"For female dogs there is an increased risk of mammary cancer if they are allowed to go through multiple heat cycles, which is one of the main reasons we spay them before their first heat cycle. We see intact female dogs get an infected uterus called a pyometra, which can be life-threatening and requires surgery to remove the uterus," Proietto told The Dodo.
"For male dogs they have an increased risk of prostatitis (inflammation of the prostate) as well as testicular cancer because they remain intact," he added.
The ASPCA agrees, and cites the high fatality rates of breast cancer in dogs.
4. "Spaying or neutering will cause my pet unnecessary pain."