Breeding Soundness Examination of the Stallion

ByPatricia L. Sertich, VMD, DACT, Department of Clinical Studies-New Bolton Center, School of Veterinary Medicine, University of Pennsylvania
Reviewed/Revised Feb 2021

Also see Breeding Soundness Examination of the Male. The breeding soundness examination should begin with a thorough history, including information regarding libido, mating ability, number of mares bred, pregnancy rates, prior illness or injury, and any medications administered. A general physical examination should be performed, noting lameness (particularly of the back and hindlimbs) and heritable conditions that may affect breeding ability or desirability as a sire.

The penis and prepuce should be free of lesions. The testes and epididymides should be evaluated for size, shape, and consistency. The testes should be freely movable within the scrotum and have a total scrotal width >8 cm. The length (L), width (W), and height (H) of the testicles can be determined using calipers or ultrasonography to calculate the volume of each testicle using the equation: 0.523 (L × W × H) = testicular volume. The stallion's daily sperm output can then be predicted by the equation: total testicular volume × 0.024 – 0.76 = daily sperm output. The internal genitalia, inguinal rings, and aorta and iliac vessels are evaluated by palpation and ultrasonography per rectum. Various semen collection and evaluation techniques and criteria can be considered for determination of breeding soundness classifications.

Swabs of the external genital tract (penile fossa and the urethra before and after ejaculation) can be obtained and submitted for aerobic culture to detect the presence of possible pathogens. If consistent heavy growth of potential pathogens such as Pseudomonas aeruginosa, Klebsiella pneumoniae, or Streptococcus zooepidemicus is seen on aerobic culture of swabs of the penile fossa or urethra and there is a history of repeated infection in mares bred, it may be necessary to breed by artificial insemination using semen extender containing an appropriate antibiotic to which the isolated organism is sensitive. If natural service is required, semen extender containing that antibiotic may be infused into the mare’s uterus before servicing.

Stallions found to have an overgrowth of P aeruginosa on the external genitalia (penis, prepuce) may benefit from a once-daily rinse of the penis and prepuce using a dilute acid solution prepared by mixing 10 mL 38% HCl (concentrated) with 4 L of water. Rinsing should be repeated daily until P aeruginosa is no longer isolated. Stagnant water should be removed from the stallion's environment.

If a heavy growth of K pneumoniae is isolated from the stallion's penis and prepuce, a daily rinse of the penis and prepuce may be considered using a dilute bleach solution prepared by mixing 45 mL 5.25% sodium hypochlorite (bleach) with 4 L of water. This rinsing should be performed daily for 1–2 weeks and then discontinued for 4 days, after which swabs should be taken from the penile fossa and preputial fold for aerobic culture. Treatment can be repeated if needed. Stagnant water and sawdust bedding should be eliminated from the stallion's environment.

If Taylorella equigenitalis is present, the stallion should not be used for breeding. The isolation of T equigenitalis requires special culture conditions; the organism will not grow in routine aerobic cultures. Stallions with lesions of coital exanthema should not be used for breeding until skin ulcers are completely healed.

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