Comparison of Surgical Vasectomy or Chemical Epididectomy as a Sterilizing Method for Feral Horse Populations

Introduction
Overpopulation of feral horses contributes to rangeland depletion and water source damage through fecal and urine contamination. They also suffer from starvation, dehydration and predation leading to welfare concerns. Sterilization is an important part of managing the feral horse population. While ovariectomy and immunization against porcine zona pellucida (pZP) have been used for mares, castration has been the only method used for stallions. This technique upsets the herd social hierarchy because dominant stallions are displaced. Surgical vasectomy or chemical epididectomy have been used in other species for male sterilization without altering social hierarchy. The objective of this study was to evaluate these methods for controlling feral horse populations. The hypothesis was that sterilizing stallions by surgical vasectomy or chemical epididectomy would be as effective as castration without reducing testosterone.

Methods
Stallions used in this study were part of the feral horse herd managed in accordance with the U.S. Fish and Wildlife Service comprehensive conservation plan for the Sheldon National Wildlife Refuge. Stallions were either surgically vasectomized (n=28), chemically epididectomized (n=16) or untreated (n=34) in the five years prior to this study. For the chemical epididectomy, 10 ml of 1% chlorhexidine in 90% DMSO (v/v) was injected into the tail of each epididymis. In 2013, stallions were gathered for routine castration. Prior to castration, venous blood samples were collected. Immediately following castration, a sample of fluid was collected from each vas deferens and evaluated for the presence of sperm by direct microscopic examination. Total serum testosterone concentrations were determined by chemiluminescence (Immulite ® 1000, Siemens). The effect of treatment on the presence of sperm in the vas deferens fluid was evaluated using Fisher's exact test and the effect of treatment on testosterone concentration (mean±SD) was evaluated using a Kruskal-Wallis rank sum test. Statistical tests were performed using R (version 3.0.2) and significance was defined as p<0.05.

Results
There was no significant difference in testosterone concentration between treatment groups (surgically vasectomized median 23.30 [25th and 75th percentile 20, 30.75] ng/mL; chemically epididectomized 29.25 [20, 58.325] ng/mL; untreated 20 [20, 20] ng/mL; Kruskal-Wallis p=0.08). Surgically vasectomized stallions had no sperm present in the remaining proximal vas deferens. All of the untreated stallions and chemically epididectomized stallions had sperm present within the vas deferens fluid samples. The difference in number of animals with the presence of sperm between treatment groups was highly significant (p<0.001).

Discussion
It is not clear why the chemical epididectomy was not an effective method for sterilizing the stallions. Additional research is needed on alternative methods (such as intratesticular injections) for nonsurgical sterilization in feral horse populations.