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Study: Some Mares Find Rectal Ultrasounds Stressful

Transrectal examinations might just be part of the routine in the breeding industry. But is that routine really acceptable for animal welfare?

A group of German and Swiss researchers have recently looked into the stress levels that pregnant and lactating mares experience during transrectal ultrasound exams and found that nonlactating Thoroughbred mares perceived the exams as modest temporary stressors.

“In the interest of equine welfare, our study confirms that wherever abdominal ultrasounds can replace rectal ultrasounds—generally speaking, after 90 days of gestation—the abdominal ultrasound should be performed,” said study author Hanno Schönbom, DrMedVet, an assistant at the Unit for Reproductive Medicine in the University of Veterinary Medicine, in Hannover, Germany.

In their study of 25 Thoroughbred broodmares, Schönbom and his fellow researchers noted significant increases in stress parameters when transrectal ultrasounds were performed on pregnant, nonlactating mares. The procedure caused notable increases in salivary cortisol concentrations and in heart rate variability, indicative of acute stress, Schönbom said. Transabdominal ultrasounds, effective without clipping and with application of isopropyl alcohol, resulted in no such increases in cortisol level, he added.

However, the lactating mares in the study did not show signs of increased stress during transrectal exams, Schönbom said. Both pregnant and nonpregnant lactating mares experienced the procedure without any significant elevation of stress parameters.

“Our study supports what previous research has already noted in mammals, including humans, that lactation leads to an attenuation of stress reactions in otherwise stressful situations,” said Schönbom.

That doesn’t mean lactating mares are in a constant state of lower stress than nonlactating mares, however. In their study, the team found that baseline stress levels (normal stress levels at rest) were about the same between lactating and nonlactating mares.

Still, for ethical reasons, Schönbom recommended veterinarians avoid the transrectal route when possible.

“Transrectal ultrasonographic examination is perceived as a modest temporary stressor in nonlactating Thoroughbred mares,” the study authors stated. “Pregnancy checks by transabdominal ultrasonography inflicted a lower stress reaction than the transrectal technique in nonlactating mares. With regard to animal welfare, this technique should be preferred during mid-gestation in nonlactating mares.”

The study, “Influence of transrectal and transabdominal ultrasound examination on salivary cortisol, heart rate, and heart rate variability in mares,” was published in Theriogenology.