Equine Estrous Cycles & Seasonality Flashcards
What is unique about the equine ovary?
entirely internal development of follicles, oocytes, and CL with eruption of oocytes through the ovulatory fossa
- cannot palpate CL or stigma
What are 5 unique aspects of the equine female reproductive tract?
- larger uterine body
- cervix is low in cartilage
- less prominent intercorneal ligament
- transverse fold = main barrier to contamination
- clitoral fossa contains small sinuses where samples can be taken for venereal diseases
When does puberty most commonly occur in horses? What 5 things affect onset?
12-24 months —> able to detect progesterone from the CL of the first ovulation
- environment - season
- social factors
- body size
- nutrition
- genetics
When is it recommended to breed horses? What happens if this is not done?
wait until 3 y/o for complete maturation of stature and reproductive capacity
high rate of pregnancy loss
How can the high rate of pregnancy loss in mares younger than 3 years old be circumvented?
embryo transfer from young mare into a mature mare
What is the seasonality of horse breeding?
long-day seasonal breeders
- 85% seasonally polyestrous
- 5% year-round polyestrous
- 10% irregular
Where are horses most likely to cycle year-round?
equatorial regions
What are the 4 stages of equine seasonality? What causes them to be long-day breeders?
- breeding season
- fall transition
- winter anestrus
- vernal/spring transition - longer daylight, but not enough to cycle
melatonin created from the pineal gland in dark periods inhibits GnRH secretions
What is equine winter anestrus? What 2 hormonal changes are observed? What is not seen?
short days = reproductive incompetence
- minimum or absent GnRH
- low LH, estrogen, and progesterone
minimal change in FSH —> few small follicles are still produced, but stay small and unable to go pasted recruitment due to low GnRH
Long vs. short days, equine cycling:
What 2 physical changes to the reproductive tract is seen in mares during the winter? Behavior?
- few follicles - <5-10 mm, mostly stromal tissue in ovaries
- flaccid reproductive tract
variable - permissively receptive due to lack of progesterone
What happens in mares during the vernal/spring transition? How long does this last? What behavior is seen?
daylight begins to increase from January to March —> 1-2 months (55-60 days)
erratic estrus behavior - inconsistent, wait until after the first ovulation of the year for breeding
What 2 hormonal changes are seen in mares during vernal/spring transition? How do ovaries typically appear?
- gradual increase in GnRH, FSH, and LH, resulting in follicle growth
- low LH due to follicle regression without ovulation
(steroidogenically incompetent)
active
What marks the end of the vernal transition in mares?
first ovulation of the year —> onset of seasonal cyclicity
Breeding season, mare:
melatonin induces seasonality!
What is considered day 0 in the equine estrous cycle?
day ovulation occurs (24-48 hours before disappearance of estrous behavior)
Estrus vs. diestrus in mares:
diestrus behavior due to increase in progesterone!
What behavioral signs are seen in mares in estrus? What are these signs associated with? Why is this significant?
- stallion receptivity (+ Flehmen)
- raising tail
- winking
- passive urination
- squatting
absence of progesterone —> may be seen during anestrus (winter)!
What is the expected size of light breeds, Drafts/Friesians, and ponies/minis preovulatory follicle?
30-45 mm
> 40-50 mm
<30 mm
What is the average size of the equine ovulatory follicle? When are they seen? What forms following stage?
40-45 mm (>35 mm)
24-48 hours prior to the end of behavioral estrus
corpus hemorrhagicum formation
What are 3 signs of estrus in mares on ultrasound? What system is stimulated?
- increased coiling of glands
- endometrial folds thicken
- edema pattern obvious throughout uterus
immune system
How can edema be scored during estrus in mares?
- 0 = no edema, typical in diestrus or anestrus
- 1 = slight edema (“spoke wheel”), more evident in uterine horns
- 2 = moderate edema, peak of estrogen effect
- 3 = heavy edema, exaggerated, pathological (can be seen in vernal transition)
increased echogenicity around follicle seen
How does cervix appearance compare in estrus and diestrus?
ESTRUS = pink, moist, relaxed
DIESTRUS = pale, dry, light (less blood flow)
What behavior changes are seen in diestrus in mares? What causes this?
- squealing
- laid-back ears
- switching tail
- kicking or biting stallion —> NOT receptive
presence of progesterone
What is characteristic of diestrus in mares? What are 3 common physical exam findings?
presence of CH <72 hrs after ovulation or a CL —> produces progesterone
- CH/CL
- toned, round uterine lacking edema (no estrogen!)
- toned (high, tight, white, and dry) cervix
What 3 effects does progesterone production during diestrus have on the mare’s reproductive tract?
- inhibits LH release and GnRH production (NOT FSH) - inhibits secondary ovulations, postpones follicular maturation and growth (1-2 follicular waves)
- immune suppression
- glandular development and secretion of uterine milk (histotroph) to prepare for pregnancy
What 2 things happen in the ovaries during fall transition in mares?
- ovulation fails and may become hemorrhagic (Autumn follicles) —> can look like a follicle on U/S, but has “dots” due to the presence of blood and fibrin
- abbreviated luteal function - luteotropic and luteolytic function have decreased efficiency resulting in a spontaneously prolonged luteal phase —> can’t maintain CL, don’t have mechanism to lyse
