PHILADELPHIA -- Each day, Patrick McCallion takes his 13-month-old dog Stewart to the corner park, where the exuberant yellow Lab mix can run loose with his pooch pals. But the park isn't enclosed.
So McCallion took out a bit of disappearance insurance, getting a microchip the size of a grain of rice implanted under the dog's skin, between the shoulder blades.
In the last few years, millions of dogs and cats -- as well as tigers and other unusual pets -- have been implanted with these microchips, which are encoded with unique numbers to make identifying lost, stolen or abandoned animals a snap.
When a lost pet is brought to a shelter or clinic, workers can use a hand-held scanner to read the chip's number. A computer database then matches the number with the pet's owner, medical history and other pertinent information.
At Queen Village Animal Clinic, where Stewart got his chip, the injection costs about $30, plus a one-time registration fee of $12.
''Probably every day at least one dog runs away from that park,'' said McCallion, 28, of Philadelphia. ''You always see the 'missing' posters on trees and poles around the city.''
The chips have been used to reunite thousands of lost pets with their owners. In northeastern Pennsylvania, LeeAnn Perry's dog, a yellow Lab named Sara, has run away three times since getting the chip a year and a half ago.
The pooch last disappeared in November, but was back home two weeks later.
''I know when she takes off, one way or another she'll be back because she's chipped,'' said Perry, 32, of Dunmore.
Microchip implantation has been around since the 1980s but was relatively rare until the mid-1990s, when chipmakers introduced a universal scanner that could read every model.
Scanners are now found in most shelters and animal control agencies across the country, according to Mary Madsen, a customer service supervisor for AVID Identification Systems Inc.
Norco, Calif.-based AVID is one of two dominant chipmakers. As of last year, 2.5 million pets were listed in the company database.
The American Kennel Club operates the other database, which contains more than 1.1 million pets and is affiliated with Schering-Plough Animal Health, distributor of the HomeAgain chip.
Most of the pets in the AKC database are dogs (842,645) and cats (265,349). However, HomeAgain chips, made by Destron Fearing Corp., can also be found in birds, horses, rabbits, tigers, monkeys, seals and many other unusual pets.
More than 70,000 lost pets have been reunited with their owners since the AKC program's inception in 1995, said Associate Director Keith Frazier.
Veterinarians say old-fashioned pet collars are fine, but not foolproof. They can come off, fade, or be chewed.
The chips are a boon to emergency room veterinarians, who often treat injured animals that don't have identifying information. Vets then face the tough choice of putting the animal to sleep or administering costly care with no hope of getting paid.
With a microchipped pet, the pet's owner can make that decision.
''For an emergency vet, it's fabulous,'' said Dr. Jeffrey Proulx of San Francisco. ''A lot of ER practices don't have the funds to go hog wild on these things.''
The chips have a variety of applications.
Officials at the Iditarod Trail Sled Dog Race use them to help prevent illegal dog substitutions. Valuable horses are sometimes injected instead of branded. In Chicago, owners of dogs considered ''dangerous'' are required to have their pets spayed or neutered and fitted with a microchip for identification.
Professional football player Damon Moore, of the Philadelphia Eagles, was charged last month with abandoning his 3-month-old Rottweiler puppy after police found the dog and the SPCA traced the microchip to the pet shop where Moore made his purchase.
The next-generation microchip will be equipped with a sensor that reads body temperature -- eliminating the need for a rectal thermometer.
Down the road, chips will be able to store information useful in an emergency -- such as whether a dog has had a rabies shot or is allergic to any medicine.
But chipmakers say it's likely that most information will continued to be stored in a database.
Could human microchip implantation be far behind?
Some say it's inevitable. A British researcher had a chip in his arm for nine days in 1998, and U.S. researchers say a chip attached to the retina could someday give blind patients the ability to see. Chips also could be used to carry medical information or criminal history, raising privacy concerns.
But for now, it's Rover who has the chip in his shoulder.
''Stewie used to have a tag,'' said McCallion, rubbing his newly microchipped dog's head, ''but his brother Murphy bit it off.''
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