One morning I receive a phone call with a request to pay a visit and scan the 6-year-old mare Beike, because she seems to be in heat. On arrival there appears to be a sizeable follicle of over 5 cm on the ovary. The mare is inseminated and at the next check-up the follicle is still there but ovulation hasn’t taken place. In the end, the follicle doesn’t ovulate at all, which is rather strange. After consultation with the owner I decide to wait for a fortnight and then return for another check-up. But at every check-up nothing has changed, Beike does not develop a normal cycle. Naturally, the other ovary is also constantly checked. I discover a special and small, anomalous structure and it’s not instantly clear to me what this is. Even after an injection to bring her in heat, Beike’s cycle still doesn’t take off. On top of that, the owner also reports that Beike is showing unpleasant behaviour when under saddle. She is resisting and evading the leg. Sometimes she kicks out and throws in a buck. All these symptoms together make me think that this mare might have a hormone-producing tumour on the ovary. But this kind of tumour is rare in young mares and these tumours are usually quite big, balls with a diameter of up to ten to twenty cm. The anomalous structure on the ovary of the 6-year-old Beike is just around 4 cm. I decide to consult a professor at the Veterinary Faculty in Utrecht.
To get more information about the nature of the possible tumour some tests are carried out. The readings are slightly out of the normal range but are consistent with a granulosa cell tumour. This is a tumour on ovaries which is usually benign, but does cause a deviation in hormone production. Symptoms can differ widely. Some mares are not at all affected, but when the owner decides to breed a foal from the mare then it turns out that she has a big tumour on the ovary. Other mares react strongly to the hormonal changes and start exhibiting stallion behaviour, do not develop a normal cycle and are bad-tempered when ridden under saddle. Together with the owner I take the decision to take Beike to a specialist clinic for surgery. There too the doctors first have reservations about the diagnosis: the structure is indeed very small, the mare is young and the blood levels are found to be just a little out of the normal. More research is conducted at the clinic, following which the doctors decide to go ahead with an operation to remove the ovary. The ovary is sent to the pathology department of the Veterinary Clinic to establish if we are indeed dealing with a tumour. Histologic research, which is research of tissue and organs, produces confirmation that this is indeed a hormone-producing granulosa cell tumour.
Granulosa cell tumours (GCT) are the most common ovary tumours in mares. 97% of ovary tumours are GCT. They are generally not malicious and only appear on one side. In very rare cases there are tumours on both ovaries. The GCT usually produce steroid hormones, which can explain the changes in behaviour and is also the reason for the absence of a normal season. Most GCTs are between ten and twenty cm, but they can range from 6 – 40 cm. So in Beike’s case we came across a very small tumour. The prognosis after removal of the ovary is excellent. After a period of recovery breeding with such a mare is still possible.
Now Beike has one ovary left. The owners still have the wish to breed a foal from her. After allowing her to recover over winter we have started the new breeding season with good hopes. Experience has shown that mares with only one ovary have no problem giving birth to a foal. Mares with just one ovary usually have normal cycles every three weeks, but it also happens that such a mare may from time to time skip a cycle. After an intense year with an operation from which Beike has fortunately fully recovered but was not able to produce a foal, the owner can now finally look forward to a foal for next year.
