Page 35 - NMHBA Spring 2022

much stress for them (and they lose weight in early pregnancy) they’ll end up slipping the pregnancy. Hot weather can also be a stress factor, especially with mares that have been trained in the north and come south for breeding. It’s a big adjustment. So, breeding these mares takes some management and individual care. Minimizing stress is very important for success.”
HORMONE IRREGULARITIES IN RACE FILLIES - Stress
can sometimes cause cystic ovaries; the ovary does not receive an appropriate message from the pituitary gland, and the follicles on the ovary do not mature and ovulate. Bob Douglas, PhD, a reproductive physiologist at BET Reproduction Laboratories (Lexington, Kentucky) says that in one type of cyst, the follicle tries to ovulate but doesn’t get the job done, then undergoes a process called luteinization.
“This would normally occur after the egg is released from the follicle in a normal ovulation; the cells that were secreting estrogen undergo this transformation and become luteal cells, capable of producing progesterone. Sometimes when there’s a mess-up in the message from the pituitary, the follicle thinks it has already ovulated, so it becomes half follicle and half corpus luteum (CL) and produces a much lower amount of progesterone. It may also release some estrogen and some androgens (male hormones). As that follicle just sits there and ages, it becomes abnormal and will secrete an abnormal mix of hormones,” says Douglas. This may cause abnormal sexual behavior or merely a total lack of estrus.
“In other cases, the follicle grows to a normal or near normal ovulatory size, then doesn’t get enough message, in terms of LH (luteinizing hormone), from the pituitary. Then the receptors (the proteins that are chemical structures on the follicle wall that can recognize luteinizing hormones from the pituitary) don’t mature. The follicle gets large and becomes dominant but just sits there and can’t ovulate.” It may keep producing estrogen, however, and the mare may seem to be in constant or sporadic heat.
Occasionally a young mare (especially in the 2 to 5 year old age) in hard training may develop hormonal irregularities that cause erratic mood swings and aggressive behavior. The trainer may blame this on heat cycles or a possible ovarian tumor. An exam will show no abnormalities in the ovaries, though they may not be ovulating at all, just as very fit women in strenuous sports may stop having menstrual cycles.
Once the filly is removed from strenuous performance, the problem generally resolves, but it may take some time. During the stressful athletic career, however, when the mare’s erratic behavior is occurring, her hormone levels go wild. This condition is sometimes treated with prednisone (to smooth the erratic behavior while she
is racing), given orally for 3 to 4 weeks. Prednisone (a steroid) will interfere with normal heat cycles, though, if she is to be bred. If she’s been given prednisone while on the track, she will need several months (and preferably a year) before you plan to breed her.
Exercise stress can create various abnormalities in the reproductive cycle, and it’s primarily at the pituitary level (in the brain) where things get messed up, says Douglas. “Women who run 5 miles a day are more likely to have low levels of LH (and erratic cycles) and be infertile. The ovary and follicle do not function properly,” he says.
“About the only time we see this in mares is on the racetrack, due to exercise stress. Those mares may be constantly in heat. It’s
a paradox since they do have progesterone circulating in the blood. This is the only time you ever see progesterone levels above 1, in a mare, unless she is pregnant. The only time you see a mare showing estrus behavior with a progesterone level higher than 1 is when she has an ovarian cystic structure present. The literature tells us there is no such thing, in a broodmare (estrus behavior in spite of high progesterone level), but in our diagnostic lab we occasionally see this,” says Douglas.
“If the trainer or racetrack vet is complaining that a filly is constantly in estrus (never goes out of heat) and yet they cannot palpate anything large on the ovary in terms of an ovulatory follicle, we do a hormone assay and looks at progesterone; usually it will
be between 1 and 2, in those cases. Then we know we have a
cystic structure – this lueinized follicular structure that is secreting estrogen, androgen and progesterone,” he says.
“Normally one shot of prostaglandin will get rid of luteal structure in a mare, but in these we often have to give it twice a day, sometimes for 5 or 6 days, to get them straightened out. The receptor is down- regulated for the prostaglandin, and it’s like giving them water; it has a very little effect. You must first turn over the receptor population again, by giving the prostaglandin often – twice a day, and sometime 3 times a day, if you are in a hurry to resolve the problem. The prostaglandin eventually will chemically destroy that luteinized follicle. The mare will stay in heat for a few more days, then go right out of heat and not have that problem again,” says Douglas.
“We think the problem is due to stress, and we may even feed these mares thyroxin to help resolve it. If their thyroxin is low, we may put them on thyroid hormone to get the thyroid levels up, and usually we can take care of the problem. Many race fillies are under a lot of stress and will tend to have depressed thyroid function.” Thyroid supplementation can be a help.
A MOMENT IN TIME
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