Page 508 - Equine Clinical Medicine, Surgery and Reproduction, 2nd Edition

Reproductive system: 2.1 The female reproductive tr act 483

VetBooks.ir large in size. Inhibin, released by the neoplastic tis- ovary is usually firm and small and often shows no
signs of activity.
sue, is thought to suppress the release of pituitary
FSH, leading to almost complete atrophy of the
normal ovarian tissue of the contralateral ovary and Differential diagnosis
cyclic inactivity. Occasionally, the ovary can con- Ovarian haematoma; cystadenoma; teratoma; other
tinue to function normally, but conception is rare. ovarian neoplasia, lymphoma or lymphosarcoma.
Neoplastic thecal cells may produce testosterone,
leading to stallion-like behaviour. Progesterone Diagnosis
levels are low due to ovarian inactivity. Rarely, the Diagnosis is made on the basis of the history, clini-
neoplastic ovarian tissue causes raised oestrogen cal signs and palpation per rectum of one grossly
levels, which may lead to abnormal mammary gland enlarged, non-cycling ovary and a small, firm, vari-
development and signs of nymphomania. The pat- ably cycling contralateral ovary, combined with
tern of hormone production may alter as the tumour ultrasonography results. Ultrasonography reveals
grows and develops. the structure of the affected ovary to be multicystic
(‘honeycomb’) in the majority of cases (Fig. 2.92)
Clinical presentation but can vary from a single large fluid-filled cyst to a
A variety of signs are exhibited according to the dense homogeneous mass. Occasionally, one or more
nature of the hormones released by the tumour.
Masculinisation may occur, with the mare showing
stallion-like behaviour of aggressiveness and inter- 2.92
est in cycling mares or, in the chronic case, develop-
ing a more masculine muscle distribution and even
clitoral enlargement. The oestrous cycle is often
affected, with the mare exhibiting anoestrus charac-
teristics, constant oestrus (nymphomania) or irregu-
lar cycling. On rare occasions the unaffected ovary
may continue to function and the mare cycles nor-
mally. Tumours have occasionally been diagnosed in
pregnant mares.
The affected ovary is usually grossly enlarged on
rectal palpation, with no palpable ovulation fossa,
and it may be cystic (Fig. 2.91). The contralateral

2.91

Fig. 2.91 Post-mortem specimen showing the cut Fig. 2.92 Transrectal ultrasonographic view of an
section of a granulosa cell tumour of the ovary. Note ovary with a multicystic ‘honeycomb’ granulosa cell
the multiple honeycombed structures. tumour.