ten day course helps mares to come into heat and ovulate within 10-14 days after the end of treatment.

Over time, the drug has been used in a variety of other ways as well:

• To synchronise the cycles of embryo transfer donors and recipients

• To reduce the behavioural signs of oestrus

• To manage inflammation of the placenta during late pregnancy

• To support pregnancy and avoid early death of embryos

Progesterone in the pregnant mare

Pregnancy support is quite a common reason why a mare may be prescribed Regumate. Once ovulation has occurred, the hollow that is left behind on the ovary is rapidly filled with new cells and within four to five days, this corpus luteum (CL) begins to produce progesterone.

As the progesterone levels rise, the lining of the uterus begins to thicken and the muscle layers in the wall stop contracting. These changes prepare the uterus to support a pregnancy. If the mare hasn’t become pregnant, the CL begins to regress or die back after about 10-14 days. Progesterone levels fall and the ovary starts to grow a new follicle in preparation for the next cycle.

If the mare is pregnant, she needs to keep producing progesterone for the next 11 months. Progesterone production to support a healthy equine pregnancy can be described as a relay race. The CL is the first to pick up the baton: producing progesterone for the first two weeks. By that stage, the fertilised egg has developed into an embryo which migrates around the uterus.

By day 18, the embryo attaches itself to the uterine lining, where its presence helps the CL to persist and prevents the mare coming back into oestrus. Next up are the endometrial cups. These structures form in the uterus by day 35 and produce a hormone called eCG. eCG stimulates the growth of additional CLs on the ovaries, thereby further boosting progesterone production. The endometrial cups start to die off by day 100 to 150. By this stage however, the placenta and the developing foal have started to produce progesterone and a range of other hormones that support the pregnancy right up until foaling.

Pregnancy maintenance

Mares with a history of early pregnancy loss can be blood sampled to check their progesterone levels. Regumate may be prescribed to supplement progesterone levels in an effort to help maintain a healthy pregnancy. In these scenarios, treatment is typically continued up until day 120, by which time the foal and placenta are the primary source of the pregnancy-supporting hormones.

Regumate is a liquid that can either be added to feed or syringed into the mouth once daily. Most mares will readily eat it in feed, so it’s relatively easy to integrate its administration into the daily routine of broodmare care.

Precautions

Regumate is soluble, and contact with human skin must always be avoided to prevent the drug being absorbed into the bloodstream. The bottle is designed to allow the product to be dispensed without skin contact. Wear disposable or non-porous (washing-up) gloves when handling it, and wash your hands with soap and water afterwards. Women of child-bearing age and pregnant individuals should avoid handling this product. Refer to the package leaflet for more information on safe usage.

Contra-indications

Regumate should never be given to male animals. Care must be taken if adding it to feed. Specific buckets and feed pots should be reserved for exclusive use by the animal(s) being treated, to avoid cross-contamination. Any uneaten feed must be securely binned and not given to any other animals.

The use of Regumate is specifically outlawed under the rules of racing. Best practice in thoroughbreds is to restrict its usage to broodmares that have been permanently retired from racing. It should never be given it to young animals or fillies out of training. Regumate is classified as a Controlled Medication by the FEI for the specific purpose of suppressing oestrus in performance mares. Like any medication, it’s important that Regumate is used correctly, to protect animal health and welfare while minimising the risk of a failed medication test in an athlete.