Family of rare ‘intersex’ horses discovered

Koko is a male who appeared to be female.
Koko is a male who appeared to be female.

Equine researchers at a Canadian university have identified a family group of horses with a rare genetic abnormality which means they are termed as ‘intersex’.

Koko, a pinto mare, appeared to be typical of her sex, but when she began acting overly aggressive and trying to mount her female counterparts, her owner became suspicious.

After a barrage of tests at the University of Guelph’s Ontario Veterinary College, researchers discovered that Koko was in fact a male horse that appeared female.

The scientists tested Koko’s relatives and found her sister Sequoia and her cousin Pandora were also intersex.

Researchers Allan King, Tracey Chenier and Daniel Villagomez are the first to identify a family of horses with a rare genetic abnormality called pseudo-hermaphroditism, which causes genetically male horses to appear female on the outside.

Initially, they thought Koko had an ovarian tumour, which can cause mares to exhibit stallion-like behaviour because of heightened levels of testosterone. But when they examined the animal’s reproductive system, the researchers found Koko had internal testes.

“It was a very exciting discovery,” said Chenier. “To be expecting a tumour but discover an entirely different internal system than anticipated sparked a lot of questions.”

The research team then conducted a karyotype analysis, which is a detailed study of an animal’s chromosomes, and results showed that Koko had a male genotype (XY).

“Because these horses are from the same family line, the condition is likely a genetically heritable one,” said Chenier.

“A very similar condition can occur in humans with what is known as androgen insensitivity syndrome. It’s linked recessively to the X chromosome and passed through the female line.”

The researchers suspect Koko’s mother has a mutation on her X chromosome that causes male offspring to appear female. Female offspring show no abnormalities but can, in turn, pass it on to their male offspring.

Currently, the team of researchers is looking for the genetic mutation that would cause androgen receptors – those that respond to the male hormones that are responsible for secondary sex characteristics – to be underactive.

Once the mutation is located, a better tool could be developed to test animals suspected of having the condition.

“We are studying the DNA, molecule by molecule, to try and determine what the mutation is and how we can more easily diagnose this disorder in horses,” said King.

After realizing Koko had internal testes, researchers performed surgery to remove them in the hope that she would become less aggressive.

“Before the surgery, she was so aggressive that it was too dangerous to ride her,” said owner Sam Campbell. “But she is completely different now. She is an incredibly lovely horse.”

Since then both Sequoia and Pandora have undergone the same procedure at the Ontario Veterinary College.

Although sex development disorders are rare, two standardbred horses in the United States were recently diagnosed with a similar intersex condition. The two animals turned out to be related to one another, but not to the horses found in Ontario, said King.

The two cases in the US were discovered after repeated drug tests found high testosterone levels.

“It was initially assumed the high levels of testosterone were due to steroid use, but when examined more closely it was determined the horses had an intersex condition.”

Finding related horses with this condition raises questions about how common this genetic abnormality is and whether it is fair for horses with this condition to race, he added.

“Nobody knows who they should race against – fillies or colts.”

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4 thoughts on “Family of rare ‘intersex’ horses discovered

  • July 18, 2014 at 3:45 am

    I would be interested knowing if it would be a compassionate and healthy decision to remove the undescended testes.

    • July 18, 2014 at 12:24 pm

      not only compassionate for the sake of a horse with a steadier emotional state but more importantly healthier. Internal testis have a very high risk of becoming cancerous and it is always better to get them removed. It occasionally happens that otherwise totally normal healthy colts will have undecended testis and need to have them removed for the same reasons.

  • May 1, 2015 at 11:12 pm

    Rather misleading to suggest this is new. I learned of other families of intersex horses at university classes in 1980. Easier to test DNA and share information now. Professor Milton Diamond estimates that 1 in 100 humans have an intersex condition although only 1 in 2,000 are easily detected. Other mammals commonly have some intersex offspring with inbreeding raising the likelihood of this being found in families (like pure bred cats).

  • January 14, 2017 at 10:48 am

    Have actually loaned a horse exactly like this…her owner didnt know and just said she was a bit ‘dominant’. I took her to my yars and she was complete nutter. Mounting the mares in the field,attacking the geldings and the owners of the mares if they were trying to catch them. She would pace in her stable and kick the door if the mare next to her was in season. After i got the vet out he suggested a hormone imbalance, i spoke to her owner who decided to take her home and have it looked into and after a thorough examination it was discovered she had testes inside although she appeared to be a mare on the outside and did have both male and female reproductive organs. She had everything removed including the testes and last i heard she was doing really well. Had totally calmed down,was great in the field with other horses and great under saddle.


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