Ovarian Enlargement, Physiologic
Epidemiology
Species, Age, Sex
• Unilateral or bilateral enlargement of ovaries might be the result of physiologic or pathological conditions.
• Physiologically, ovarian size varies considerably depending on the season of the year, age, and pregnancy.
• Generally, a young maiden mare’s ovaries are about 2 to 4 cm long and 2 to 3 cm wide during the winter anestrous and increase to 5 to 8 cm by 3 to 4 cm in the spring.
• Older broodmares’ ovaries are usually larger; during the anestrous period, they are about 3 to 6 cm long and 4 to 5 cm wide, and during the breeding season, the ovaries vary from 6 to 12 cm long and 6 to 9 cm wide.
Genetics and Breed Predisposition
• Physiologic enlargement of the ovaries may occur in mares of any breed.
• Spontaneous multiple ovulations appear to be higher in some breeds such as Thoroughbred mares. A retrospective study reported a more than 30% incidence of multiple ovulations (double and triple) in this breed.
• Multiple ovulations are more frequent in older mares, with an incidence of 22% in young mares (2–4 years old) and 51% in older mares (17–19 years old). Because of the relative large size of the equine preovulatory follicle (35–70 mm), the presence of multiple preovulatory follicles results in significant enlargement of the ovaries.
• Large Equine breeds tend to develop larger ovulatory follicles (≤55–70 mm) compared with pony breeds (≤35–45 mm); consequently, large horse breeds with multiple ovulatory follicles develop larger ovaries.
Geography and Seasonality
• In equatorial regions, mares tend to cycle year round if good nutrition is provided.
• In the Northern hemisphere, mares exposed to natural daylight cycle regularly from April to September.
• In general, spring and autumn transitional periods last approximately 40 to 60 days and occur immediately before and after the period with regular estrous cycles.
• Mares in the Southern hemisphere cycle from October to April and have transitional periods as described above.
Clinical Presentation
History, Chief Complaint
• A detailed reproductive history, including changes in behavior, estrous cycle characteristics, sexual behavior, and the last observed estrus, should be obtained.
• During the transitional period, mares may be presented with the complaint of persistent estrus, a history of reduced performance, and intolerance of riding.
• Mares in early gestation can be presented with a history of stallion-like behavior or exhibit estrus signs. In addition, if mating was missed by the owner, mares may be presented with a history of anestrous.
Physical Exam Findings
• During palpation per rectum, the ovary (or ovaries) will be enlarged; the size varies from a softball size to a basketball size.
• Transrectal ultrasonography is the method of choice to identify ovarian structures and to determine the cause of enlarged ovaries.
• Anovulatory follicles are usually as large as or larger than preovulatory sizes. The appearance of the follicular wall and its contents can vary.
• In luteinized follicles, the walls of the follicles have increased echogenicity and thickness, as opposed to a thin and less echogenic wall of nonluteinized follicles.
• Follicular fluid may contain hyperechoic particles (red blood cells), strands (fibrin), or both.
• In pregnant mares, the ovaries will contain multiple follicular structures that may undergo normal ovulation or luteinization. Therefore, the appearance of some of these structures may be indistinguishable from the luteinized anovulatory follicles. However, the fetus can be observed within the uterus.
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