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Basics ⬇

Outline

BASICS

Definition!!navigator!!

ET traditionally refers to the removal of an embryo from the uterus or oviduct of 1 mare (the donor) and placement into the uterus or oviduct of another (the recipient). Oocyte transfer and IVF are other, related assisted reproductive technologies also being further developed and used in horses.

Systems Affected!!navigator!!

Reproductive

Signs!!navigator!!

General Comments

Indications/Potential Donors

  • Mares >15 years with a history of conception failure, EED, and/or abortion
  • Mares with systemic disease or structural abnormalities which prevent them from carrying a foal to term
  • Young mares with valuable genetics or those in competition
  • Certain extraspecific matings, e.g. zebra transferred into a horse recipient

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Diagnostic Procedures!!navigator!!

Prebreeding/Embryo Collection Evaluations

  • Indicated in individuals with a history of conception failure, EED, or abortion
  • Transrectal US, vaginal examination (both digital manual and speculum), endometrial cytology/culture, and endometrial biopsy to detect evidence of anatomic defects, endometritis, and fibrosis, which may predispose a mare to conception failure or EED
  • Similar procedures should be performed on mares being screened as potential ET recipients. These mares should have minimal reproductive abnormalities and physically be capable of carrying a foal to term

Transrectal/Transvaginal US Examinations

  • Transrectal US is used to evaluate follicular development and ovulation and to determine the appropriate timing for donor mare insemination/breeding. The ovulations of the donor and recipient mares should be synchronized with one another, with the recipient mare ovulating within 48 h of (and preferably after) the donor; there is some clinician preference based on the individual's experience
  • Transrectal US examination is indicated for the donor and recipient mares at the time of flushing and transfer, respectively, to determine the absence of intrauterine fluid and the presence of a CL
  • Transvaginal US procedures have been used in the aspiration of oocytes

Other Diagnostic Procedures

  • Checking the recipient's progesterone concentration may be indicated prior to transfer and at the recipient's initial pregnancy examination to check for a functional CL
  • ELISA or radioimmunoassay for acceptable progesterone levels vary from >1 to >4 ng/mL, depending on the reference laboratory

Pathologic Findings!!navigator!!

An endometrial biopsy can demonstrate the presence of moderate to severe, chronic endometritis, endometrial periglandular fibrosis with decreased normal glandular architecture, and/or lymphatic lacunae.

Treatment ⬆ ⬇

Outline

TREATMENT

Appropriate Health Care!!navigator!!

Most ETs are best handled in a hospital setting with adequate facilities and personnel. It is important that all of the reusable and disposable equipment, as well as supplies that will potentially come into contact with the embryo, be free of bacterial contamination and embryocidal residues.

Embryo Recovery Procedures

  • For optimal success embryos are collected 6–8 days post ovulation, depending on the circumstances, logistical considerations, and clinician preference
  • The flushing solution used for ET is commonly a modified phosphate-buffered saline solution with added fetal or newborn calf serum, ± antibiotics
  • Nonsurgical uterine flushing 6–8 days after ovulation
  • Surgical oviductal flushing 2–4 days after ovulation
  • Laparoscopic and, especially, transvaginal US-guided recovery of oocytes

Identification of Embryos

  • After the flushing medium is run into the donor mare's uterus, it is collected, usually in conjunction with some type of filtering device
  • Equine embryos can vary in size and appearance depending on their age
  • Embryos are evaluated and graded using a standard grading scheme. Grade 1 denotes a high-quality embryo, and grade 4 denotes a very poor quality embryo
  • Multiple embryos can be collected, if there are multiple ovulations, and unfertilized oocytes are also occasionally recovered
  • Embryos are washed and can be transferred immediately

ET Procedures

  • Embryos can be transferred immediately, or they can be cooled and shipped to an ET center that maintains recipient mares. The mares are examined regularly throughout the breeding season to identify the best match (synchrony) with embryos as they are received. Smaller facilities may work closely with the shipping entity to ensure there will be multiple synchronized recipients close in ovulation time to the donor mares’ embryos
  • Embryos are shipped in containers developed to transport cooled semen or in specifically designed containers for embryos. The type of medium in which the cooled embryos are shipped might differ from that medium used to flush the donor mare (consult with the ET center prior to breeding the donor)
  • Nonsurgical intrauterine transfer—most commonly used technique; logistically simpler and less expensive than surgical transfer
  • Surgical intrauterine transfer—initially more successful than nonsurgical transfer before improved nonsurgical equipment and methods became available
  • Oocyte collection and IVF, using intracytoplasmic sperm injection to produce a zygote, followed by laparoscopic or surgical oviductal transfer of zygotes (ZIFT)
  • Oocyte collection followed by laparoscopic or surgical oviductal transfer of gametes (GIFT) into inseminated/bred recipient whose own follicles have been aspirated
  • Cryopreservation of embryos and unfertilized oocytes; manipulation, transport, and cryopreservation schemes have improved greatly over the last 10 years

Nursing Care!!navigator!!

Generally required after more invasive procedures in donor and recipient mares.

Activity!!navigator!!

  • Generally restricted after more invasive procedures in donor and recipient mares
  • Preference may be to restrict activity of donor mares in competition because of the possible impact of stress on pregnancy maintenance

Diet!!navigator!!

Normal diet, unless contraindicated by concurrent maternal disease or exercise restriction.

Client Education!!navigator!!

  • ET procedures are not approved by all breed registries
  • Success rates can be less than expected when donor mares are older, subfertile mares
  • Emphasize to the client that communication between the individuals breeding the donor mare and those performing the embryo collection and transfer is essential
  • Recipient mares generally need to be fairly closely synchronized with donor mare, depending on the procedure (within 0–2 days)
  • If the number of normal, synchronized recipients is limited, embryos can be transported to commercial facilities with large numbers of recipient mares
  • Embryo-freezing procedures are improving

Surgical Considerations!!navigator!!

  • Surgical intervention might be indicated to repair anatomic defects predisposing a mare to endometritis
  • Surgical oviductal recovery and implantation

Medications ⬆ ⬇

MEDICATIONS

Drug(s) of Choice

  • Progestins, antibiotics, anti-inflammatory medications, and/or intrauterine therapy may be used in donors and, possibly, in recipient mares, depending on the circumstances, procedures involved, and clinician preference
  • Equine follicle-stimulating hormone has also been used with some success to superovulate mares for embryo collection. However, superovulation remains a challenge with horses

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Accurate teasing records
  • Reexamination of donors diagnosed and treated for endometritis before ET
  • Early transrectal US of recipient mare for pregnancy
  • Transrectal US to monitor embryonic and fetal development in the recipient mare

Possible Complications!!navigator!!

  • Recipient EED or abortion
  • Endometritis in donor mares after the uterine flushing procedures
  • Pregnancy in donor mare following an unsuccessful flush (i.e. no embryo retrieved and follow-up prostaglandin injection not administered post flush to the donor)

Expected Course and Prognosis!!navigator!!

  • Prognosis for successful pregnancy depends on the quality of the oocyte and/or embryo and the reproductive health of the recipient mare
  • Prognosis for successful recovery of intrauterine embryo at 6–8 days after ovulation is ≈70% in normal mares (less in subfertile mares)
  • Prognosis for successful surgical intrauterine transfer of embryos resulting in a successful pregnancy is ≈70–75% for embryos from normal mares (depends on facility and clinician; less from subfertile mares)
  • Nonsurgical intrauterine transfer of embryos originally was less successful (large individual and facility variation) than surgical transfer, but has become very widespread, with improved equipment and techniques
  • Other embryo, early zygote, and gamete procedures might have lower success rates than traditional ET, but still are being improved
  • Embryo cryopreservation techniques have been developed and continue to be improved
  • Oocyte collection, manipulation, and cryopreservation techniques are improving

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Synonyms!!navigator!!

  • GIFT
  • IVF
  • ZIFT

Abbreviations!!navigator!!

  • CL = corpus luteum
  • EED = early embryonic death
  • ELISA = enzyme-linked immunosorbent assay
  • ET = embryo transfer
  • GIFT = gamete intrafallopian tube transfer
  • IVF = in vitro fertilization
  • US = ultrasonography, ultrasound
  • ZIFT = zygote intrafallopian tube transfer

Suggested Reading ⬆ ⬇

Carnevale EM. Advances in collection, transport and maturation of equine oocytes for assisted reproductive techniques. Vet Clin North Am Equine Pract 2016;32(3):379–399.

Hartman DL. Embryo transfer. In: McKinnon AO, Squires EL, Vaala WE, Varner DD, eds. Equine Reproduction, 2e. Ames, IA: Wiley Blackwell, 2011:2455–2478, 2871–2879.

McCue PM, LeBlanc MM, Squires EL. eFSH in clinical equine practice. Theriogenology2007;68(3):429–433.

McCue PM, DeLuca CA, Wall JJ. Cooled transported embryo technology. In: McKinnon AO, Squires EL, Vaala WE, Varner DD, eds. Equine Reproduction, 2e. Ames, IA: Wiley Blackwell, 2011:2455–2478, 2880–2886.

Moussa M, Duchamp G, Daels PF, et al. Effect of embryo age on the viability of equine embryos after cooled storage using two transport systems. J Equine Vet Sci 2006;26(11):529–534.

Rader K, Choi YH, Hinrichs K. Intracytoplasmic sperm injection, embryo culture, and transfer of in vitro-produced blastocysts. Vet Clin North Am Equine Pract 2016;32(3):401–413.

Squires EL. Breakthroughs in equine embryo cryopreservation. Vet Clin North Am Equine Pract 2016;32(3):415–424.

Author(s) ⬆

Author: Tim J. Evans

Consulting Editor: Carla L. Carleton