The inflammatory process during insertion and occurrence of migration are major concerns of horse owners when determining the usefulness of microchip identification. The objectives of a study conducted at Pennsylvania State University were to characterize the inflammatory response after microchip insertion, evaluate pain response and swelling at the microchip insertion site, and measure migration of the microchips.
Eighteen mature Quarter Horse mares were assigned randomly to three treatment groups. The microchip group (n = 7) had microchips inserted using a sterile needle and syringe; the "sham" group (n = 7) had a needle inserted but no microchip; and the control group (n = 4) had no insertion. The insertion site was visually determined by a veterinarian to be within the nuchal ligament on the left side of the horse, with a transverse position halfway between withers and poll, and dorsal position several centimeters below the crest of the neck.
The inflammatory response was measured over a 2-week period by measuring dermal temperature, response to pressure and swelling at the insertion site, and plasma serum amyloid A (SAA). SAA is expressed in response to inflammatory stimuli.
For the migration component of the study, radiographs of the seven microchipped horses were taken over 6 months after insertion. These radiographs allowed measurement between a select vertebral point and the microchip.
The microchip and sham insertion did not cause a detectable increase in temperature. Algometer readings, used to quantify pressure necessary to induce a pain threshold response, indicated that microchip insertion area was more sensitive than sham insertion at 2 hours on day 1, and day 3 post insertion. Visible swelling began 2 hours post-insertion and resolved by day 3. SAA concentrations were affected by day following insertion, but not by treatment group. Increases in SAA concentration could not be matched with local insertion reactions. Migration was not detected in any of the horses during the 6 months.
Microchip identification is a viable alternative form of identification for equids. It does not cause excessive inflammation or continued tissue irritation after insertion. It also does not migrate if implanted within the nuchal ligament on the left side of the horse halfway between the withers and poll.
However, it may be important to continue to assess the microchips for migration in young growing animals. For some breed registries, identification has to be obtained on registration, so microchip insertion will have to be performed at less than 1 year of age. Although the microchip may not actually be moving, the development of the tissues around the microchip could put it in a different position than anticipated. Future studies should investigate ideal location as affected by age for microchip insertion.
Summarized by Krishona Martinson, PhD, University of Minnesota