Dr. Fayrer-Hosken locates the longissimus muscle, where the injection will be given

ATHENS, Georgia (CNN) -- If the thought of causing your pet pain, combined with the expense of sterilization surgery, has kept you from getting your pet spayed, take heart: a less painful, less expensive alternative is on the horizon.

One day, you may be able to get your female pet spayed with a simple injection.

Richard Fayrer-Hosken, a veterinarian at the University of Georgia, has developed a vaccination that uses a female mammal's immune system to chemically sterilize her.

Fayrer-Hosken's spay shot is a genetically engineered animal protein which travels to the ovaries and invades the zona pellucida, the harmone-rich gel that covers the eggs of all mammals.

The invasion prompts the animal's immune system to respond by counterattacking with a flood of white blood cells. The end result: the zona pellucida is stripped away, blocking the production of key reproductive harmones, and the eggs wither.

"And when this happens, the animal is truly chemically sterilized without surgery," Fayrer-Hosken told CNN. ( 168K/6 sec. AIFF or WAV sound)

The veterinarian says his vaccine proved to be safe and effective in a small clinical trial, using dogs as test subjects. He wants to complete a larger trial -- using a thousand or more dogs -- before asking the U.S. Food and Drug Administration for approval of the spay vaccine.

"We know it works, we know it works 100 percent. So, we've just got to get the (test) number bigger to satisfy ourselves that it is safe, that it is practical, and (that) it can be translated into a product," he said. ( 196K/7 sec. AIFF or WAV sound)

Fayrer-Hosken says he's also tried his spay vaccine on elephants at Kruger National Park in South Africa.

The protected African elephant prospers too well on the park's limited lands and resources. That has forced gamekeepers to kill entire elephant families, in order to preserve greater numbers of the population.

The treatment seems to work on any mammal

In their search for an alternative population control method, park officials allowed Fayrer-Hosken to test his vaccine. He said his method proved twice as effective as others being used to stop the elephants from reproducing.

"So it is incredibly satisfying that we are helping such a magnificent and incredible animal," he said. ( 364K/15 sec. AIFF or WAV sound)

Fayrer-Hosken says because the vaccine appears to have the same effect on all mammals, it may eventually be used on dogs, cats and other animals. It could also eventually become an alternative contraception for humans.

He also thinks it won't be long before his method wins FDA approval.

"I think the release is very imminent, but what we would want to do is do the science absolutely correctly first, rather than be rushed by an obvious goal -- save the animals," he said.

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CNN's Rick Lockridge reports.
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Veterinarians say they encourage the spaying and neutering of pets to stop overpopulation, and that they charge about half of what they should in hopes that more pet owners will seek the surgery.

But the operations are still expensive enough that many pets never get them.

Humane societies say they about 30 percent of the people who adopt pets fail to honor the adoption contract, which requires that the animal be spayed or neutered. Failure to honor the contract means already crowded shelters are forced to continue housing animals that would otherwise be adopted out.

Veterinarians and humane societies say they would welcome a faster, cheaper sterilization method that may lead more pet owners to get their pets spayed.

"For veterinarians, it would make life a lot easier to have a safe, efficacious vaccine that you could use to sterilize dogs. There is no doubt about it. I would welcome it," said Dr. Peter Mueller, of Atlanta.

Correspondent Rick Lockridge contributed to this report.