Changes in blood testosterone concentrations following surgical and chemical sterilization of male free-roaming dogs in southern Chile

There is a growing interest in chemical sterilization as an alternative to surgical castration in large-scale sterilization campaigns to control canine populations. An important step toward understanding the short- and long-term effect of chemical sterilants is to determine their impact on blood testosterone concentrations, particularly as these could influence dog behavior post-treatment. A field trial was conducted with 118 free-roaming male dogs in the Chilean Patagonia, where 36 dogs were chemically sterilized using EsterilSolTM, 39 dogs were surgically castrated, and 43 dogs remained intact as controls. Blood testosterone levels were determined at four time periods: upon enrollment six months prior to treatment (t-6m), at the time of treatment (t0, within one hour after surgical castration or chemical sterilization and during a concurrent two-week period for the control group), four (t+4m), and six (t+6m) months post-treatment. Intrinsic and temporal factors were evaluated: age was significantly associated with testosterone, where dogs 2 to 4 years old had the highest testosterone concentrations (P=0.036), while body weight and body condition scores were not associated with testosterone; testosterone concentration was not influenced by time of day, month, or season. After treatment (t+4m and t+6m), all of the surgically castrated dogs had testosterone concentrations below 1.0 ng/ml. Based on this cut point (<1 ng/ml), testosterone remained unchanged in 66% of the chemically sterilized dogs at both t+4m and t+6m; remained low for 22% of dogs at both t+4m and t+6m; was unchanged at t+4m but low at t+6m in 9% of dogs; and, was low at t+4m but reverted back to unchanged at t+6m in one dog (3%). Incidentally, testosterone in chemically sterilized dogs increased dramatically within 1 h of treatment (t0), more than doubling (131%) the concentration of control dogs at the time of treatment (t0), likely due to severe necrosis of interstitial cells. The use of EsterilSolTM as a method of sterilizing dogs had a variable effect on blood testosterone concentrations. Approximately 30% of chemically sterilized dogs had a reduced testosterone concentration (actual maximum 1 ng/ml) after six months, similar to that of surgically castrated dogs. The majority of the chemically sterilized dogs, however, showed no long-term changes in blood testosterone concentrations.