Clinton Vet Service: Week 3

Day 1

Day 7

My time with Clinton Vet continued to sprint by during week three.  This week we saw everything from lameness cases, to sick horses, to impressive wounds.  There never seem to be two days that are the same here.  The week started out with a 'Big' horse that had an unfortunate run in with a fence.  I first saw the case with Dr. Russ on Sunday evening and have gotten to recheck the case every couple of days since then.  When we first saw the injury we took care to clean it thoroughly, remove dead tissue, and ensure that no underlying structures were involved.  Based on location, our biggest fear was whether or not the joint was involved.  But after further exam it seemed that all was well in the joint capsule.  The wound is going to take a while to heal, but the Big has remained sound and is continuing to be a champ about the frequent bandage changes and exams.  All this excitement and it was only Monday!


One of the largest parts of my internship thus far have been reproductive cases.  Not only does Clinton Vet service the MSU Horse farm, but Dr. Cynthia also assists with the breeding of a number of local, privately owned, mares.  Some of the work is done on their personal farms, while others are boarded at the clinic for breeding so that we can check them daily and track their cycles.  My reproductive background is in cattle, so it has been interesting to learn the different nuances involved in breeding mares.  Like cattle, horses have a 21 day estrous cycle that lasts from the ovulation of the previous follicle, through the growth, development and eventual ovulation of a new follicle (see images of different sized follicles).  Unlike cattle though, horses ovulate much larger follicles.  Cows will ovulate follicles that are between 10-15 mm while horses  range from 40-50 mm.  This was a surprising find the first time I looked at an equine ultrasound and thought she had a cyst!

Various sized follicles with a large (close to ovulatory sized follicle on the right-hand side)

Ovulatory sized follicle (approximately 50 mm in diameter)

When breeding horses with either fresh cooled or frozen semen it is crucial to time insemination as close to ovulation as possible.  There are many ways to achieve a successful timed ovulation and it often involves the use of hormones to induce various stages of the estrous cycle.  Typically, the first manipulation involves induced regression of the corpus luteum (which is made up of the remnants of the previous ovulatory follicle).  Corpus luteum, or CL, release the hormone progesterone which effectively keeps follicles from ovulating and is involved in the maintenance of pregnancy if it is present.  In the absence of pregnancy, without any intervention, the CL will regress between day 14-16 of the estrous cycle.  In an effort to shorten the natural cycle, you can administer prostaglandins to induce regression of the CL and allow faster ovulation. (see image of new CL).

Corpus luteum after a recent ovulation (<24 h post ovulation)

Another key aspect to timing insemination, is precise ovulation.  The hormones involved in this event are GnRH or gonadotropin releasing hormone.  GnRH is an endogenous hormone released from the hypothalamus and stimulates leutenizing hormone (LH) release from the pituitary gland.  Today, we have synthetic formulations of GnRH that can be administered to induce ovulation of properly sized follicles within 24-48 hours of application.  These formulations include deslorelin acetate (SucroMate) or histrelin (a compounded GnRH product).  Ideally, we will time our breedings so that the mare receives a dose of semen 6-12 h pre and post ovulation.

Uterus with edema

As mares begin the process of ovulating a follicle, her estrogen increases (driven by the presence of a large follicle) and her progesterone decreases (now that the CL is gone).  These changes induce both behavioral and biological changes in the mare.  Mares in estrus, or heat, will begin to be more antsy, can be come moody, and will display signs that she is ready to be bred (winking, frequent urination, and mucous secretion from her vulva).  On ultrasound and palpation you can detect that her cervix is relaxed and open and that her uterus has formed edema.  This is prime time for breeding the mare and setting her up for successful conception.

 Once the mare has successfully been bred and been checked for ovulation she is hopefully all set to maintain a healthy pregnancy. We try and ultrasound mares 16 d post breeding to look for pregnancy.  If she is pregnant we will return 30 d post breeding and do a 'heart-beat check' to ensure the pregnancy is progressing as planned.

16 d pregnancy

30 d pregnancy

  Unfortunately, even if all goes as planned and the mare ovulates on schedule, pregnancy is never guaranteed.  If a mare is found to be "open" or not pregnant we have a few options moving forward.  If it is at her 16 d check, and a CL is present, we can administer PGs and then begin breeding 3-5 days later.  If she has been bred multiple times and still not conceived we can take a culture of her uterine contents and check for signs of infection (which will significantly decrease her chances of conceiving).  If that is the case she can be lavaged with antibiotics and re-synchronized for breeding once the infection is cleared.  One of the mares that Dr. Cynthia has been working with cultured positively for a slight infection and needed to be lavaged.  Dr. Cynthia let me perform the lavage and documented the process for me!

You have to maintain a sense of humor when performing reproductive work!

Breeding horses can be considered more of an art than a science.  At times it can be extremely frustrating when things go as planned and a mare still fails to conceive.  I hope to one day become proficient at equine reproduction and have been very fortunate to get to see so much repro work with Dr. Cynthia.  It is hard to believe that I only have two weeks left, but I am sure they will be filled with even more adventures!  Thanks for following along.