Measurements of reproductive function in stallions treated with trimethoprim-sulfamethoxazole and pryrimethamine

Objective-To evaluate the effects of trimethoprimsulfamethoxazole and pyrim ethamine treatment on various measures of reproductive function in healthy pony stallions. Design-Randomized complete block study. Animals-12 healthy, mature pony stallions. Procedure-Stallions were assigned to treatment and control groups balanced for age and various characteristics of reproductive function. The treated g roup received trimethoprim-sulfamethoxazole and pyrimethamine for 90 days d uring summer and fall; the control group was not treated. Semen characteris tics, sexual behavior, testicular volume, and sperm production efficiency w ere evaluated before treatment started and at 30-day intervals until 60 day s after treatment ended. Results-Effects of treatment were not detected for semen characteristics, t esticular volume, sperm pro production efficiency, libido, erection, and qu antitative measures of ejaculatory efficiency. At 30, 60, and 90 days, 4 of 6 treated stallions had unsteadiness upon mounting, clumsy or weak thrusti ng, failure to flex the back, and thready or inapparent ejaculatory pulses that resulted in dribbling of semen rather than forceful expulsion. Conclusions and Clinical Relevance-Although treatment with trimethoprim-sul famethoxazole and pyrimethamine may not affect semen quality, testicular vo lume, sperm production efficiency, erection, or libido of healthy stallions , treatment may induce changes in copulatory form and agility and alter the pattern and strength of ejaculation. Stallions that develop neurologic sig ns during treatment should be used with caution for breeding.