LOS ANGELES -- When Stephen Zawistowski got his first dog 50 years ago, she was the only dog in the neighborhood that was spayed.
"She had an incision that must have been a foot long and was sewn up with what looked like piano wire," says Zawistowski, science adviser for the New York-based American Society for the Prevention of Cruelty to Animals.
It took years of campaigning to change thinking about sterilizing pets, but it has paid off. This year fewer than 4 million unwanted dogs and cats will be euthanized, down from as many as 20 million before 1970.
There are several reasons: Aggressive adopt-a-pet campaigns are carried out every day in cities all over the country and breed rescues save many dogs. But animal experts believe spaying and neutering has played the biggest role in saving so many lives.
Nearly every public shelter, private rescue or animal welfare organization in the country donates money, space or time to low-cost spay and neuter clinics.
Spaying and neutering has become the law in some states, counties and cities. Many states require all shelter animals to be sterilized. Rhode Island requires most cats to be sterilized, and Los Angeles requires most dogs and cats to be spayed or neutered by the time they are 4 months old.
While shelters are firmly onboard, the biggest problem has been selling sterilization programs to pet owners.
When pets are sterilized, their reproductive organs are removed so they can no longer breed. Some people consider that unnecessary mutilation of their pets.
There are those who say: "You won't do that to my dog because I wouldn't want it done to me," Zawistowski says.
And it wasn't just pet owners who had to be convinced -- so did veterinarians, he says.
Medical procedures have caught up in the last half-century and a lot of people have changed their thinking.
"Now they make a one- or two-inch incision and use self-absorbing sutures" that mean a much quicker recovery for the animals, Zawistowski says.
Aimee Gilbreath, executive director of Found Animals, a Los Angeles-based charity, agrees. "It's become a tenant of responsible ownership," she says of spaying and neutering.
"I'm 36. My childhood dog was never neutered. We lived in some rural places and he roamed the countryside. Who knows how many litters he fathered," Gilbreath says.
"Today my dog is neutered. My parents' dog is spayed. In my family, it's gone from something you didn't do to something that is automatic."
In addition to eliminating shelter kills, spaying and neutering can make pets easier to manage, less aggressive and healthier, said Andrew N. Rowan, president and CEO of Humane Society International and chief scientific officer for the Humane Society of the United States.
So what drove the changes, and what now?
The first public spay and neuter clinic in the U.S. was opened in Los Angeles in 1969 and was so successful that there was a four-month waiting list. Another opened four years later and charged $17.50 for spaying female cats and dogs and $11.50 for neutering males, said Linda Gordon, director of facility development for the city's Department of Animal Services.
A low-cost shelter today charges about $55 to neuter a male cat and $60 to spay a female cat, around $150 to spay a female dog under 30 pounds and between $150 and $250 (depending on size) to neuter a dog, said Carrie Harrington, communications director for the Marin Humane Society in Novato, north of San Francisco.
The first private low-cost clinic was opened in 1973 by the Marin shelter, using $42,000 raised by its women's auxiliary thrift store. This year, the group will raise money to help sterilize about 2,300 pets, Harrington said.
The steep decline in the number of animals being euthanized each year comes even as the pet population has boomed. In 1970, there were about 62 million companion pets and today there are about 170 million, Zawistowski said.
But 4 million animals put to death is still 4 million too many, said Betsy Banks Saul, co-founder of Petfinder.com, an online adoption database that has helped with 17 million adoptions since it started in 1996.
The next step may be in the form of an affordable pill, implant or vaccine to sterilize cats and dogs.
Dr. Gary Michelson, a billionaire orthopedic spinal surgeon and founder of Found Animals, posted a $25 million prize in 2008 for the creator of such an affordable chemical sterilant that works in male and female cats and dogs. Michelson has also put up $25 million for grants to allow scientists to do the research.
Found Animals has received 150 grant applications so far and 14 projects have been approved for funding, Gilbreath said.
"When we first saw grant proposals coming in, we saw old ideas that had been laying around for 15 or 20 years. What we are seeing now are proposals based on cutting edge science -- areas related to cancer and stem cell research. The level and sophistication of the science has moved to a higher level," said Zawistowski, who is on the prize board.
In 2003, the Food and Drug Administration approved the first sterilant for male dogs. But at about $50 a shot, Neutersol was too costly. It was reworked, the price was cut to about $6 a dose and it was again approved by the FDA under the name Esterilsol, Zawistowski said. It's been used in trials around the world and is expected to be available in the United States later this year.
Miami-based 600million.org, named for the number of stray dogs that can be found around the world on any given day, is working on a contraceptive pill for dogs. The most promising and closest to completion is the female sterilization pill, said group founder Alex Pacheco.