Breeding

Estrus Suppression

A review of the vaccine used to prevent heat in mares.

Mares come into heat or estrus due to rising estrogen levels, which are produced by developing ovarian follicles. Mares typically do not show heat during the winter when they have small inactive ovaries. Heat is also not exhibited after ovulation. Ultimately, expression of behavioral estrus occurs in the presence of estrogen and the absence of progesterone. Absence of estrus is a function of low estrogen levels and/or elevated progesterone levels. The development of follicles and ovulation are initiated by hormonal signals from the brain. Gonadotropin-releasing hormone is produced by the hypothalamus and secreted in pulses into the bloodstream. The anterior pituitary gland near the base of the brain receives the hormone signals. The pituitary secretes a follicle-stimulating hormone that stimulates follicular growth and luteinizing hormone that causes follicle maturation and induces ovulation.

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Attacking the Hormone

Vaccines have been developed that target the gonadotropin-releasing hormone in this cascade of events. A vaccine is typically used to stimulate the immune system to produce antibodies against one or more specific infectious disease agents. However, vaccines can also be developed against other substances, including hormones. Vaccination against the gonadotropin-releasing hormone results in production of antibodies that bind to the hormone circulating the blood stream and block the biological activity of the molecule.

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As a result, the lack of bioactive gonadotropin-releasing hormone results in a reduction of follicle-stimulating hormone and leutinizing hormone production by the anterior pituitary and, ultimately, a reduction of ovarian follicular development. The absence of follicular growth is associated with estrogen levels that are insufficient to cause behavioral estrus. Equity is the name of a commercial vaccine licensed in Australia "for use in the control of estrus and estrus-related behavior in fillies and mares not intended for breeding." The vaccine is administered in a two-dose series. Research shows that vaccinated mares develop an antibody response that peaks approximately two weeks after the second vaccination. Immunization resulted in a reduction of ovarian activity. Vaccination also was associated with a reduction in estradiol levels and a decrease of behavioral heat.

Vaccination

Most mares had an increase in ovarian activity and a return to estrus as the concentrations of antibodies in circulation decreased over time. However, some young mares vaccinated against the hormone when they were in training failed to develop follicles or ovulate when their performance career was over. The incidence rate of this issue has not been determined. The Australian vaccine is clearly labeled for fillies and mares not intended for breeding. Holding to that principle would negate any adverse effects on ovarian function. However, the use of a horse can change over time, as horses are bought and sold. In addition, once cannot predict the end of an athletic career and the start of a breeding career.

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In summary, vaccines against the gonadotropin-releasing hormone offer great potential for a medically safe and straightforward means of blocking estrus or inducing contraception in mares. However, there might be serious issues regarding future reproductive performance in a limited percentage of vaccinated fillies or mares. Currently there are no vaccines against gonadotropin-releasing hormone approved for horses in the United States.