Thornton, CO Animal Ordinances: Mandatory Spay/Neuter (2026)

There is no general requirement in Chapter 6 that every dog or cat be sterilized. Instead, spay/neuter becomes mandatory as a court-imposed sentencing condition once an owner is convicted of certain violations. Under Section 6-7(c)(2), an owner convicted of letting an animal habitually run at large (three or more at-large incidents in 12 months) must, in addition to a fine of at least $100 and up to one year in jail, have the animal spayed or neutered by a licensed veterinarian or licensed shelter and permanently microchipped with the registration filed with animal control.

The same mandatory spay/neuter and microchip conditions apply to convictions for keeping an aggressive animal under Section 6-20(a)(2)-(3) and for an animal attack causing bodily injury under Section 6-24(a)(2)-(3), where the attack fine starts at $500. Outside these court-ordered scenarios, the city separately runs a trap-neuter-return program for community cats under Section 6-42, under which feral cats are sterilized, vaccinated, eartipped, and released, and Vietnamese potbellied pigs must be neutered or spayed by six months of age under Section 6-4(a) as a vaccination-related health requirement, not a general pet mandate.