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A rig is a horse that shows stallion-like behaviour but has no palpable testes. There are two types of rig: a true rig (cryptorchid), which has some retained testicular tissue and may be fertile; and a false rig, a gelding with no remaining testicular tissue, both testicles having been completely removed by castration.

A wide spectrum of behavioural problems is associated with rigs, from biting and being bargy with people on the ground, through to attempting to separate off an in-heat mare before mounting and actually covering her. These tendencies most often manifest themselves in spring and summer, when mares are coming into heat.

There appears to be no association with how early or late a colt is castrated and subsequent rig-like habits. A rig may not have expressed this behaviour before, even though he has been in the same ownership for several years. It is thought to be instinctive behaviour that may or may not have been allowed in the past.

Hence, a horse can emerge with rig-type tendencies with no apparent previous history. His presence in a yard could put other horses at risk of injury by being kicked or herded into fences, or through mating.

Tests and treatment

IT is advisable to test a horse if he displays this behaviour, to establish whether he is a true or false rig and therefore how to proceed. The recommended rig test, with 100% reliability, is the single blood test to measure anti-mullerian hormone (AMH). This is a specific test for the presence of testicular tissue in horses of any age.

The use of AMH in cryptorchid donkeys has not been validated, but it is a current topic of research. Other tests that can be used are the measurement of blood oestrone sulphate in horses of more than three years of age. This is not reliable, however, in younger horses.

The human chorionic gonadotrophin (HCG) stimulation test is recommended for all donkeys and for any doubtful results in horses and ponies. This requires a baseline testosterone blood test, followed by the intravenous injection of HCG and a second blood sample 30 minutes to 48 hours later to measure any increase in testosterone.

About 90% of horses tested for stallion-like behaviour are found to be false rigs. While a negative rig test result will put an owner’s mind at rest that any mating activity should not result in a pregnant mare, the behaviour still has to be managed.

Cryptorchidism, the retention of one or both testicles resulting in a true rig, is the most common developmental defect in the horse. It is estimated that between 5% to 8% of all male foals are affected. If one testicle is retained there is reduced sperm production; if both are retained the horse is usually infertile. A retained testicle  is thought to be more at risk of developing neoplastic (cancer) cells, as a result of the warmer temperature inside the body compared with the scrotum.

The missing testicle or testes can be located by ultrasound scanning of the caudal (rear) abdomen and inguinal (groin) regions, in order to decide the most appropriate surgical approach for removal. Most commonly, inguinally retained testicles are removed under a short general anaesthesia, either at home or at the clinic.

The recent development of standing laparoscopic surgical techniques is proving less invasive for the removal of abdominally retained testicles, and is thought to carry less risk of herniation postoperatively.

Vets are trained to leave an existing testicle in situ until the missing one has been located and removed, to avoid a horse being misidentified as a gelding and sold as such. Rogue (non-qualified) castrators still exist, however, and may be less scrupulous. Owners should be dissuaded from considering the use of these people for cheap castration, as this compromises equine welfare and increases the risk of the sale of known true rigs.

Legal implications

Rigs are often sold in winter, when they are less likely to show unwanted behaviour. These animals can be a danger to other horses and handlers, so it is not safe to sell them without disclosing the information.

If the horse is good at his job and the buyers are knowledgeable about managing these types, then an appropriate deal may be agreed. It’s advisable to have the horse tested, however, to ensure that he is not a true rig, as this could incur further costs for surgical removal of the retained testicle.

Where do you stand as a buyer if you discover that you’ve been sold a rig without prior knowledge? If you purchased the horse from a dealer and you find out within 30 days of both ownership and the horse being delivered, then as a consumer you have a short-term right to return the horse. If you wish to do this, you must state in writing that you are ending the contract between you and the dealer. It is advisable to have a written contract of sale: check with a solicitor about details that should be included.

A buyer could have the right to return a known false rig to a dealer if his behaviour is such that it renders him unfit for the purpose for which he was sold, or to return a true rig if he was advertised as a gelding.

Even if someone is aware of behavioural problems, such as rig-like behaviour, as a private seller they are not legally required to disclose these unless specifically asked. If it can be shown, however, that the seller did not give honest answers to your questions, which you relied upon when buying the horse, then you are likely to have the right to return.

Is it inherited?

Further research is needed to establish how large a role genetics and hereditability play in equine cryptorchidism. This would allow the development of a more universal policy among breeding organisations about the acceptability of registering unilateral (one-sided) cryptorchids as breeding stallions.

Work is under way on gene sequencing of the equine genome to progress our knowledge about the mode of inheritance of this condition, providing more information for the future of breeding.

Ref Horse & Hound; 24 November 2016