The removal of abnormal ovaries has long been a remedy for aggressive behavior in mares. Curiously, though, the excision of “normal” ovaries also reliably mitigates unwanted behaviors. Now German researchers may be closing in on the reason why: Those ovaries might be not normal at all.

Unilateral ovariectomies are commonly performed after imaging reveals the presence of an ovarian tumor, says Tanja Witte, DVM, of the Ludwig Maximilians University in Munich. “Granulosa cell tumors (GCT) might produce testosterone and therefore are responsible for ‘stallion-like behavior’ in mares,” she says.

Yet, Witte notes, even when there is no evidence of tumors, many owners opt for the removal of both ovaries (bilateral ovariectomy)—and that also leads to a reduction of unwanted behavior. “Why are behavioral improvements often seen after removal of ovaries that show no clinical detectable signs of tumors? A physiological explanation for this positive outcome is lacking,” she says. “That’s why we performed this study looking for the histopathological explanation.”

The study

The study was based on 30 mares under veterinary care between 2019 and 2022 because of unwanted behaviors. “The problematic behavior was not only estrus-related,” says Witte, “but also behavioral abnormalities independent from estrus.” The three behaviors most often cited by the owners of study horses were “moodiness,” “stressed manner” and “unwillingness to be ridden.”

In 10 of the mares, diagnostic imaging showed evidence of GCT, and only the affected ovary was excised. No signs of tumors were found in the remaining 20 mares; nonetheless, both ovaries were removed.

Prior to the surgeries, researchers examined each mare, took blood samples for hormonal analysis, and documented behavioral and treatment history. They found elevated blood hormone levels in mares found to have tumors and variable hormonal levels in mares with ovaries that appeared normal.

The findings

After ovariectomies were performed, an independent laboratory analyzed the ovarian tissues immunohistochemically using markers characteristic of ovarian tumors. Expression of those markers revealed no significant differences between the “normal” ovaries and those that had tumors.

Further analysis revealed “early neoplastic changes” (ENC) in 15 percent of the mares that underwent bilateral overiectomies. And 30 percent of them had “anovulatory-like follicles,” evidence of a lack of egg release or interruption in ovulation. Both are suspected to be precursors for neoplastic changes.

These findings suggest that the apparently normal ovaries may have been in the early stages of tumor development, Witte says. “ENC are known to develop from anovulatory follicles, and we found anovulatory-like follicles in a high amount of the bilateral ovariectomized ovaries. So we assume that they contribute to the high success rate after removal of both ovaries.”

In follow-up interviews, 85 percent of owners of the mares that had both ovaries removed reported improvements in behavior—a rate also seen in earlier research.

Witte says this study may explain why removing seemingly normal ovaries can improve behavior in mares but calls for further study.

Reference: “Pathohistological findings after bilateral ovariectomy in mares with behavioral problems,” Animals, October 2024