Suppression of testicular function using two dose rates of a reversible water soluble gonadotrophin releasing hormone (GnRH) vaccine in colts.

This research aimed to study how two different doses of a reversible gonadotrophin releasing hormone vaccine affected the testicular function in colts, with results suggesting they can suppress testicular function, and that these effects are reversible.

Methodology

• The researchers injected either a 200 ng or 400 ng dose of a gonadotrophin releasing hormone (GnRH) vaccine into colts. The injections were administered twice, spaced four weeks apart, with six colts in each dosage group.
• If the colt’s antibody levels dropped below 1000, a third injection was administered to maintain the suppression until the end of the breeding season.

Results

• The results showed that effective antibody titres were present for 12 to 27 weeks.
• Testosterone concentrations decreased from 2.22 to 0.31 nmol/L six weeks after the primary vaccination.
• Androstenedione concentrations decreased from 1.78 to 0.28 nmol/L five weeks after vaccination.
• Testosterone and androstenedione concentrations rose above 0.69 and 0.87 nmol/L respectively, 31 to 43 weeks after vaccination.
• Mean scrotal widths and lengths decreased over 29 weeks from 9.2 cm and 9.7 cm to 6.7 cm and 7.6 cm respectively. At the time of surgical castration, these dimensions were at 10.1 cm and 11.0 cm respectively.
• Semen characteristics were consistent before and after vaccination, with only minor decline in gel-free volume and slight increase in sperm concentration and total sperm per ejaculate, with significantly improved percentage of live normal spermatozoa after recovery.
• The study also found that the testes continued to produce sperm histologically, with no evidence of halting spermatogenesis.
• The mean testicular parenchyma weights for the 200 and 400 ng groups were 129.0 g and 109.8 g respectively.
• Daily sperm production per testis and per gram of testis showed a decrease in the 200 ng group compared to the 400 ng group.

Conclusions

• The study concluded that both dose rates were successful in suppressing testicular function in colts.
• The effects of the vaccine were found to be reversible, indicating that it could be a viable non-permanent contraceptive option for managing equine population.
• The hosts’ individual immune response was found to be less varied with the 200ng dosage compared to the 400ng one.
• Looking forward, the researchers indicated a need for additional study to decrease the variable response in the immunosuppression of testicular function to achieve more consistent results.