Several systems are available for individually identifying research mice. Depending on the circumstances, either temporary or permanent methods may be used.
Several methods are particularly well suited for identifying cage mates. A felt tip marker may be used for marking an ear or tail; such marks usually disappear in 1-2 days. Food coloring may be used to dye a patch of fur; such marks generally last for 1-2 weeks but can be used only on albino and light-colored mice. A patch of fur on the back or side of the mouse may be shaved; such marks generally last 1-4 weeks (depending on stage of the hair cycle) and can be used on any color mouse.
Methods for permanently identifying mice include ear punching, ear tagging, tattooing, and subcutaneously implanting a numerically coded microchip.
Ear punching, commonly used for identification of rodents, involves using a special punch to either produce a small (0.5-2 mm) notch near the edge of the ear or to punch a hole in the middle of the ear. The holes and notches are placed according to a predetermined code (Dickie 1975; Ingalis 1980; Stark and Ostrow 1991). Before using an ear punch, it should be tested to make sure it works properly and the cutting edge should be disinfected. The punched-out ear tissue may be sufficient to perform PCR genotyping. Mice do not need to be anesthetized to be ear-punched. Ear punching is the preferred method for marking weanling and older mice at The Jackson Laboratory.
Ear tagging involves using a special applicator to place an "earring," or metal tag with an identification number, in the pinna of a mouse's ear. Because tags are relatively large and mouse ears develop quickly, ear tagging is suitable only for weanling and older mice. The applicator tip and ear tags should be disinfected before they are used. Ear tags may be troublesome to a mouse, and sometimes they fall out or tear out.
Tattooing is the preferred method for identifying neonatal mice. The tattoo is placed on a toe or tail. Tattoos may also be used to mark weanling and older mice. The tattoo is placed on the tail, toe or ear. While being tattooed, neonatal mice are manually restrained. Older mice are either placed in a restraining device or sedated. Before you tattoo a mouse, disinfect its skin and the tattoo needles. Use pigment approved by the United States Food and Drug Administration.
Subcutaneously implanting a microchip is another method of permanently identifying a mouse. Microchips are about the size of a grain of rice and transmit a unique alphanumeric code for each mouse. The codes are easily distinguished from one another by a computer. Because the implants are relatively large, they are not recommended for newborns, but they are acceptable for identifying weanlings and adults. Before being implanted, mice are either sedated or anesthetized. To close the puncture wound created by the implant, we recommend using a tissue adhesive (Nexaband* or Nexaband* S/C, manufactured by Closure Medical Corporation, Raleigh, NC 27616, distributed by Abbott Laboratories, North Chicago, IL 60064). Sutures or wound clips are not recommended. Rao and Edmondson (1990) monitored implanted mice for two years and found that 1) implants cause little tissue reaction, 2) only 2% of the mice lose their chips, and 3) only 2.8% of the chips fail.
Resources for instruments
Following are the names and addresses of some companies that sell animal identification tools. Other companies may also sell them.
Ear tags (model 1005-1)
Tattoo kits (include a micro-tattoo instrument, tattoo paste, a planchette, hypodermic needles and a carrying case)
Dickie, MM. 1975. Keeping Records. In: Green EL, editor. Biology of the Laboratory Mouse. New York, New York: Dover Publications; 706 p.
Ingalis, JK. 1980. Introduction to Laboratory Animal Science and Technology. New York, New York: Pergamon Press; 323 p.
Rao GH, J Edmondson. 1990. Tissue reaction to an implantable identification device in mice. Toxicologic Pathology 18:412-16.
Stark, DM and ME Ostrow, editors. 1991. AALAS training manual series, vol 1: Assistant laboratory animal technician. Tennessee: Cordova; 186 p.