The Modern Equine Vet
March 2024
Vol 14 Issue 3 2024
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Technician Update

Ultrasonography for Oocyte Recovery

By Cara Linse, BS

Transvaginal aspiration (TVA) is an advanced reproductive procedure that uses an ultrasound-guided needle to puncture the ovary and its follicles to recover oocytes. The oocytes will be fertilized by intracytoplasmic sperm injection, and we hope to produce an embryo that can be transplanted into a recipient mare to carry and produce a live foal.

Transvaginal aspiration carries inherent risks, however, and previous studies have explored the effects of this procedure on blood values and abdominal fluid parameters, the timing of the procedure, the number of punctures to the ovary, and the probability of the formation of ovarian abscesses.

We wanted to characterize normal findings and baseline values of free abdominal fluid prior to and following TVA to aid in appropriate and early identification of pathology in case the mare’s health declines after the procedure.

We did TVA on 17 healthy mares (7 mares were aspirated twice during the length of this study), recording the signalment, sedation, medication and time, and used transabdominal ultrasonography to measure the depth and character of free abdominal fluid.

For consistency, this measurement was taken at the most ventral aspect of the abdomen on midline using a 5-7.5MHz linear rectal transducer (Sonosite Edge II). A larger scan of the abdomen was also performed to determine the distribution of the fluid throughout the abdomen. This scan was repeated at the conclusion of the TVA and the fluid pocket was measured again in similar fashion. The volume of fluid (EmCare Complete Flush Media) infused and recovered was measured based on weight to calculate a presumable volume of fluid lost in the mare from the procedure.

Prior to the TVA, the mean fluid was 0.56 cm, with a range from 0.0 to 1.54 cm. Post-aspiration, the mean depth of the abdominal fluid pocket was 1.17 cm with a range of 0.0 to 3.02 cm. The mean volume of unaccounted fluid post-procedure was 85.7 mL and the mean time of aspiration was 28 minutes.

Pre-aspiration observations of the fluid distribution yielded 42% (13/31) with no fluid; 26% (8/31) had a focal pocket; 13% (4/31) were multifocal; and 19% (6/31) diffuse. Post-aspiration observations of the fluid distribution was 23% (7/31) no fluid; 32% (10/31) a focal pocket; 42% (13/31) multifocal; and 3% (1/31) diffuse.

Pre-aspiration observations of fluid character were 42% (13/31) no fluid; 26% (8/31) anechoic; 32% (10/31) slightly echogenic; and 0% (0/31) hyperechoic. Post-aspiration, fluid character was 23% (7/31) no fluid; 39% (12/31) anechoic; 35% (11/31) slightly echogenic; and 3% (1/31) hyperechoic.

All mares were healthy and clinically normal in the 24 hours following the transvaginal oocyte aspiration procedure.
These findings help to understand the character, measurements and changes in ultrasound appearance of abdominal fluid following oocyte aspiration. This study size was not large enough to determine the significance of changes in abdominal fluid but does suggest there are some changes in fluid seen following aspiration.

The amount of measurable free abdominal fluid is 42% (13/31). Aspirations began with no significant fluid noted in the abdomen at 23% (7/31), and ended with no measurable fluid or 77% (24/31) having measurable fluid post-TVA. As one of the goals was to establish normal baseline values, an important measurement was the maximum amount of fluid measured in a mare’s abdomen at any point in time was 3.02 centimeters, and that occurred post-aspiration.

There was also generally an increase in multifocal distribution of fluid, and the fluid was most commonly anechoic or slightly echogenic in character post-aspiration. The more echogenic fluid visualized was also not associated with a negative outcome. This study saw similarities to previous studies in that all mares were clinically healthy and there were no adverse health effects such as fever, inappetence, or colic in the 24 hours following an uncomplicated TVA procedure, regardless of the time or number of follicles punctured. MeV

About the Author
Cara Linse’s career began in Lexington, Ky., nearly a decade ago working on Thoroughbred breeding farms foaling out mares and raising those foals. Her interactions with veterinarians on those farms piqued her interest in veterinary medicine, and in 2018 she acquired a position at Rood & Riddle Equine Hospital.

Since then, Cara has progressed within the clinic, learning throughout her time on the nursing staff, to the Internal Medicine Department, and is now working within the theriogenology group where she has been able to further her technical skill set assisting in advance theriogenology procedures such as oocyte aspiration and hysteroscopy.

Additionally, she has worked for Equine Analysis Systems, measuring their cardiac ultrasound scans as part of evaluations of performance potential for Thoroughbred public auction sales.

Outside of work, she spends as much time as possible in the outdoors with her Golden Retriever, Rossi, and her off-the-track Thoroughbred, “Paul With All.”

For more information:
Orellana-Guerrero D, Dini P, Santos E, et al. Effects of transvaginal aspiration of oocytes on blood and peritoneal fluid parameters in mares. J Equine Vet Sci. 2022;114:103949.
Velez IC, Arnold C, Jacobson CC, et al. Effects of repeated transvaginal aspiration of immature follicles on mare health and ovarian status. Equine Vet J. 2012;43:78-83.

Performing ultrasonography.

Checking the ultrasound image.

Pre-TVA scan and post-TVA scan.
Images courtesy of Cara Linse and M. Schnobrich