Sound and Pictures

One of the most significant advances in equine management in recent years has been the advent of ultrasonography, or ultrasound. Through this technology, which bounces repeating sound waves off tissues and structures in the horse's body and converts them to a visual image based on the length of time it takes for the wave to return, veterinarians can examine the inner workings of soft tissues such as tendons and ligaments. Perhaps even more exciting, they can finally monitor with accuracy the mare's reproductive cycle--and literally shed light on a mare's unborn foal.

Like any other diagnostic tool, ultrasound has its limitations--its sound waves cannot penetrate bones, for example, translating them as white lines. Also, accurately interpreting ultrasound images, as with radiographs (X rays), requires expertise and practice. However, as a soft tissue diagnostic, ultrasound currently has no equal.

Other imaging methods do exist, but none, thus far, are as accessible as ultrasound--or as flexible. Portable ultrasound machines now are part of the regular arsenal of equipment toted by any veterinarian with an interest in reproductive work. In the approximately 15 years that ultrasound has been a part of general veterinary practice, both the level of the technology, and the expertise of the practitioners interpreting the images, have steadily improved.

An ultrasound image appears as a three-dimensional, black-and-white series of shapes, that, with practice, an operator can analyze to identify normal tissue and organs easily, and discern abnormalities. The lighter the object on the imaging screen, the more solid and dense it is, and the slower the sound waves are moving. Dark areas on the screen usually indicate fluid, through which sound waves move faster. Movement also can be detected via ultrasound--as many delighted expectant mothers have discovered upon their own visits to an ultrasound clinic.

While ultrasound is widely used to diagnose soft-tissue injuries such as strained tendons, it's with regard to broodmares that it probably makes its most significant contributions. Ultrasound can be used to good advantage at virtually every stage of a mare's reproductive cycle. Many circumstances that previously could be diagnosed only by rectal palpation (itself a technique that requires much practice) now can be observed with far more accuracy using ultrasound. Veterinarians use a combination of both techniques to provide them with information both tactile and visual--they can now see what before they only could feel.

The most important use of ultrasound technology for broodmares is in pregnancy detection. A developing embryo might be observed fairly easily by 15 or 16 days after conception (when it has reached 1.5 cm to 2 cm in size). According to Michael Zajac, DVM, whose Toronto-area practice focuses on reproductive work in sport horses, an experienced ultrasound operator can detect a fetus's presence (or absence) as early as 11 or 12 days after conception. By contrast, the average wait for accurate pregnancy diagnosis by rectal palpation is about 30 days.

When a mare is examined for pregnancy via ultrasound, she has her tail wrapped, is allowed to evacuate her bowels, then has the transducer--a probe that emits the sound waves--inserted rectally. Using a 5.0 Megahertz transducer (a probe that allows fairly deep penetration through the tissues, and provides good resolution of the uterus and surrounding structures), what is visible within the uterus at the 15- to 21-day mark is a small, dark, fluid-filled sac that contains the fetus and its membranes. Somewhere between Day 15 and 17, the fetus stops being mobile within the uterus and becomes ‘fixed’ in one position on the uterine wall--usually making it easier to find. By Day 24, the fetus itself usually can be observed, and sometimes it's possible to see a heartbeat.

If a mare is scanned at an earlier stage (11 to 12 days), pregnancy is best detected when the transducer is moved slowly, so that the image does not pass over the fluid vesicle that is only 2 to 3 mm wide at this point in development. (Vesicles at this stage also sometimes are mistaken for uterine cysts--more on these later.)

The early diagnosis of pregnancy is of importance to breeders, as it allows managers to make the decision to re-breed at the next estrus if the mare has not ‘caught.’ But later scanning also can be useful, particularly if you would like to know the sex of the foal. This is usually possible in the 60-78 day range, when the fetal genital tubercle (FGT), the precursor of the sex organs, migrates closer to the umbilical cord in colts, and back toward the anus in fillies. New research has shown that fetal sex can be determined at a later date. Also, from Day 80 onwards, the fetus can be scanned with an external transducer along the mare's abdominal wall in the region of the udder. (For more information on fetal sexing see Up Front in this issue or The Horse of May 1995, page 20.)

The absence of a heartbeat in an otherwise normally developing fetus is one problem that can be detected only via ultrasound--manual examination invariably indicates a normally developing pregnancy. If early embryonic loss is suspected, two ultrasounds spaced 24 hours apart should be performed to confirm the diagnosis. Provided the pregnancy is proceeding as planned, Zajac generally does not ultrasound mares past the 65 day stage--but if the mare is an older individual (in her late teens or 20s), or if she has had problems carrying foals in the past, periodic ultrasound monitoring of the fetus might be recommended.

Twinning: No Cause For Celebration

The detection of twin embryos is another crucial use for ultrasonography. While the idea of twin foals might be beguiling, most breeders know that a surplus of embryos greatly increases the risk factor of the pregnancy. Some researchers have suggested that up to 94% of twin pregnancies result in the abortion or stillbirth of both fetuses.

Often twin pregnancies begin normally, but because the distribution of the two placentas in the uterus is rarely equal in area, one fetus does not receive its share of the nutrients and dies. If this happens early in the pregnancy, the embryo might mummify and the placenta of the living twin can expand and enclose it. The end result is the birth of a single live foal with no one the wiser as to its twin. More commonly, the pregnancy will proceed until the eighth or ninth month, at which point the smaller of the two fetuses dies, triggering abortion of the live foal as well. Even if the mare manages to carry both foals to term, only 8% of twins born alive survive longer than two weeks.

As a result of these statistics, the recommendation of most veterinarians is that twin pregnancies be treated by ‘pinching’ (terminating) one embryo so that the other can be carried successfully to term. This is a simple procedure, performed without much stress to the mare, but it has a small window of opportunity. It is best done before the 17th day of gestation. To treat twinning after that date significantly increases the risk of losing both embryos.

This is where ultrasound is an extremely valuable tool, for it can detect twinning earlier, and more accurately, than rectal palpation. Prior to the availability of ultrasound, veterinarians would search for evidence of twinning by examining the uterine horns for bilateral enlargements, or a single enlargement that was larger than normal (as occurs when both twins are sited in one horn). Double corpora lutea on the mare's ovaries also are a tip-off. Even with this care, many twin pregnancies were missed, and even when they were detected, it was often too late to do anything about it.

By contrast, ultrasound can reveal the presence of twins very early (although it should be noted that if the twin fetuses are side by side in one uterine horn, or on top of each other relative to the transducer, they might be difficult to see), and it can help guide the veterinarian through the pinching process.

Certain mares seem to be prone to double ovulations, and thus they are at higher risk for twin pregnancies. The incidence is particularly high in Thoroughbreds, where it's estimated that 3.5% are predisposed toward twinning. Standardbreds come in second at a 1% risk rate, and the risk is lowest for the pony breeds. Any mare with a history of twinning should be monitored closely--to the point that she might be given an ultrasound examination during ovulation for evidence of double corpora lutea (a sign that two ova were released). Certainly she should be scanned between Days 12 and 15 in order to perform manual embryo reduction if necessary. On most Thoroughbred farms, ultrasound examinations to prevent twinning are standard procedure.

Detecting Estrus

For those owners with maiden mares, or mares which undergo ‘silent’ heat periods, ultrasonography can be a blessing. It can detect the status of pre-ovulatory follicles, and help your veterinarian accurately predict the onset of ovulation even if the mare shows none of the classic behavioral signs.

Equine ova have a very short time frame for viability (only about 12 hours once released from the ovary), whereas sperm can remain alive and viable for up to 72 hours in the female reproductive tract--so the best strategy for successful breeding is to inseminate the mare 12 to 24 hours prior to ovulation, which is shortly before she is due to go out of heat.

Timing can be particularly crucial when the mare is to be bred with shipped semen (often obtained at a considerable cost), so ultrasound can be a useful tool to pinpoint the optimum time for insemination. Just before ovulation, the follicle appears on an ultrasound screen as a very dark circle; once ovulation has occurred, the screen is often dark as well, due to increased fluid accumulation that goes along with the process.

Zajac points out that rectal palpation is still an important adjunct for the detection of ovulation.

‘One thing ultrasound can't do,’ he explains, ‘is give you an idea of the consistency of the follicle. Just before ovulation, there's a softening, and it feels mushy. That's something you can only tell by palpating.’

Fertility Problems Revealed

Yet another avenue in which ultrasound has proven invaluable is in the diagnosis of infertility problems in mares. Uterine cysts are one common occurrence, especially in older mares. The cysts make it difficult for a conceived embryo to attach itself to the uterine wall. Present before breeding, these cysts usually are fluid-filled and thus show up black on an ultrasound scan.

‘They're not uncommon,’ says Zajac, ‘and they're an indication that there's been a uterine pathology--perhaps a pregnancy complication--in the past. They can be in practically any position in the uterus. Sometimes they're innocuous and don't interfere with a pregnancy, but sometimes they do.’

What's particularly tricky about uterine cysts is that they might masquerade as a pregnancy. On an ultrasound, they can appear to be very similar to the vesicle that indicates early pregnancy, although Zajac says with practice, veterinarians can learn to distinguish between them.

‘They don't have the same 'echo' a pregnancy does,’ he says.

Inexperienced practitioners, however, might mistake a cyst for the real thing. To confirm the presence of cysts, endoscopic examination might be necessary. Ultrasound also can be used to determine the presence of ovarian abnormalities.

Another common cause of infertility is endometritis, which causes a persistence of excess fluid that pools in the uterus instead of being flushed out of the tissues. Endometritis is a response to chronic inflammation, either from infection or as a response to the irritating qualities of stallion semen itself, to which some mares are more sensitive. Often its effects worsen over time and can be most troublesome in older mares.

Because ultrasonography is so good at detecting fluid, it can demonstrate endometritis very well--particularly about 24 hours after a mare has been bred, when the irritation from semen is at its peak. Depending on the source of the inflammation, mares with endometritis can be treated with oxytocin, which causes short-term contractions and helps eliminate the excess fluid, or with antibiotics aimed at eliminating a persistent infection. (See related article on sub-fertile mares, page 22.) Some mares might be ready for breeding on the next cycle. Determining the degree of improvement also can be accomplished with ultrasound.

A final use for ultrasound in the reproductive cycle of the mare is in post-partum care, particularly if there were foaling complications or if there is an intention of breeding her back shortly after foaling. Uterine size and tone easily can be examined by scanning transrectally, which can help breeding managers make realistic plans for the mare's future pregnancies.

All of these applications help justify the considerable cost to an equine veterinarian of a portable ultrasound machine (which can cost in the neighborhood of $30,000 for top-of-the-line machines). More expensive models also tend to be more versatile. With the use of a different probe, usually a 7.5 Megahertz type rather than 5.0 MHz, the unit can be used to diagnose injuries to tendons and ligaments.

The technology that produces ultrasound machines continues to improve, as well--can color pictures be far in the future?

About the Author

Karen Briggs

Karen Briggs is the author of six books, including the recently updated Understanding Equine Nutrition as well as Understanding The Pony, both published by Eclipse Press. She's written a few thousand articles on subjects ranging from guttural pouch infections to how to compost your manure. She is also a Canadian certified riding coach, an equine nutritionist, and works in media relations for the harness racing industry. She lives with her band of off-the-track Thoroughbreds on a farm near Guelph, Ontario, and dabbles in eventing.

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