Smaller Horses, Bigger Reproductive Problems
- Mar 1, 2002
Big gifts come in small packages. That's a fine sentiment when it comes to presents, but when it refers to miniature horse reproduction, that big gift/small package combo can be a little tough on the mare. It's not so great for the foal, either. In fact, a "big gift" is one of several reproductive concerns unique to miniature horses. For sure, miniature horses are hardy creatures, but their small stature can lead to reproduction problems and increased difficulties with veterinary procedures. Following we will discuss some of the problems that are common with miniature horses.
Small Feet, Small Head, Small...
Similar to their full-sized cousins, miniature horses risk fewer reproduction issues through good breeding and management x techniques, including screening breeding prospects against undesirable characteristics (such as limb abnormalities or dwarfism), teasing properly, maintaini0ng health management and veterinary care throughout the breeding and pregnancy, and having experienced people attending the foaling. Still, dystocia and abortion are more common in minis than in full-size horses due to foal size and fetal abnormalities.
Unique to minis, size matters. For example, a small mare bred to a medium or large stallion can result in a fetus too large for the mare to deliver normally. Breeding young mares to larger stallions also can result in foals which are too large for the mare to deliver normally (which happens more often in minis than in regular-sized horses anyway). Notes Philippe Vervaet, DVM, in the department of gynecology and obstetrics at the University of Ghent in Belgium, "Ten to 15% of the miniature horses here are foaling with an oversize foal. In half of the cases, it is due to the owner who bred a mare that is too young and the pelvic diameter was still too small."
In regards to the mare size vs stallion size problem, Carla L. Carleton, DVM, MS, Dipl. ACT, an associate professor in equine theriogenology at Michigan State University, says that although a mare has the internal ability to modulate the size of the fetus that's developing in her to some degree, "When you get a discrepancy and the stallion is quite a bit bigger than the mare, the mare's internal control over fetal size may be insufficient to the task. If the fetus' size exceeds that which may easily pass through the mare's birth canal, it will present as a dystocia due to fetal/maternal disproportion.
"Dystocia occurs because the fetus is too large to pass through the birth canal. If you can't get it out through vaginal manipulation (meaning with expert help by someone used to working with dystocias) or through a Cesarean section, the mare may die or her genital tract can become damaged in the efforts to remove the fetus."
Pre-partum ultrasound will not reveal the fetal/maternal disproportion, she noted. Ultimately, breeders should consider the size of the two breeding prospects as well as the mare's age when making breeding decisions.
The small size of the mare can make the timing of breeding as well as pregnancy diagnosis tricky. Says Dale Paccamonti, DVM, MS, Dipl. ACT, a professor of theriogenology at Louisiana State University, "Due to the small size of the mare, it is often difficult to monitor follicular development (since the horse might be too small for a rectal exam). This is important with sub-fertile mares when you would like to minimize the number of breedings, thereby reducing the number of times the uterus is subjected to the inflammatory process resulting from insemination. It is also challenging to time the breeding close to ovulation if you have a sub-fertile stallion. You need to evaluate the mare after breeding to detect ovulation, and to evaluate uterine clearance post-breeding. Furthermore, it's difficult to detect and monitor early pregnancy and check for twins by ultrasound per rectum, and it's also difficult in late gestation to perform an evaluation of the placenta near the cervix for placentitis."
Size matters, too, for the stallion. "Miniature stallions have smaller testicular size and smaller sperm output (than standard-size horses), so one must take this x into account when doing a breeding soundness exam on a miniature stallion," says Paccamonti. "Many miniature stallions will not meet the expectations for a 'full-size' stallion such as an Arab or Quarter Horse; guidelines for breeding soundness evaluation (i.e. testicle size, total sperm numbers) were developed from full-size horses."
Miniature mares often cycle differently than standard-size horses. They might cycle year-round, exhibit longer periods of anestrus, or have prolonged estrus, any of which hinders a successful breeding, says Robert Judd, DVM, Dipl. ABVP, owner of the Judd Veterinary Clinic in Hewitt, Texas. Judd authored the chapter "The Miniature Equine" for A Colour Handbook of Exotic Species (edited by F.L. Frye, tentatively scheduled for publication in early 2002 by Manson Publishing, London).
Anestrus--Judd notes that about a third of the miniature mares he examines are affected by behavioral anestrus. "Behavioral anestrus is caused by the tremendous emotional attachment by (miniature) mares to their foals," he says, "and is the most common cause of anestrus in miniature mares in our practice."
Diagnosis is made by history, teasing records, rectal palpation, and/or ultrasound examination.
"While palpation reveals normal ovarian function and ovulation, these mares are indifferent to breeding and are extremely belligerent toward the stallion."
As artificial insemination is now accepted for registered miniature horses, this is an option. To limit this emotional attachment, Judd recommends weaning foals at four months of age.
"Usually, these mares will show clinical signs of heat four to six weeks post-weaning."
The second most common cause of anestrus is a persistent corpus luteum (CL), Judd reports. This prolonged period of infertility occurs when the progesterone-producing corpus luteum fails to degenerate as is normally the case when a pregnancy does not develop. The continued production of progesterone prevents the mare from coming into heat. Diagnosis is made via history, examination of the reproductive tract, and response to prostaglandins. For treatment, Judd recommends intramuscular injections of prostaglandin to regress the CL followed by daily teasing two days after the injection. Mares should return to estrus within four to six days.
Hypothyroidism (decreased thyroid function) can also potentially cause anestrus in miniature horses. Hypothyroidism is a difficult diagnosis in horses as serum thyroid concentrations can be variable, there are no validated diagnostic tests in the horse, and many extrinsic (outside the horse) factors can affect thyroid hormone levels. Also, there is controversy among veterinarians about the actual existence of hypothyroidism in the horse.
Clinically, Judd says that he has seen an apparent, although not proven, response of restored cycling in some miniature mares with exogenous thyroid hormone therapy. He recommends initiating therapy with oral sodium levothyroxine administered at 20 µg/kg once daily. Serum T4 levels should be checked every 60-90 days and dosage adjusted depending on the serum concentration as well as clinical response.
Prolonged Estrus--Reports vary on x the normal length of the estrous cycle for the miniature mare. Vervaet finds that the heat cycle in most miniature mares lasts about six to 10 days, whereas Judd puts that figure at five to seven days. Some miniature mares remain in estrus for eight to 14 days or more, he adds.
"Prolonged estrus in standard-size mares generally occurs during the transitional phase of the breeding season (coming out of or going back into winter anestrus," he states. "Although this same syndrome occurs in miniature mares during the transitional period, a large number of miniature mares cycle year-round in our practice area.
"This non-fertile period of prolonged cycling is usually characterized by one large follicle that reaches maturity (greater than 1.2 inches or 30 mm), but does not ovulate. Without treatment, the follicle will eventually regress, but remains approximately at that size for seven days or longer, with clinical estrus exhibited the entire time."
After confirming diagnosis by ultrasound examination every other day during estrus, a 750-1,000 international unit (IU) injection of human chorionic gonadotropin (HCG) should be administered when the follicle reaches 30 mm, with the mare bred 24 hours after the injection, recommends Judd. Ovulation usually occurs 36-40 hours after the injection. If ovulation has not occurred, a second HCG injection can be given and the process repeated.
Endometritis--This inflammation of x the uterus is a leading cause of infertility in both standard-size and miniature mares. Diagnosis and treatment of minis is the same as larger horses, although special equipment might be needed for them, says Judd. He recommends using a nine-inch length of PVC plastic pipe with an outside diameter of one inch, a wall thic x kness of one-eighth inch, and a beveled end for speculum examination. He says small-diameter swabs should be used for uterine cultures as it is difficult to manually dilate mares that are not in heat. He also uses small endometrial biopsy forceps for endometrial biopsies.
As noted earlier, confirming pregnancy in minis can sometimes be difficult because rectal palpation is more difficult. Dana M. Pantano, DVM, owner of Black Pond Veterinary Service in Norwell, Mass., sees a high percentage of miniature horses in her practice. She also is a veterinary columnist for Miniature Horse Voice magazine. She has found that ultrasound exams, uterine cultures, and uterine lavage present certain challenges to the equine practitioner due to the mini's small stature.
"It is important to realize that while some miniatures are too small for these types of procedures, most times, the practitioner can be successful with a patient owner, a tranquilized horse, and some modifications to standard-size horse equipment."
Adds Carleton, "If the veterinarian's hand can safely be passed through the anal sphincter to do a palpation, there's a fair amount of room inside to do a proper examination. While it may not be possible to carry the ultrasound probe (transducer) in as deeply as in a 1,000-pound mare, the veterinarian may be able to extend the probe slightly beyond the reach of his or her hand to image features otherwise out of range, taking particular care because of the greater risk of rectal tears."
About 80% of miniature mares in Judd's practice can be rectally palpated, as long as the veterinarian's hand is small to medium size (7 1/2 glove size or less). He recommends using well-lubricated latex examining gloves, which are less bulky than regular palpation gloves, and advancing slowly into the rectum.
"Depending on the practitioner's forearm size, the uterus and both ovaries can be examined," Judd says.
"In mares too small to be rectally palpated, ultrasound examination is more difficult due to lack of accurate external transducer control," Judd continues. "In these cases, use a lubricated transducer holder designed for use with llamas to guide the transducer."
Additionally, pre-foaling mammary development is not always as advanced in miniature mares, says Paccamonti. "This makes monitoring milk electrolytes very difficult and somewhat unreliable. I've seen minis foal without having calcium rise
pre-partum and without any colostrum in their udders."
In other words, with a full-size (non-maiden) mare, one can feel pretty confident that if her milk calcium is low in the afternoon or evening, she won't foal that night. In minis, that's no guarantee she won't foal.
Vervaet says that miniature horses have a higher incidence of some dystocias due to postural problems--unilateral shoulder flexion posture in about 4% of the cases and lateral deviation of the head in 3%
"A dog-sitting position of the foal has an incidence of 1%, about the same as standard-size horses," he says.
Depending on the kind of dystocia, repositioning the fetus might help the mare deliver naturally, although a cesarean section might be required to deliver the foal.
"The veterinarian must take extra care when trying to correct dystocia by repositioning the fetus in a miniature horse," warns Paccamonti. "Due to their size, there is less room for manipulation, which can result in trauma or tears to the reproductive tract."
Prognosis varies. "Unilateral shoulder flexion posture has mostly a good prognosis for mare and foal when the owner is early to call his veterinarian," says Vervaet. "A dog-sitting position yields a poor prognosis for the foal, but usually a good prognosis for the mare."
Carleton reports that miniature mares have a higher incidence of detached placentas, premature placental separation, and other mishaps related to the higher incidence of dystocia.
"Parturition takes longer, and therefore mares are at a greater risk," she notes. "You either resolve the problem early--either late stage one or early stage two parturition--or you're probably too late to save the foal."
Notes Paccamonti, "If a mini experiences dystocia, retained placenta, or other post-partum complications such as metritis, vaginitis (inflammation of the vagina), or cervicitis (inflammation of the cervix), she is at much more risk than full-size horses of developing hepatic lipidosis (a disorder involving deposition of fat in the liver), which can result in a very sick (or dead) mare."
At Vervaet's clinic, the development of vaginal adhesions after a hard delivery is the most common postpartum difficulty seen in miniature mares.
"Clinical signs appear a few weeks after foaling," he states. Diagnosis is made through vaginal examination; treatment is removal of the adhesion generally through blunt dissection. Prognosis depends on the size and location of the adhesions.
"When adhesions are only in the vagina and the cervix is not involved, there is a good fertility prognosis. But, when the cervix is involved, adhesions can cause permanent infertility," he says. Vervaet advises that administering corticosteroids for seven days following a hard delivery might minimize the risk of this problem.
Post-foaling colic is also more frequent and severe in miniatures. "Up to 30% of miniatures after foaling will be in severe enough pain to need treatment," says Judd.
Pantano agrees: "Miniatures seem to become more colicky than the average large mare, resulting in danger to the foal (from her rolling around) and more importantly, making it nearly impossible for the foal to nurse in a timely fashion."
Suffocation due to the inability to break out of the fetal membrane also occurs more frequently with miniature foals than with standard-size foals.
"Normally at birth, the fetus breaks through this sac as it is being expulsed from the birth canal," explains Pantano. "Once out, the umbilical cord is broken, and the foal has to breathe air. In minis, due to the low birth weight of foals, sometimes the sac doesn't break, and the foal suffocates. Upon examination, the sac might seem thicker or tougher than the average, freshly delivered sac of a big horse."
If the foal fails to break through the sac, the veterinarian or lay person can break the sac with fingers or scissors. "Just peeling the sac back so the head is exposed is all that's necessary to ensure that the foal can breathe," Pantano says. "The rule of thumb is that as long as the head is fully out and not going back in, you can break the sac, certainly immediately after the foal is clear of the birth canal."
Although miniature horses might experience a somewhat higher incidence of breeding and reproduction problems, owners and veterinarians can increase success rates and minimize problems by following proper breeding and management protocols, being knowledgeable about increased risks, adapting veterinary procedures for the miniature, and making sure foalings are attended by experienced people. Knowing these potential problems and addressing them with proper care and preparation can save your mare and foal in a problem situation.
About the Author
Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.