An RFID chip (also known as PIT tag) next to a grain of rice.
A microchip implant is an identifyingintegrated circuit placed under the skin of an animal. The chip, about the size of a large grain of rice, uses passive RFID (Radio Frequency Identification) technology, and is also known as a PIT tag (for Passive Integrated Transponder) .
Externally attached microchips such as RFID ear tags are commonly used to identify farm and ranch animals other than horses. Some external microchips can be read with the same scanner used with implanted chips.
Animal shelters, animal control officers and veterinarians routinely look for microchips to return lost pets quickly to their owners, avoiding expenses for housing, food, medical care, outplacing and euthanasia. Many shelters place chips in all outplaced animals.
Microchips can activate some pet doors programmed to recognize specific animals.
Some countries require microchips in imported animals to match vaccination records. Microchip tagging may also be required for CITES-regulated international trade in certain endangered animals: for example, Asian Arowana are tagged to limit import to captive-bred fish. Also, birds not banded who cross international borders as pets or for trade must be microchipped so that each bird is uniquely identifiable.
Information about the implant is often imprinted on a collar tag worn by a pet
Microchips can be implanted by a veterinarian or at a shelter. After checking that the animal does not already have a chip, the vet or technician injects the chip with a syringe and records the chip's unique ID. No anesthetic is required. A test scan ensures correct operation.
An enrollment form is completed with chip ID, owner contact information, pet name and description, shelter and/or veterinarian contact information, and an alternate emergency contact designated by the pet owner. Some shelters and vets designate themselves as the primary contact to remain informed about possible problems with the animals they place. The form is sent to a registry, who may be the chip manufacturer, distributor or an independent entity; some countries have a single official national database. For a fee, the registry typically provides 24-hour, toll-free telephone service for the life of the pet. Some veterinarians leave registration to the owner, usually done online, but a chip without current contact information is essentially useless.
The owner receives a registration certificate with the chip ID and recovery service contact information. The information can also be imprinted on a collar tag worn by the animal. Like an automobile title, the certificate serves as proof of ownership and is transferred with the animal when it is sold or traded; an animal without a certificate could be stolen.
Authorities and shelters examine strays for chips, providing the recovery service with the ID number, description and location so they may notify the owner or contact. Multiple registries may have to be consulted; see Pet recovery service for further discussion of related issues. If the pet is wearing the collar tag, the finder does not need a chip reader to contact the registry. An owner can also report a missing pet to the recovery service, as vets look for chips in new animals and check with the recovery service to see if it has been reported lost or stolen.
Many veterinarians scan an animal's chip on every visit to verify correct operation. Some use the chip ID as their database index and print it on receipts, test results, vaccination certifications and other records.
Components of a microchip
A microchip implant is a passive RFID device. Lacking an internal power source, it remains inert until it is powered by the scanner.
Most implants contain three elements: a 'chip' or integrated circuit; a coil inductor, possibly with a ferrite core; and a capacitor. The chip contains unique identification data and electronic circuits to encode that information. The coil acts as the secondary winding of a transformer, receiving power inductively coupled to it from the scanner. The coil and capacitor together form a resonant LC circuit tuned to the frequency of the scanner's oscillating magnetic field to produce power for the chip. The chip then transmits its data back through the coil to the scanner.
Example of an RFID scanner used with animal microchip implants.
These components are encased in biocompatible soda lime or borosilicate glass and hermetically sealed. Barring rare complications, dogs and cats are unaffected by them.
In dogs and cats, chips are usually inserted below the skin at the back of the neck between the shoulder blades on the dorsal midline. According to one reference, continental European pets get the implant in the left side of the neck. The chip can often be felt under the skin. Thin layers of connective tissue form around the implant and hold it in place.
Horses are microchipped on the left side of the neck, halfway between the poll and withers and approximately one inch below the midline of the mane, into the nuchal ligament.
Birds are implanted in their breast muscles. Proper restraint is necessary so the operation requires either two people (an avian veterinarian and a veterinary technician) or general anesthesia.
Microchips are not yet universal, but they are legally required in some jurisdictions such as the state of New South Wales, Australia. Some countries, such as Japan, require ISO-compliant microchips or a compatible reader on imported dogs and cats.
In New Zealand, all dogs first registered after 1 July 2006 must be microchipped. Farmers protested that farm dogs should be exempt, drawing a parallel to the Dog Tax War of 1898. Farm dogs were exempted from microchipping in an amendment to the legislation passed in June 2006. A National Animal Identification and Tracing scheme in New Zealand is currently being developed for tracking livestock.
In April 2012 Northern Ireland became the first part of the United Kingdom to require microchipping of individually licenced dogs. England will have mandatory microchipping of all dogs by 2016.
The United States uses the National Animal Identification System for farm and ranch animals other than dogs and cats. In most species except horses, an external eartag is typically used in lieu of an implant microchip. Eartags with microchips or simply stamped with a visible number can be used. Both use ISO 15 digit microchip numbers with the U.S. country code of 840.
Cross-compatibility and standards issues
In most countries, pet ID chips adhere to an international standard to promote compatibility between chips and scanners. In the US, however, three proprietary types of chips compete along with the international standard. Scanners distributed to US shelters and vets well into 2006 could each read at most three of the four types. Scanners with quad-read capability are now available and are increasingly considered required equipment. Older scanner models will be in use for some time so US pet owners must still choose between a chip with good coverage by existing scanners and one compatible with the international standard. The four types include:
The ISO Conformant Full Duplex type has the greatest international acceptance. It is common in many countries including Europe (since the late 1990s) and Canada. It is one of two chip protocol types (along with the "Half Duplex" type sometimes used in farm and ranch animals) that conform to International Organization for Standardization standards ISO 11784 and ISO 11785. To support international/multivendor application, the 3-digit country code can contain an assigned ISO country code or a manufacturer code from 900 to 998 plus its identifying serial number.[Note 1] In the US, distribution of this type has been controversial. When 24PetWatch.com began distributing them in 2003 (and more famously Banfield Pet Hospitals in 2004) many shelter scanners couldn't read them. (Some still can't; asking local shelters about this is still a good idea.) At least one Banfield-chipped pet was inadvertently euthanized.
The Trovan Unique type is another pet chip protocol type in use in US pets since 1990. Patent problems forced the withdrawal of Trovan's implanter device from US distribution and they became uncommon in US pets, although Trovan's original registry database "infopet.biz" remained in operation. In early 2007, the American Kennel Club's chip registration service, AKC Companion Animal Recovery Corp., "akccar.org", which had been the authorized registry for HomeAgain brand chips made by Destron/Digital Angel, began distributing Trovan chips with a different implanter. These chips are read by the Trovan, HomeAgain (Destron Fearing), and Bayer (Black Label) readers. Despite multiple offers from Trovan to AVID  to license the technology to read the Trovan chips, AVID continues to distribute readers that do not read Trovan or the ISO compliant chips.
A third type sometimes known as FECAVA or Destron[Note 2] is available under various brand names. These include, in the US, "Avid Eurochip", the common current 24PetWatch chips, and the original (and still popular) style of HomeAgain chips. (US HomeAgain and 24Petwatch now can supply the true ISO chip instead on request.) Chips of this type have 10 digit [hexadecimal] chip numbers. This "FECAVA" type is readable on a wide variety of scanners in the US and has been less controversial, although its level of adherence to the ISO standards is sometimes exaggerated in some descriptions. The ISO standard has an annex (appendix) recommending that three older chip types be supported by scanners, including a 35-bit "FECAVA"/"Destron" type. The common Eurochip/HomeAgain chips don't agree perfectly with the annex description, although the differences are sometimes considered minor.[Note 3] But the ISO standard also makes it clear[Note 4] that only its 64-bit "full-duplex" and "half-duplex" types are "conformant"; even chips (e.g., the Trovan Unique) that match one of the Annex descriptions are not. More visibly, FECAVA cannot support the ISO standard's required country/manufacturer codes. They may be accepted by authorities in many countries where ISO-standard chips are the norm, but not by those requiring literal ISO conformance.
Finally, there's the AVID brand Friendchip type, which is peculiar due to its encryption characteristics. Cryptographic features are not necessarily unwelcome; few pet rescuers or humane societies would object to a design that outputs an ID number "in the clear" for anyone to read, along with authentication features for detection of counterfeit chips. But the authentication in "Friendchips" has been found lacking and rather easy to spoof to the AVID scanner. Although no authentication encryption is involved, obfuscation requires secret information to convert transmitted chip data to its original label ID code. Well into 2006, scanners containing the secrets were provided to the US market only by AVID and Destron/Digital Angel; Destron/Digital Angel put the decryption feature in some, but not all, of its scanners possibly as early as 1996. (For years, its scanners distributed to shelters through HomeAgain usually had full decryption, while many sold to vets would only state that an AVID chip had been found.) Well into 2006, both were resisting calls from consumers and welfare group officials to bring scanners to the US shelter community combining AVID decryption capability with the ability to read ISO-compliant chips. Some complained that AVID itself had long marketed combination pet scanners compatible with all common pet chips except possibly Trovan outside the US. By keeping them out of the US, it could be considered partly culpable in the missed-ISO chips problem others blamed on Banfield.[Note 5] In 2006, the European manufacturer Datamars, a supplier of ISO chips used by Banfield and others, gained access to the decryption secrets and began supplying scanners with them to US customers. This "Black Label" scanner was the first four-standard full-multi pet scanner in the US market. Later in 2006, Digital Angel announced that it would supply a full-multi scanner in the US.[Note 6] In 2008 AVID itself announced a "breakthrough" scanner, although as of October 2010 AVID's is still so uncommon that it's unclear whether it supports the Trovan chip. Trovan itself also got the decryption technology in or before 2006, and now provides it in scanners distributed in the US by AKC-CAR. (Some are quad-read, but others lack full ISO support.)
Numerous references in print state that the incompatibilities between different chip types are a matter of "frequency". One may find claims that early ISO adopters in the US endangered their customers' pets by giving them ISO chips that work at a "different frequency" from the local shelter's scanner, or that the US government considered forcing an incompatible frequency change. These claims were little challenged by manufacturers and distributors of ISO chips, although later evidence suggests the claims were disinformation. In fact, all chips operate at the scanner's frequency. Although ISO chips are optimized for 134.2 kHz, in practice they are readable at 125 kHz and the "125 kHz" chips are readable at 134.2 kHz. Confirmation comes from government filings which indicate that supposed "multi-frequency" scanners now commonly available are really single-frequency scanners operating at 125, 134.2 or 128 kHz.) In particular, the US HomeAgain scanner didn't change excitation frequency when ISO-read capability was added; it's still a single frequency, 125 kHz scanner.
Scanner Compatibility table for chip types used in pets
Expected results for chip type (OK=Good read NR=No read DO=Detect Only with no number given)
Scanner to test
ISO Conformant Full Duplex chip
AVID Encrypted "FriendChip"
Original U.S. HomeAgain, AVID Eurochip,[Note 7] or FECAVA
"Trovan Unique" and current AKC CAR chips
Minimal ISO Conformant Scanner (also must read HALF Duplex chips common in livestock ear tags)
OK but extra step required (web-based decryption service)
(For users requiring Shelter-Grade certainty, this table is not a substitute for testing the scanner with a set of specimen chips. One study cites problems with certain Trovan chips on the Datamars Black Label scanner. In general the study found none of the tested scanners to read all four standards without some deficiency. The study predates the most recent scanner models, however.)
Reported adverse reactions
RFID chips are used in animal research, and at least three studies conducted since the 1990s have reported tumors at the site of implantation in laboratory mice and rats. Noted veterinary associations responded with continued support for the procedure as reasonably safe for cats and dogs, pointing to rates of serious complications on the order of one in a million in the U.K., which has a system for tracking such adverse reactions and has chipped over 3.7 million pet dogs. A recent study found no safety concerns for microchipped animals with RFID chips undergoing MRI at one Tesla magnetic field strength. In 2011 a microchip-associated fibrosarcoma was reported found in the neck of a 9-year old, neutered-male cat. Histological examination was consistent with postinjection sarcoma, but all prior vaccinations occurred in the hindlegs.
^For display, typically the three country/manufacturer code digits are followed by twelve digits of serial number to make a 15-digit numeric string.
^Curiously, an actual matching descriptive specification from the Federation of European Companion Animal Veterinary Associations, or one from Destron Corporation, remains illusive.
^The differences are obvious and easily surmounted by someone trying to make a scanner for a FECAVA chip specimen, so the Annex is still quite useful. (The actual FECAVA frequency-modulated signals are inverted (backwards) from the Annex specification.)
^This is found in clauses 2 and 6 of ISO 11785; the two actual conformant 64-bit types are described in clauses 6.1 and 6.2.
^Few of the petitioners bothered to ask AVID to add Trovan-chip compatibility at that time, as these chips would remain uncommon and obscure until 2007 in the U.S.
^In addition to its current scanners with full support for ISO full duplex chips, and maybe ten years production of earlier scanners with no ISO support, Destron/Digital Angel Corp. is also reported to have made in-between models circa 2006, one that gives a detection indication but no number for ISO chips, and one model that gives either simple detection or full number readout depending perhaps on the chip's manufacturer or some other factor. These models may be hard to discern without a lot of specimen chips; upgrades may be available, especially to current customer partners of HomeAgain.
^A mention of a chip type called "AVID Travelchip" has been removed from this heading. It appears that "Travelchip" was actually a trademark not of AVID itself but of a chip distributor, which used it as a blanket term for several different chip types sold in value-added kits- firstly AVID Eurochips, later HomeAgain types both regular and ISO.
^U.S. FCC database search form (Submit the form with "Grantee Code" and "Product Code" for each individual scanner; for the new universal Digital Angel/HomeAgain Scanner, still operating at 125 kHz (0.125 megahertz) codes "C5S" and "HS9250L"; for a recent AVID scanner, operating at 134.2 kHz (0.1342 megahertz), codes "IOL" and "-134-AV1034I" .)