Physical, behavioral, endocrinologic, and cytogenetic evaluation of two Standardbred racehorses competing as mares with an intersex condition and high postrace serum testosterone concentrations.

The research focuses on two Standardbred racehorses identified as females, which showed high postrace serum testosterone concentrations and displays of male behavior. Upon physical, endocrinologic, and cytogenetic evaluation, it was confirmed that they were pseudohermaphrodites, genetically male but possessing ambiguous external genitalia. This finding led to speculation that intersex conditions may be more prevalent in racing Standardbreds than previously thought.

Initial Situations and Observations

• The racehorses, initially recorded and competing as mares, displayed serum testosterone concentrations exceeding the acceptable limit for females in postrace drug tests.
• Initial physical evaluation by a veterinarian revealed indeterminate external genitalia and the possibility of testes located intra-abdominally, suggesting male pseudohermaphroditism.

Further Examination

• Both horses were referred for further sex evaluation. Further physical examination of external genitalia validated initial findings.
• Transrectal palpation and ultrasound examinations revealed gonads with an appearance consistent with testes.

Cytogenetic Analysis

• Upon cytogenetic profiling, both horses exhibited a 64,XY karyotype within their cells, which associates with male sex determination in mammals.
• The horses showed eight intact Y-chromosome markers and five SRY gene markers, indicating they were genetically male and confirming an intersex condition.

Behavioral and Endocrinologic Findings

• Both horses displayed some male-type behaviors and endocrinologic characteristics common to sexually intact males.

Final Conclusion

• These diverse evaluations certified both horses as male pseudohermaphrodites. They returned to racing, this time classified as males.
• The Pennsylvania Horse and Harness Racing Commissions, in October 2008, implemented a postrace drug testing policy, which included scanning blood samples for steroids and setting a maximum tolerable testosterone concentration for geldings and females.
• As a result of this new policy, within 8 months these two racehorses were declared to have intersex conditions, raising the possibility of a higher prevalence of such conditions in Standardbred racehorses than initially suspected.