"600 Million" is actively fundraising for a single-dose oral sterilant for male and female cats and dogs. Unfortunately, the campaign lacks transparency, data, and results.
Calcium chloride is being championed as an injectible sterilant for male dogs and cats. Read ACC&D’s position statement on use of this approach to sterilization.
In 2013, JAVMA published results of a modeling study advocating vasectomies and hysterectomies (versus traditional spay/neuter or lethal control) in feral cat management.