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Basics ⬇

Outline

BASICS

Definition!!navigator!!

  • Gestation exceeding the normal range for the individual mare (can be significant variation between mares)
  • Gestation that appears to be lengthened by abnormal characteristics of the fetus
  • Normal range of gestation is 320–355 days, but it is not rare for gestational length to fall outside this range
  • Fescue toxicosis may extend gestation length to >360 days

Pathophysiology!!navigator!!

Primary, Individual, Mare Variation

May be attributable to placental function or dysfunction, endocrine changes; may also involve damage to the endometrium, thus reducing the nutrient supply to the fetus; still able to sustain life, but resulting in fetal intrauterine growth retardation.

Fescue Toxicosis

  • Fescue toxicity caused by an endophyte-infected perennial grass
  • The endophyte Neotyphodium coenophialum produces multiple toxins (those affecting animal health—lolines, ergopeptine alkaloids)
  • Ergot alkaloids act as dopamine D2 receptor agonists on prolactin secretory cells of the anterior pituitary; results in decreased prolactin concentrations and hypogalactia or agalactia in the mare
  • May block fetal corticotropin-releasing hormone, adrenocorticotropic hormone, and cortisol, resulting in a prolonged gestation, leads to a larger fetus
  • Fetus suffers intrauterine hypoxemia
  • Fetus may fail to properly position itself for parturition; resulting in an increased incidence of dystocia; dysmature, weak, or stillborn foals
  • Placental compromise (thickened, edematous); increasingly difficult for membranes to tear during parturition (further fetal compromise)
  • Prolonged gestation is but one manifestation attributed to infected fescue

Systems Affected!!navigator!!

Reproductive

Genetics!!navigator!!

N/A

Incidence/Prevalence!!navigator!!

  • No statistics are available regarding incidence
  • Other than fescue toxicosis, prolonged pregnancy is not a common reproductive problem

Signalment!!navigator!!

  • Females
  • All breeds
  • All of breeding age

Signs!!navigator!!

General Comments

  • Differs from other domestic species, in which prolonged pregnancy is linked with iodine deficiency, increased progesterone, and inherited factors
  • May be caused by abnormal fetal pituitary and adrenal development or by lack of fetal hypothalamic maturity at term

Historical Findings

  • Individual mare—usually appears to be a fetal problem, so historical information may have limited value
  • Fescue toxicosis—has been grazing on endophyte-infected pasture within last 30+ days of gestation

Physical Examination Findings

Primary, Individual, Mare Causes

  • Mare—no abnormalities unless excessive uterine fluid has accumulated
  • Fetus—postpartum examination usually reveals a dead, dysmature or very weak fetus; smaller than normal; may appear undernourished

Fescue Toxicosis

  • Pregnant mare >330 days of gestation, grazing endophyte-infected pasture; no signs of imminent foaling (failure of mammary development, no softening of tissues around tail-head and perineum in preparation for foaling)
  • US may reveal thickened placental membranes:
    • Transrectal US of cervix and caudal uterine body to assess possible abnormalities—thickness, placental detachment
    • Transabdominal US to monitor placental integrity, fetal viability

Causes!!navigator!!

Individual Mare

  • Hormonal—the pituitary or adrenal glands are most commonly involved
  • Not a mare problem; mares can be bred again with little concern for recurrence
  • Fescue toxicosis
    • Grazing/being fed endophyte-infected pasture the last 30+ days before parturition

Risk Factors!!navigator!!

See Causes.

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

  • Accurate history combined with transrectal palpation
  • Outward gross appearance of the mare
  • Appearance of the fetus—prepartum, postpartum, and/or necropsy

Differential Diagnosis!!navigator!!

Hydrops

  • Prolonged pregnancy with a hydrops is invariably shorter than the lengthened gestation associated with a fetus having a higher center defect
  • Known breeding and ovulation dates facilitate differentiation

CBC/Biochemistry/Urinalysis!!navigator!!

N/A

Other Laboratory Tests!!navigator!!

Peritoneal Tap

To ifferentiate the location of excessive fluid accumulation between prolonged pregnancy and hydrops:

  • If hydrops amnii—results from a fetal defect (segmental aplasia within GI tract, may be multiple affected loci; location(s) involved affect speed and volume of accumulation); excessive fluid is amniotic; failure of fetus to swallow and process amniotic fluid across GI tract as it is produced; excess fluid would normally be eliminated as waste into chorioallantoic fluid. Dam is not the cause
  • If hydrops allantois—caused by a uteroplacental defect; excessive fluid is allantoic. The dam's uterus is incapable of normal placental function, not a good future prospect for breeding (rarely linked to a particular stallion)

Imaging!!navigator!!

US imaging, to determine:

  • If excessive fluid is present
  • If the endometrium is thickened
  • If the approximate lengths of fetal extremities differ from what is normal for known gestational length
    • Evaluate the combined thickness of uterus and placenta, a measure of placentitis
    • Assess the appearance and approximate volumes of allantoic and amniotic fluids in relationship to the fetus for a known gestation length

Other Diagnostic Procedures!!navigator!!

When >30 days postpartum, perform an endometrial biopsy of the mare to determine her endometrial status before breeding again.

Pathologic Findings!!navigator!!

  • The uterus may or may not be larger than normal
  • If the fetus is dead or dies postpartum, a necropsy should be done to determine if its adrenal and pituitary glands and hypothalamus are normal

Treatment ⬆ ⬇

Outline

TREATMENT

  • Parturition induction is an option. However, it is essential that the breeding date is accurate, and that records can confirm its validity, to avoid inducing a preterm, nonviable foal
  • Remind owners that fetal survival after a prolonged gestation will be in question. The actual circumstances of survival may not be known until after delivery
  • Routine care for the mare during the postpartum period
  • With the exception of fescue toxicosis, emphasize to owners that prolonged pregnancies can occur and that most affected mares deliver normal foals
  • Remind owners that gestational length may fall outside the normal range and still be normal for that individual

Fescue Toxicosis, Management/Treatments

  • Remove mares from infected feed by 300 days of gestation
  • If the sole source of feed is infected, institute treatment of the mare during the final 2–4 weeks of pregnancy
    • Domperidone—1.1 mg/kg/day PO. Can also treat post foaling, every 12 h for several days to increase milk production, but best to start treatment during the last month of pregnancy
    • Reserpine—0.01 mg/kg every 24 h PO; only effective in resolving postpartum agalactia
    • Sulpiride—3.3 mg/kg/day; to resolve postpartum agalactia, less effective than domperidone
  • Monitor mammary gland development; plan for an observed parturition as delivery assistance will most likely be necessary, be prepared to assist neonate (treat as for a high-risk/critical care patient; likely FTPI so supplement immunoglobulin G, IV plasma, or other supplemental feeding)

Appropriate Health Care!!navigator!!

N/A

Nursing Care!!navigator!!

N/A

Activity!!navigator!!

N/A

Diet!!navigator!!

N/A

Client Education!!navigator!!

N/A

Surgical Considerations!!navigator!!

N/A

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

Refer to Fescue Toxicosis management/treatments, above.

Contraindications!!navigator!!

N/A

Precautions!!navigator!!

N/A

Possible Interactions!!navigator!!

N/A

Alternative Drugs!!navigator!!

N/A

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

  • Monitor mares once they are suspected to be “overdue”
  • Take no action unless pregnancy goes beyond the expected due date in the absence of external evidence of advancing gestation and approaching parturition
  • A pluriparous mare should be close to a previous “term gestation” length in which she delivered a normal foal before considering inducing parturition
  • Failure to show evidence of mammary development:
    • If there are intact horses (males) on the farm, must rule out every likelihood of the mare having been bred on a subsequent estrus, unobserved by owner/farm personnel
    • The suspected prolonged gestation may be of normal length, but the result of a later breeding
    • Delivery of a fetus 3 weeks early will be too immature to survive. Can determine parentage after the fact by DNA testing

Prevention/Avoidance!!navigator!!

  • Unknown, because the major causes involve abnormal fetal pituitary, adrenals, or hypothalamus
  • If grazing/being fed fescue-contaminated feed, remove from that source of feed during the last month of gestation

Possible Complications!!navigator!!

  • Prolonged pregnancy could result in dystocia
  • Because the fetus usually is smaller than normal and has no ankylosis of joints, dystocia usually is not a problem

Expected Course and Prognosis!!navigator!!

Postpartum

  • Normal postpartum examination of the mare
  • The fetus is expected to be smaller than normal and has a low probability of survival
  • Fescue toxicosis results in a dysmature foal (premature, mature; dysmature = overly long)
  • The dysmature foal will be at high risk for FTPI
  • Treat as would be done for other critical care neonates

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

N/A

Age-Related Factors!!navigator!!

N/A

Zoonotic Potential!!navigator!!

N/A

Pregnancy/Fertility/Breeding!!navigator!!

Occurs only during gestation.

Abbreviations!!navigator!!

  • FTPI = failure of transfer of passive immunity
  • GI = gastrointestinal
  • US = ultrasonography, ultrasound

Suggested Reading ⬆ ⬇

Vandeplassche M. Obstetrician's view of the physiology of equine parturition and dystocia. Equine Vet J 1980;12:45–49.

Wolfsdorf KE. Fescue toxicosis. In: Carleton CL, ed. Blackwell's Five-Minute Veterinary Consult Clinical Companion, Equine Theriogenology. Ames, IA: Wiley Blackwell, 2011:259–265.

Author(s) ⬆

Author: Carla L. Carleton

Consulting Editor: Carla L. Carleton

Acknowledgment: The author/editor acknowledges the prior contribution of Walter R. Threlfall.