Page 566 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.2 The male reproductive tr act 541

VetBooks.ir on standing, sedated animals or under general Table 2.7 Guidelines for total scrotal width by age
anaesthesia in dorsal recumbency.
for light horse stallions
Detailed explanations of surgical methods are AGE MINIMUM (MM) NORMAL RANGE (MM)
available elsewhere, and the reader is referred to 2–3 years 81 81–111
surgical textbooks for specific information on these 4–6 years 85 85–115
methods.
>7 years 95 95–124
Prognosis
Exposure of the retained testicle to high tempera-
tures results in increased likelihood of neoplastic Table 2.8 Guidelines for testis length and width for
transformation. Affected stallions should not be mature light horse stallions
used for breeding purposes due to the hereditary
nature of the condition. DIMENSION RECOMMENDED MINIMUM (SD)
Left width 57.8 mm (5.2 mm)
TESTICULAR HYPOPLASIA Left length 103.1 mm (82 mm)
Right width 55.8 mm (5.8 mm)
Definition/overview Right length 107.5 mm (8 mm)
Testicular hypoplasia is defined as underdevelop-
ment of one or both testes. SD = standard deviation.

Aetiology/pathophysiology
Hypoplasia of the testes is fairly common in the stallions must not be erroneously diagnosed with
stallion and usually is the result of inherited genetic hypoplastic testicles before growth is complete.
aberrations or chromosomal defects. The condition Note that it is common during reproductive evalua-
may also result from cryptorchidism or exposure tion of the stallion to observe that one testis (usually
to teratogens, toxins or possibly infections during the left) is larger than the other.
fetal life. The gonads arise following migration of
primordial germ cells from the embryonic yolk sac. Diagnosis
Factors that prevent or disrupt this migration, or Examination of a young, post-pubertal stallion
affect the germinal epithelium following formation with small testes, small epididymides, oligozoo-
of the primitive gonad, lead to testicular hypoplasia. spermia or azoospermia and a history of poor
libido and infertility is highly suggestive of testic-
Clinical presentation ular hypoplasia. Testicular biopsy may be helpful
Testicular hypoplasia may be unilateral or bilateral, in confirming the diagnosis (see Testicular degen-
and ranges from mild to severe. A young stallion eration, p. 552).
with testicular measurements well below the mini-
mum recommended for its age may have testicular Management
hypoplasia (Tables 2.7 and 2.8). Stallions with the No effective treatment is known.
condition have low libido, small testes, low sperm
numbers, poor semen quality and a history of infer- Prognosis
tility or subfertility. Stallions with the condition appear to be predis-
posed to TD with advancing age. Use of an affected
Differential diagnosis stallion for breeding purposes is discouraged since
The condition must be differentiated from acquired the condition is likely to be hereditary and often the
conditions of the testis such as TD. Prepubertal result of chromosomal aberrations.