You sometimes hear stories of very difficult mares who really behave more like stallions. This can lead to annoying and sometimes dangerous situations. The people will have tried everything: different feed, training, different stabling, supplements… But the behaviour remains. There is a chance that your mare has an ovarian tumour. 

An ovarian tumour is a tumour on a mare’s ovaries, which is a rare tumour in all animal species. There are several types of ovarian tumours, but the most common is the granulosa theca cell tumour. In principle, this type of tumour can occur in any mare, but is mainly seen in mares between five and ten years old. Often an ovarian tumour occurs on only one ovary, but the hormones that are released do affect the normal functioning of both ovaries. What causes it is unknown. 

The problem with this tumour is that it produces testosterone (a male reproductive hormone), which can cause the mare to show stallion-like behaviour or aggressive behaviour. In addition to the behaviour, the mare can also develop a heavy, muscular ‘stallion neck’. Furthermore, the tumour can influence the estrus cycle (fertility cycle/season). How this is influenced can differ per mare. In some mares the normal cycle is disrupted so that they do not come into estrus at all, while in others the signs of estrus are persistent. If the tumour is very large, it can cause a lot of pain in the mare and sometimes even colic. 

When there are suspicions of an ovarian tumour, the vet will perform a rectal examination. They will palpate the ovaries with their fingers and to determine how big they are and what shape they have. If there is a tumour, one ovary will be abnormally large. The size and structure of the tumour can be viewed by means of an ultrasound. An ovarian tumour is often surgically removed, where the ovary removed from the abdomen. Another method is to operate the horse lying down under general anaesthesia. This method is used less often because of the risks of the anaesthesia and there is more chance of bleeding. There is also the option of removing the tumour via laparoscopic surgery (scoping of the abdomen), this method is particularly advantageous for smaller tumours. 

After treatment, the mare’s behaviour will stabilize quite quickly. If the other ovary is intact and functioning normally, the mare will return to a normal cycle. This may take some time, but eventually (in most cases) you will be able to breed your mare again. 

Sometimes you can’t tell if your mare has an ovarian tumour, especially if it’s not a broodmare. Not all mares give clear signals, no problem you would think, right? That was not the case with the horse We Are, a racehorse in France. The mare was disqualified after winning a race. Tests done after the race showed that the mare had a high level of testosterone in the blood, which therefore resembled doping. The owner and trainer knew she had not been given any testosterone supplements. Testing revealed that the high testosterone levels were due to an ovarian tumour, but race officials took no action. The mare remained disqualified.