Table of Contents

  • Sex Chromosome DSD
    • Chimaerism
  • XX DSD
    • Gonadal dysgenesis
    • Ovarian
    • Testicular
    • Ovotesticular
  • XY DSD
  • Sex Chromosome DSD


    Chiamerism is very seldom reported. Anaya et al (2018) reported finding 5 blood chimeras from 23 live twins from 14 twin pregnancies from 21,097 purebred Spanish horses. These did not appear to have a reduced reproductive capacity.

    Anaya G, Fernández ME, Valera M, Molina A, Azconab F, Azorc P, Soléc M, Moreno-Millán M, Demyda-Peyrás D. Prevalence of twin foaling and blood chimaerism in purebred Spanish horses. The Vet J 2018; 234: 142-144

    XX DSD

    Gonadal dysgenesis

    Villagomez et al (2011) reported six cases of XX DSD. They were all SRY -negative. The majority were probably gonadal dysgenesis. Two were identified as having ovotestes and the others were not examined histologically. All had a female phenotype.



    There is a single case report by Kelly and Newcombe (2009) on this entity. The animal was a 3 year old infertile mare. Chromosomal analysis

    Kelly GMM and Newcombe JR (2009) Uterus bicorpora bicollis as a possible cause of infertility in a mare Vet Rec. 2009; 164: 20-21

    Vaginal septum

    A vaginal septum is band of tissue in the vaginal that extends dorsoventrally. It is often an incidental finding.



    McCarthy MB. Townsend KS. Johnson PJ. LaCarrubba AM. Voelkl DL. Volkmann DH. Occurrence of a vaginal septum in a foal diagnosed with pyometra. Equ vet Edu 2021; 33: e273-e279




    Tories et al 2014) reported on a single horse that had abdominal testes without epididymis and a female phenotype with ambiguous genitalia. An attempt to measure testosterone done but none was found. The genotype was XX, SRY -negative.


    Villagomez et al (2011) reported six cases of XX DSD. They were all SRY -negative. The majority were probably gonadal dysgenesis but two were identified as having ovotestes and the others were not examined histologically. All had a female phenotype.

    XY DSD


    Many horses are now being genotyped for parental verification. Martinez et al (2020) reported finding 49 of 209,000 potential XY SRY- females. 33 were confirmed.


    Martinez MM, Costa M, Ratti C. Molecular screening of XY SRY-negative sex reversal cases in horses revealed anomalies in amelogenin testing. J Vet Diagn Invest. 2020;32(6):938-941. doi:10.1177/1040638720952380

    Gonadal dysgenesis

    Villagomez et al (2011) had six cases of XY DSD but none of them had histological evaluation of the gonads. All were SRY -negative and all had a female phenotype and gonadal dysgenesis was assumed.





    Gonadal Dysgenesis




    Female Phenotype

    Villagomez et al (2011) reported on five cases of XY DSD that were SRY -positive. Four had abdominal testes. All had a female phenotype. No measurement of testosterone receptor was possible but it can be assumed that the primary defect is in a failure of masculinisation of the genitalia because of a lack of testosterone production or a lack of the testosterone receptor.

    Male Phenotype

    Both scrotal testes present - Testes normal

    Hypospadias and fistulas

    Harrison and Mushtaq (2017) and Harrison et al (2018) report on the surgical correction of [a] foal[s] with extensive hypospadias and bifid scrotum. One was 3 months of age and the other 6 months of age and the photographs were remarkably similar. The second paper did not quote the first case report. I assume these 2 reports were of the same case.

    Harrison J, Corley K, Kearney C, Mushtaq I. Single stage urethroplasty for perineal hypospadias in a foal. Equine Vet Educ 2018; 30: 527-530.

    Harrison J, Mushtaq I. Single stage urethroplasty for perineal hypospadias in a horse: A case report. J Pediatr Urol. 2017; 13: 84-85.

    Hereditary infertility

    Schrimpf et al (2015) reports on a condition of Hanovarian stallions that have single nucleotide polymorphisms (SNPs) within the within FK506-binding protein (FKBP6) that effects fertility and conception rates. There are no reported abnormalities but have impaired acrosomal reaction.

    Schrimpf R, Metzger J, Martinsson G, Sieme H, Distl O. Implication of FKBP6 for Male Fertility in Horses. Reprod Dom Anim 2015; 50: 195–199.

    Congenital phimosis

    Penile abnormalities are rare!

    Canisso et al (2020) reported on a case of a newBorn cold could not urinate through the penis because the head of the penis was attached to the inner mucosa of the prepuce obstructing the urethra and urethral fossa. Freeing of this connection allowed the colt to urinate normally. This colt also had marked distention of the urachus in the umbilical cord so that large distended fluid filled pockets were formed.


    Canisso IF, Ellerbrock RE, Wilkins PA. Congenital phimosis causing preputial swelling in a newborn foal. Canadian Vet J 2020; 61: 247-250




    Both scrotal testes present - Testes small

    Testicular hypoplasia

    Testicular hypoplasia is where the testis does not develop to its normal size. This is always accompanied by a failure of the epididymis to acquire its normal size. Hypoplasia is almost always seen as a failure of the prepubertal testis to enlarge, but there may be cases where even the prepubertal testis is smaller than normal. Hypoplasia is best diagnosed clinically by identifying that the testis has not increased in size from puberty. Microscopic changes of hypoplasia include cessation of spermatogenesis at some stage, with excessive apoptosis of the germinal epithelium. The most severe cases of hypoplasia have a Sertoli cell-only pattern or they have some normal and abnormal tubules intermixed, often with affected tubules being in the more dorsal part of the testis.

    One scrotal testis present

    The failure to find one or both testes in the scrotum is a clinical challenge. There are several possibilities including

    If the results of endocrinological testing indicate functional testicular tissue, the first condition to exclude is a retained testis (also called cryptorchidism). Surgery to remove the retained structure will hopefully reveal a testis. Pathologists get involved to confirm that any tissue removed contains testicular tissue or some vestige thereof.


    Cryptorchid male horses are XY SRY+ with testes and a male phenotype. Cryptorchidism is infrequently reported in stallions but the prevalence is much higher. It can be unilateral or bilateral.

    Retention of the testis and epididymis is mostly in the subcutaneous/inguinal region. Cryptorchid horses have an equal distribution of the undescended testis - there is an equal number affected on the left and right side. Those that are on the left side are usually intra-abdominal. Those on the right side are located just inside the inguinal ring, within the inguinal ring or just outside the inguinal ring.and occurs about twice as often on the left as compared to the right side. Hartman et al (2015) reported on 604 surgical cases. 280 were left sided and 240 were on the right. In unilateral cases (87%) , the left abdominal (35%) and right inguinal location (28%) was most common. For bilateral (13%), 60% were intraabdominal.

    Little is known of the pathogenesis of cryptorchidism in stallions, although it is believed to be hereditary. There is a lack of evidence however.

    The histological appearance of the cryptorchid testis is similar to that of hypoplastic testes and epididymides. Neoplasia in cryptorchid testes is very rare, with an interstitial cell tumor and a fibrolipoma reported.

    Santschi et al (1989) reported on 3 horses with cryptorcidism that were considered to be monorchidism with epididymis present. This may be previous testicular degeneration or necrosis in a cryptorchid animal.

    Almeida et al (2013) found that cryptorchid testes had increased AMH and AMHR2 immunolabelling when compared with normal testes. This suggests a failure of maturation of Sertoli cells and/or lack of testosterone suppression. Failure of Sertoli cell maturation in the cryptorchid testis may also be attributed to AR abnormalities and/or a consequence of lack of testosterone suppression due to decreased 3bHSD. Cyclin-dependent kinase (CDKN1B)was not expressed in Sertoli cells of cryptorchid testes suggesting that Sertoli cells are still proliferating, which is also a characteristic of the immature testis. In addition, Cx43 expression is decreased in the cryptorchid testis, indicating a disruption in intercellular communication.

    Gardner et al (2017) reported on a single case of a cryptorchid horse with testicular necrosis. They assumed that the cause of cryptorchidism was testicular necrosis. This unfortunately ignores evidence that cryptorchid testes can undergo degeneration, necrosis, and torsion because they are cryptorchid.

    Almeida J, Conley AJ, Ball BA (2013) Expression of anti-Müllerian hormone, CDKN1B, connexin 43, androgen receptor and steroidogenic enzymes in the equine cryptorchid testis. Equine vet J 2013 45: 538-545

    Gardner AK, Santschi EM, Aeffner F, Pigott JH, Russell DS. Testicular ischaemic necrosis as a cause of equine cryptorchidism. Equine Vet Edu 2017; 29: 314-317

    Hartman R, Hawkins JF, Adams SB, Moore GE, Fessler JF. Cryptorchidectomy in equids: 604 cases (1977-2010).J Am Vet Med Assoc. 2015; 246(7): 777-784

    Santschi EM, Juzwiak JS, Slone DEMonorchidism in three colts. J Am Vet Med Assoc 1989; 194: 265-266.

    Stout TAE, (2013) Can and should we do more to reduce the incidence of cryptorchidism? Equine vet J 2013 45: 531-532

    Torres A, Silva JF, Bernardes N, Sales Luı´s J, Lopes da Costa L (2012). 64, XX, SRY-negative, Testicular DSD Syndrome in a Lusitano Horse. Reprod Dom Anim 2012,

    Villagómez DAF, Lear TL, Chenier T, Lee S, McGee RB, Cahill J, Foster RA, Reyes E, St John E, King WA. (2010) Equine Disorders of Sexual development in 17 mares including XX, SRY -negative, XY, SRY -negative and XY, SRY -positive genotypes. Sex Develop 2011;5(1):16-25.

    Marked unilateral degeneration

    Marked unilateral necrosis


    Unilateral anorchia/aplasia or monorchidism

    The presence of one testis only in a horse is a very rare. The 3 terms used for this situation all imply that the testis is missing and never developed.

    Confirmation of aplasia is a diagnosis of exclusion. There must not be any indication of testicular or epididymal tissue. Sometimes there are remnants of the testicular capsule and some primitive tubules, but no indications of lipofuscin containing macrophages or hemosiderin (as would occur with testicular necrosis or total degeneration).

    I have only seen several cases that I was convinced were true aplasia/anorchia/

    Strong et al (1997) reported on a case that did not have any reproductive structures on one side. In contrast, Santschi et al (1989) reported on monorchidism with epididymis present. This latter situation may be previous testicular degeneration or necrosis in a cryptorchid animal.

    Strong M, Dart AJ, Malikides N, Hodgson DR. Monorchidism in two horses. Aust Vet J 1997; 75: 333-335.

    Santschi EM, Juzwiak JS, Slone DEMonorchidism in three colts. J Am Vet Med Assoc 1989; 194: 265-266.


    No scrotal testes present

    Anorchia - Aplasia


    Previous surgical removal

    When there has been previous surgical removal, the end of the deferent duct can be found, and usually the ductus is well developed and at a size commensurate with the size of the ductus at the age of castration or removal. No epididymal tissues or embryonic remnants should be present.

    Congenital phimosis