Top Equine Reproduction Studies of 2013

While your veterinarian is stitching wounds, delivering foals, and monitoring colics, researchers from around the world are publishing research that often advances the collective of horse health care. So to bring busy practitioners up to speed on the top studies in a variety of fields, a panel of veterinarians presents a news-type program each year at the annual American Association of Equine Practitioners' Convention.

Pat McCue, DVM, PhD, Dipl. ACT, a professor of equine theriogenology at Colorado State University's Equine Reproduction Laboratory, described the reproduction studies he deemed most important and useful to a veterinary audience during the Kester News Hour. This year's lecture took place Dec. 7 at the convention, held in Nashville, Tenn.

First McCue described a retrospective study in which researchers evaluated the bacteria isolated from mares' reproductive tracts, along with the efficacy of the drugs used to treat them. McCue said the researchers identified potentially pathogenic organisms (such as Escherichia coli and β-hemolytic Streptococcus equi subsp. zooepidemicus) in 31% of the 8,296 samples evaluated; additionally, 12.6% of the samples contained multiple organisms.

But the key point McCue drove home was that the researchers found that several pathogens had increased antimicrobial resistance to a variety of drugs. This, he said, reinforces the importance of using susceptibility testing (or evaluating how susceptible an isolate is to specific medications) before implementing treatment.

Davis HA, et al. Uterine bacterial isolates from mares and their resistance to antimicrobials: 8,296 cases (2003-2008). J Am Vet Med Assoc. 2013 Apr 1;242(7):977-83. 

Next, McCue reported on research evaluating whether laparoscopic application of the hormone prostaglandin E2 (or PGE2) to a mare's uterine tube surface could improve fertility in some subfertile mares. The team tested their theory in 20 barren embryo donor mares and eight barren mares bred to carry their own foals.

McCue said 17 of the 20 donor mares produced an embryo and seven of the eight mares bred to carry the foal became pregnant after veterinarians applied 0.2 mg of PGE2 to the mares' uterine tubes.

"This is not a panacea for all cases," he said, adding that case selection is critical, but this technique could help improve fertility in mares with unexplained infertility.

Ortis HA, et al. Laparoscopic application of PGE2 to the uterine tube surface enhances fertility in selected subfertile mares. J Equine Vet Sci. 2013 Nov; 33(11): 896-900. 

Next, McCue described study results showing that blood anti-Müllerian hormone (AMH) concentrations appear to be a good biomarker for detecting granulosa-cell tumors (GCTs, the most common type of ovarian tumor). Although mostly benign, GCTs can prevent pregnancy and cause stallionlike behavior and other problems in mares.

After collecting blood from normal mares and mares with confirmed GCTs, the researchers found that mares with GCTs had significantly higher AMH levels than normal mares. Further, they found that AMH had a sensitivity (i.e., the ability to correctly identifies mares with GCTs) of 98%.

McCue opined that the AMH screening should be a standard part of tumor diagnostic panels at diagnostic laboratories.

Ball BA, et al. Determination of serum anti-Müllerian hormone concentrations for the diagnosis of granulosa-cell tumours in mares. Equine Vet J. 2013 Mar;45(2):199-203. 

Moving forward, McCue touched on another study in which researchers looked at fungal isolates' susceptibility over time to various antifungals in an effort to improve treatment efficacy in mares with fungal endometritis.

Out of 102 isolates cultured from 92 uterine samples, yeast (69%) and mold (26%) were most common. The researchers found that molds and yeasts were 95 to 100% susceptible to amphotericin B, natamycin, and nystatin; molds were resistant to fluconazole and increasingly to ketoconazole; and yeasts showed 90% susceptibility to ketoconazole but increased resistance to miconazole over time.

Thus, McCue recommended that "choosing an antifungal agent for a mold should be based on antimicrobial susceptibility testing."

Beltaire KA, et al. Retrospective study on equine uterine fungal isolates and antifungal susceptibility patterns (1999-2011). Equine Vet J. 2012 Dec;44 Suppl 43:84-7. 

He then described a study in which researchers sought to determine the plasma and endometrial tissue concentrations of orally administered fluconazole (an antifungal medication) and to determine if these tissue levels surpassed the minimum inhibitory concentration (MIC) for Candida spp. organisms, which are known to cause infectious endometritis.

The researchers administered a loading dose (14mg/kg) of fluconazole followed by six maintenance doses (5 mg/kg) to three mares and collected blood plasma and endometrial tissue samples before and after administration. They found that both plasma and endometrial fluconazole levels 24 hours after administration were above the recommended standard for effective therapy.

This showed that fluconazole can be used for treating Candida spp. in mare's reproductive tracts, and could be a more cost-effective option than some other drugs available, McCue said.

Scofield DB, et al. Equine Endometrial Tissue Concentration of Fluconazole Following Oral Administration. J Equine Vet Sci. 2013(33):44-50. 

Then McCue shined light on research in which scientists evaluated a mobile blue-light mask's efficacy in suppressing mares' melatonin secretions, thereby inducing estrus earlier in the year. Currently most mares are stalled and maintained using artificial light.

He said the team found that exposing mares to as little as 10 lux via a blue-light mask effectively induced estrus in four healthy Thoroughbreds.

"This could allow mares to be maintained outdoors rather than indoors and result in early cyclicity and reduced management costs" from maintaining horses in pastures rather than in stalls, McCue said.

Walsh CM, et al. Blue light from light-emitting diodes directed at a single eye elicits a dose-dependent suppression of melatonin in horses. Vet J. 2013 May;196(2):231-5 

Next, McCue shared the results of a retrospective study in which researchers looked at mares' ability to carry a foal depending upon gestational age at the time of colic surgery.

He said the researchers found the prognosis for carrying a foal to term post colic surgery was significantly better for mares younger than 15 years of age and for mares carrying pregnancies greater than 40 days gestation at the time of surgery. Losses were highest when surgery took place during the embryonic stage of pregnancy, in older mares, and when the duration of clinical signs before surgery or the duration of anesthesia increased.

"Early recognition and early intervention is important," he concluded, noting this study can help veterinarians supply better information about horses' prognosis to owners when faced with a colicking pregnant mare.

Drumm NJ, et al. Factors influencing foaling rate following colic surgery in pregnant Thoroughbred mares in Central Kentucky. Equine Vet J. 2013 May;45(3):346-9. 

Moving forward, McCue described research that evaluated factors affecting live foal rates in mares that underwent manual twin elimination. Researchers looked at 129 twin pregnancies, he said, and found that live foaling rates were not significantly different between adjacent and nonadjacent embryos; however, the mare's age impacted live foaling rates: Live foal rates in mares nine years of age or older were lower than in mares younger than nine years.

For best results, McCue stressed that practitioners deal with twin reduction immediately, rather than putting it off until another day.

Schnobrich MR, et al. Factors affecting live foal rates of Thoroughbred mares that undergo manual twin elimination. Equine Vet J. 2013 Nov;45(6):676-80. 

Along those same lines, McCue presented the results of a study in which researchers evaluated pregnancy and foaling rates after another method of twin reduction: transvaginal ultrasound-guided aspiration (TUA). The team found that 49% of the 44 mares evaluated in the study delivered one live foal after TUA and that the highest live foaling rates occurred in mares that underwent TUA before Day 42 of pregnancy.

"Decisions on twin reduction should be made before 35 days (of gestation)," McCue concluded, adding that reductions should take place between Day 30 and Day 35, if possible.

Klewitz J, et al. Evaluation of pregnancy and foaling rates after reduction of twin pregnancy via transvaginal ultrasound-guided aspiration in mares. J Am Vet Med Assoc. 2013 Feb 15;242(4):527-32. 

McCue then turned his attention to stallion care as he described a study in which researchers evaluated whether the use of single-layer density centrifugation (SLC) can improve frozen, thawed semen quality. He said the researchers found that the procedure improved total and progressive motility (movement), viability, morphology (structure), and DNA integrity compared to control samples; however, the amount of sperm recovered after SLC decreased.

Nonetheless, McCue said this technique might be beneficial for some stallions with poor sperm quality.

Stoll A, et al. Use of a single-layer density centrifugation method enhances sperm quality in cryopreserved-thawed equine spermatozoa. J Eq Vet Sci. 2013;33:547-551. 

Turning his attention back to mares, McCue described a recently published review of breeding-induced endometritis (an inflammation of the uterus lining caused by breeding). He said the researchers found breeding-induced endometritis in 10-15% of mares and that factors such as advanced age, poor perineal conformation, a pendulous (i.e., downward facing or slanted) uterus, and an altered immune response put mares at a greater risk for developing this condition.

He said the researchers identified six hours as the critical timeframe to clear breeding-induced inflammation from a mare's uterus; mares that failed to clear inflammation by six hours after breeding remained inflamed.

Woodward EM and Troedsson MH. Equine breeding-induced endometritis: a review. J Equine Vet Sci. 2013;33:673-682. 

The results of the final study McCue shared could be very useful for breeders in the near future. He said researchers set out to test whether mares' milk pH could be a useful indicator of impending foaling. The results suggest that if the milk's pH is above 6.4, she's not yet ready to foal, he explained. However, once the pH drops below 6.4, the mare will likely foal in the following few days.

McCue said owners can use test strips at their own farms to test mares' milk pH. He said for this purpose, he'd suggest using strips that focus on the mid-section of the pH scale (i.e., 5.5 to 8.0), rather than ones that can measure ranges from 1 to 10.

Korosue K, et al. Comparison of pH and refractometry index with calcium concentrations in preparturient mammary gland secretions of mares. J Am Vet Med Assoc. 2013 Jan 15;242(2):242-8. 

The Horse Managing Editor Alexandra Beckstett contributed to this report.

Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.