There are a number of factors at work, according to both breeders and rescue operations. In the last decade animal rights groups have effectively convinced the public that breeders are politically incorrect. As a consequence, breeders are producing fewer litters.
At the same time, many parts of the country have done such a good job of animal population control that there are no adoptable puppies in shelters. Also, say CDC officials, unregulated Internet sales and smuggling of international pets is on the rise.
The majority of dogs - 33 percent - are acquired through a friend or adopted as strays, according to the American Pet Product Manufacturer's Association. Another 27 percent come from breeders and 10 percent from shelters.
Patti L. Strand, president of the National Animal Interest Alliance (NAIS), a consortium of pet owners, breeders and veterinarians, said it's the newer shelters and rescue groups who are most involved in "humane relocation."
Strand said that only two years ago, when NAIA began researching the issue, foreign imports on shelter Web sites varied in age. "Today, most of them are puppies," she said. "It's is easy to speculate that … enterprising rescuers and shelter directors could help developing countries become breeding grounds for stocking U.S. shelters."
A hobby breeder, Strand worries that the practice could "diminish the responsible breeding and placement of well-bred, healthy dogs and cats."
Other animal rights groups say adopting overseas makes no sense when 4 million unwanted dogs are put to death each year.
"We are obviously barely coping with our own overpopulation crisis," said Daphna Nachminovitch, director of People for the Ethical Treatment of Animals (PETA), who said many animal shelters face little regulation, and budgets for animal control are strained.
"If a shelter is empty, it's not doing its job," said Nachminovitch. "I can take you anywhere in this country and on streets are homeless, unsprayed animals with litters. I talk to people in the trenches every day. Their doors are not wide open enough."
Even though dog overpopulation is rampant in some states, particularly in the South, successful spaying and neutering programs in the Northwest, California and the Northeast have created a dearth of adoptable puppies, say local shelters.
One rescue group -- Save a Sato (Spanish slang for mixed breed) -- has brought an estimated 14,000 dogs from Puerto Rico to the United States since its founding in 1996, according to Massachusetts volunteer Twig Mowatt.
The island is home to thousands of abandoned and abused dogs in need of homes.
"It's a beautiful thing," said Mowatt, who herself adopted a terrier named Rico. "People are really desperate for an adoptable dog, and they don't want to go to pet stores or breeders. Otherwise there would be nothing."
In Massachusetts, where many of these dogs are adopted by shelters, puppies must be held for 48 hours before being released to homes to watch for signs of illness.
But protocol failed in 2004, when one puppy was diagnosed with rabies during quarantine at a shelter in Cape Ann, Mass. The puppy -- only several months old -- developed neurological problems consistent with rabies and was euthanized, according to Donna Rheaume, a spokesman for the state department of health.
A state lab confirmed the animal had rabies, she said, and all who had handled the dog were given preventive treatment.