Husbandry and Management Fall 1999

Compiled by LEIGH ANN HOWARD

This month brings cooler weather to our breeding farms. If our major pasture grass is California Bermuda, it is starting to turn brown. Those of us who have overseeded with annual rye are seeing a bright green blush on our pastures.

The horses are putting on their winter coats. We are planning our lighting system for the maiden and barren mares. Most farms now put these mares under lights shortly after the Thanksgiving holiday. Clydene Boots, Westfield Farm, wrote an excellent article in the November 1997 issue of California Thoroughbred describing how she uses lights to trick the mares into cycling earlier. Check back in your old issues or call Michael Compton for a reprint.

Many of us have already started supplementing our early foaling mares with additional nutrients, minerals and trace elements. The nutritional needs of the third trimester mare will be discussed in the December issue.

It is also in this season that mares are more likely to abort. Dr. Jeanne Bowers, the Harris Farms resident veterinarian, has been kind enough to give us some information on the subject of pregnancy loss in mares.

Dr. Jeanne Bowers,
resident veterinarian, Harris Farms:

Now that Fall is upon us farms have weaned most of the foals and everyone is appreciating the fact that the weather is cooling off and everything is slowing down. Hopefully the pastures are full of pregnant mares just waiting to reward us with the foals of carefully planned matings.

Unfortunately not all of those mares are going to deliver a foal, and determining ways to increase our odds of getting every foal on the ground should be a serious consideration during this time of the year. Expected pregnancy loss rates for a normal mare prior to day 40 of gestation is 1-2 percent.

After day 40, approximately 6 percent of normal mares will fail to produce a foal. These figures increase to 10-15 percent when we include mares that are considered subfertile. Any mare that has taken more than two cycles to conceive when bred to a normal fertile stallion can be considered subfertile.

So what are the reasons that a mare loses a pregnancy after day 40 of gestation? Causes of abortion can be broken down into two classes:

non-infectious and infectious.

    The non-infectious reasons include:
    1) twins
    2) genetic abnormalities
    3) uterine torsions

The infectious causes include:
    1) viral agents
        (equine herpes virus and equine viral arteritis)
    2) fungal infections
    3) bacterial infections.

Twins we try to avoid with early and frequent transrectal ultrasonography. Mares should be evaluated with an ultrasound up to 45 days of pregnancy to decrease the incidence of twins.

Genetic abnormalities are impossible to predict and since DNA studies are not routinely performed on aborted fetuses we can't really say what the incidence of this cause of abortion truly is.

Uterine torsions are infrequent but early diagnosis and correction will allow foals to be delivered successfully.

Viral agents can be controlled by vaccinating the mare with a killed rhinoneumonitis vaccine and 5, 7 and 9 months of gestation or prior to breeding with an equine viral arteritis (EVA) vaccine. Additionally pregnant mares should be kept separated from the transient population on the farm to avoid exposure to viral agents.

The biggest cause of fetal loss is uterine inflammation (edometritis) and/or fibrosis. Unfortunately if a mare has a uterus that is full of glandular fibrosis there isn't much we can do for her. Establishing that fact prior to breeding by obtaining a uterine biopsy may help in eliminating some of those mares from the herd or at least realizing that we have a very poor chance of obtaining a foal. Uterine inflammation is definitely controllable and it is important in that endometritis is what leads to fibrosis. Fortunately with the advent of uterine flushes, intrauterine antibiotics and agents to increase uterine muscular activity to physically clear bacteria (e.g. oxytocin, prostaglandin) we are able to effectively control uterine inflammation in many cases. These procedures combined with the use of a Caslicks operation (suturing the vulva closed) have given us quite an arsenal against endo-metritis.

Unfortunately we still have mares that despite all of our efforts develop an ascending placentitis (infection of the placenta via the cervix). These mares will commonly abort after 5 months of gestation or may go to term delivering a compromised foal. Recognizing these mares and intervening during the pregnancy has given us the opportunity to get more of these foals delivered successfully.

Now how are we going to evaluate this pregnancy? Initially we can evaluate hormone levels, the most common being progesterone and estrogen. These can be supplemented if the values are too low. Another hormone that is being studied that offers promise for assessing placental health is Relaxin.

This is the hormone that the placenta produces that not only acts in conjunction with progesterone to maintain the pregnancy but also allows the soft tissue structures of the pelvis and the cervix to relax enabling the foal to be delivered. Low levels of this hormone have been associated with abnormal placentas. Hormonal assessment is not the best tool for complete evaluation as mares have had perfectly normal hormone levels and have aborted anyway.

So what can we do next? EKG machines have been used to study fetal heart rate. The fetus should move at some point during a 10 minute EKG session and the heart rate should increase corresponding to the fetal movement. Consistent low heart rates or a failure to note a rise in fetal heart rate during a 10 minute session may be indicative of a fetus in distress. So, we recognize that we have a fetus in distress what next? Ultrasonography. This tool has been the single greatest advance in evaluating equine pregnancies. We are able to evaluate the whole fetus and placenta as well as amniotic and allantoic fluid with the use of transabdominal ultrasonography. This requires a bit of time as the entire abdomen of the mare must be shaved and scanned to evaluate the entire foal. The advantage is that we can look at the quality and quantity of both amnionic and allantoic fluid, we can assess placental thickness in several areas and we can measure the size of the aorta in the foal to get an estimate of the foal size. We can also see how the foal is oriented.

Recently Drs. Troeddson, Renaudin, Zent and Steiner have established some normal values for evaluating the placenta transrectally. Since ascending infections begin at the cervix this area is going to be the first to show signs of any abnormality and it is easily evaluated with the use of a typical ultrasound that is used for early pregnancy detection. The study established some normal values for the combined thickness of the uterus and placenta, in an area just in front of the cervix, during mid and late gestation. They were able to demonstrate that mares that deviated from the normal range subsequently aborted or delivered compromised foals.

So how can we help once we've identified a mare with a problem?

1) If hormone levels are low they should be supplemented. This is commonly done in the form of oral progesterone analogs (e.g. Regumate) or injectable forms of progesterone, or progesterone and estrogen.

2) Antibiotics can be given based on results of cervical cultures or historical infections in the mare and other agents can be used that help improve placental function such as the drug pentoxyfylline. Pentoxyfylline is a drug that has both immunomodulatory (regulates the immune system) and rheologic (alters the blood flow) properties and it has been shown to help mares with placentitis.

3) Another important consideration for the mare at risk is making sure that she delivers her foal under supervision.

So remember that although Fall signals a time when activity on the farm may be decreasing it also is a time when the mares are beginning to head into the most crucial stages of pregnancy. It is important that we provide adequate nutrition, preventative measures are in place (such as vaccinations, dewormings and dental care) and that we have identified the mares at risk so that we can be proactive and try to increase the odds for delivery of a normal healthy foal.