Thousands of residents on both sides of the debate have contacted the mayor’s office in the past week, Hancock wrote, “passionately expressing their opinions” on an issue that has long divided local governments, housing authorities and military bases across the country.

Proponents of pit-bull bans, formally called breed-specific legislation, argue that the dogs are genetically predisposed to violence. However, animal experts say that notion is misinformed and not based in science.

Organizations such as the American Veterinary Medical Association, the Centers for Disease Control and Prevention, the American Bar Association, the Humane Society of the United States and the American Society for the Prevention of Cruelty to Animals have expressly opposed breed-specific legislation, according to a journal article for the AVMA.

Even the National Animal Care and Control Association has said restrictions on dangerous animals should be based on a dog’s individual behavior, not on its breed. “Pit bull” is not a breed but rather a generic category that can include American pit bull terriers or Staffordshire terriers and mixes, and studies have found that the label is often given arbitrarily.

Hancock said he was vetoing the ban reversal because he felt the ordinance did not realistically account for the city’s current challenges with regulating dog ownership of any kind. Less than 20 percent of pets in Denver are currently licensed, he wrote in the letter.

Under the reversal ordinance, pit bull ownership would have began with a “breed-restricted license,” which registers the animal with the city and requires proof that the dog has been microchipped, vaccinated for rabies, and spayed or neutered.

After 36 months without a violation of the city’s animal safety policies, Denver Animal Protection would have waived the need for a restricted license and allowed owners to register their pit bulls under the same requirements of any other breed.

The new ordinance would have limited pet owners to two pit bulls per household. Owners were required to notify the city within eight hours if their pit bull bit something or escaped. If the dog died or the owner moved, the owner was required to alert the city within 24 hours.

But Hancock said the city cannot allow pit bull ownership until greater progress has been made to address off-leash dogs and pet licensing and registration.

“The reality is that irresponsible pet owners continue to be a problem, and it is the irresponsible owners and their dogs I must consider in evaluating the overall impact of this ordinance,” Hancock said. “We cannot diminish the very real, very traumatic experiences of those who have reached out to me to share their stories.”

The mayor praised the ordinance’s sponsor, Councilman Christopher Herndon, but said he does not believe the proposed language “fully addresses the very real risk to severe injury that can result from attacks from these particular dog breeds, especially should they happen to a child.”

The reversal would pose an “increased risk” to the people of Denver. Passing it would do a “disservice” to the community if “harm comes to someone as a result.”

“I’m disappointed the mayor is choosing to disregard the science on the issue of breed-specific legislation,” Herndon said in a statement. “Research tells us breed-specific legislation is ineffective at keeping communities safe and experts in the field — from the local level to the national level — agree it is no longer best practice.”

If the council does not vote to override the mayor’s veto, Herndon said he will prepare a ballot measure on the ban for the November election.

“I hope my colleagues on Council will support an evidence-based approach to our animal ordinance, rather than one steeped in fear and stereotypes,” Herndon wrote.

Breed-restriction ordinances in cities across the country first gained popularity in the 1980s, when news reports maligned pit bulls as aggressive and threats to public safety, according to the AVMA. The first breed-specific ordinance in the United States was enacted in Hollywood, Fla., in 1980, but was reversed two years later when a judge ruled there was little evidence that pit bulls posed a greater threat than other dogs, the article said.

As of 2017, more than 900 U.S. cities had some form of breed-specific ordinance, the AVMA says.

Denver’s ban was enacted in 1989.

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